Programming Concepts and Logics

The systematic method of making a computer program is called programming. A programmer studies the problem, plans the steps, writes the code, tests it and fixes the errors.

Features of a good program:

A programming language is an artificial language used to give data and instructions to the computer so that it does a particular task. A computer cannot understand English, so programs are written in a programming language.

On the basis of their development, programming languages are of the following types.

Some high level languages are:

System software is a group of programs that controls and manages the computer hardware and lets the user work with it. It does not solve one particular problem of the user. It runs the computer, controls memory, CPU and other devices such as printers, and runs the application programs. Operating systems, device drivers and language processors are system software.

Language processors translate a program written in a programming language into machine language. There are three kinds:

Syntax means the grammar rules of a programming language. These rules tell how the words, symbols and statements can be written and joined. If a statement breaks a rule, it is a syntax error. Example: a missing semicolon, a misspelt keyword or a bracket that is not closed.

Semantics means the meaning of a statement, that is, what it does when the program runs. A statement can be correct in grammar but wrong in meaning. This is a semantic (logical) error. The program runs, but gives a wrong answer. Example: writing avg = a + b / 2; when you want the average of a and b. The correct statement is avg = (a + b) / 2;.

A syntax error is caught by the compiler, but a semantic error has to be found by the programmer by testing the output.

A bug is a fault in a program that makes it give wrong output or stop working. Finding and removing bugs is called debugging.

The structures that control the order in which the statements of a program are executed are called control structures. A program is designed using three control structures.

“Pseudo” means false. So pseudo-code is a false code. It is the logic of a program written in simple English statements. It looks like a program but it is not written in any real language, so it does not need the correct syntax and a computer cannot run it. It is also called structured English. Programmers write it before the real code because it is easy to write and easy to change.

Pseudo-code to find the largest of three numbers

Input a, b, c
If a >= b and a >= c then
    Largest = a
Else if b >= a and b >= c then
    Largest = b
Else
    Largest = c
End if
Print Largest

Note: the signs >= are used so that the logic also works when two numbers are equal.

Program documentation is the written description of a program and the system around it. Some of it is inside the program as comments. The rest is on paper or in files: the aim of the program, system analysis, the logic used, flowcharts, program code, limitations and a user manual.

Types of documents

Why it is needed

A flowchart is the picture of a program or an algorithm. The steps are drawn with standard symbols and joined by arrows to show the flow of the logic. It makes a problem easy to understand.

Figure: Basic flowchart symbols and what they mean

Advantages: useful when the program has to be modified, and it gives good documentation. Disadvantages: drawing takes time, a change means redrawing, and it becomes complicated for programs with many branches and loops.

Program debugging is the process of finding and removing the errors (bugs) of a program. Hardly any program runs correctly the first time, so debugging is a necessary and time-consuming part of program development.

A language is called a high level language when it has these characteristics:

An algorithm is a finite set of steps that, if followed, solves a particular problem. Every task in our daily life, such as making tea, follows some steps. When these steps are written for the computer, they form an algorithm.

Algorithm for the commission

Step 1: Start
Step 2: Read the sales amount SA
Step 3: If SA < 500 then
            commission = SA * 2 / 100
        Else if SA < 5000 then
            commission = SA * 5 / 100
        Else
            commission = SA * 10 / 100
Step 4: Print commission
Step 5: Stop

Check: for SA = 4000 the commission is 4000 × 5 / 100 = Rs. 200. For SA = 6000 it is 6000 × 10 / 100 = Rs. 600.

Algorithm. An algorithm is a finite list of steps, each step written in plain words, that solves a problem. Example: an algorithm to add any two numbers.

Step 1: Start
Step 2: Read two numbers a and b
Step 3: c = a + b
Step 4: Print c
Step 5: Stop

Flowchart. A flowchart is the pictorial form of an algorithm. The same steps are drawn with flowchart symbols and arrows. Here is the flowchart of the algorithm above.

A program design tool helps the programmer to plan the logic before writing the code. The tools are:

Computers store everything as 0s and 1s, so numbers and characters need a code.

C is a general purpose programming language developed by Dennis Ritchie at AT&T Bell Laboratories, USA, in 1972. It was first used to write the UNIX operating system. C is called a middle level language, because it has the easy statements of a high level language and also lets the programmer work close to the hardware like a low level language.

Why C is a structured language: in C a big problem is broken into small parts called functions, and each function does one job. The program is written using only the three control structures (sequence, selection and iteration), so the flow is clear and it is easy to read, test and correct. This style is called structured programming.

Features of C

A C program is written in a fixed order. The parts are:

/* Program to print a message */          <- comment
#include<stdio.h>                           <- preprocessor section
#include<conio.h>
void main()                                 <- main function
{
    printf("Welcome to C");                 <- statement
    getch();
}

Each statement ends with a semicolon, and the body of a function is placed between { and }.

The preprocessor is a part of the C compiler that works before the real compilation begins. It handles the lines that start with the # sign, called preprocessor directives. These lines do not end with a semicolon.

Header files are files that have the declarations of library functions. They end with .h. A function can be used only after its header file is included.

#include<stdio.h>
#define PI 3.14159
void main()
{
    float r = 2, area;
    area = PI * r * r;
    printf("%f", area);
}

Keywords are words that have a fixed meaning for the C compiler. They are reserved for special purposes, so they cannot be used as names of variables. They are written in lower case. Standard C has 32 keywords.

A constant is a value that does not change while the program is running. Constants are also called literals. For example, 5, 3.14 and ‘A’ are constants.

A variable is a name given to a memory location, where a value is kept. The value of a variable can change during the program. Before use, a variable must be declared with its data type, for example int x;.

For example, in x = 5; the 5 is a constant and x is a variable. Later x can hold 8 or 10, but the constant 5 is always 5.

There are two groups of constants in C: primary and secondary.

a. Primary constants

b. Secondary constants: arrays, structures, pointers, unions and enums.

A data type tells the compiler what kind of value a variable will keep and how much memory it needs.

The size of int depends on the compiler. It is 2 bytes in Turbo C and 4 bytes in most modern compilers. The modifiers short, long, signed, unsigned change the size or range. For example, long int takes 4 bytes and is printed with %ld.

A format specifier starts with the % sign followed by a letter. It tells printf() and scanf() the type of the data to print or read. It is also called a conversion specification.

int a = 10;
float b = 2.5;
char c = 'x';
printf("%d %f %c", a, b, c);

Statement. A statement is an instruction to the computer. It ends with a semicolon. The kinds are the declaration statement (int a;), the expression statement (a = b + 2;), the compound statement (many statements inside { }) and control statements (if, for, while).

Operator. A symbol that does an operation on data. The data it works on is called an operand. The groups of operators are arithmetic (+ – * / %), relational (< > <= >= == !=), logical (&& || !), assignment (=, +=, -=), increment and decrement (++, –), and conditional (? :).

Expression. A combination of operands and operators that gives one value. Examples: a + b, x * y - 5, a > b.

In an expression, brackets are solved first, then * / %, then + -, and the assignment = comes last. So 2 + 3 * 4 is 14, and (2 + 3) * 4 is 20.

Input and output functions are in the header file stdio.h. Input means taking data from the keyboard, and output means showing data on the screen.

The & sign before a variable name in scanf() gives the address of the variable, so the typed value is stored there. For a string, the & is not used.

int a;
printf("Enter a number: ");
scanf("%d", &a);
printf("You entered %d", a);

getchar() reads only one character at a time. It shows the typed character on the screen and waits for the Enter key. putchar() also prints only one character at a time. So to read or print a string such as “Nepal”, the functions must be used again and again inside a loop, which makes the program long.

For strings it is better to use gets() and puts(), or scanf("%s", name) (which stops at the first space).

A number is even when it leaves remainder 0 on division by 2. Otherwise it is odd.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a;
    printf("Enter a number: ");
    scanf("%d", &a);
    if(a % 2 == 0)
        printf("Even");
    else
        printf("Odd");
    getch();
}

7

Enter a number: Odd

The first number is compared with the other two. If it is not the largest, the second is compared in the same way. The last number is the largest if both fail. The sign >= is used so that the program also works when two numbers are equal.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b, c;
    printf("Enter any three numbers: ");
    scanf("%d%d%d", &a, &b, &c);
    if(a >= b && a >= c)
        printf("Largest = %d", a);
    else if(b >= a && b >= c)
        printf("Largest = %d", b);
    else
        printf("Largest = %d", c);
    getch();
}

12 45 30

Enter any three numbers: Largest = 45

if(condition)
{
    statements;
}

if-else: runs one block when the condition is true and another when it is false.

if(condition)
{
    statements 1;
}
else
{
    statements 2;
}

else-if ladder: used when there are many conditions. The first true condition is run and the rest are skipped.

if(condition 1)
    statements 1;
else if(condition 2)
    statements 2;
else
    statements 3;

switch: chooses one block out of many by matching the value of an expression with the case labels. The break statement stops the control from falling into the next case. The default block runs when no case matches.

switch(expression)
{
    case value1: statements; break;
    case value2: statements; break;
    default: statements;
}

while loop checks the condition first, then runs the body.

i = 1;
while(i <= 5)
{
    printf("%d ", i);
    i++;
}

do-while loop runs the body once first, then checks the condition. Note the semicolon after the condition.

i = 1;
do
{
    printf("%d ", i);
    i++;
} while(i <= 5);

for loop keeps the initial value, the condition and the update in one line.

for(i = 1; i <= 5; i++)
{
    printf("%d ", i);
}

1 2 3 4 5

#include<stdio.h>
#include<conio.h>
void main()
{
    int marks;
    printf("Enter marks: ");
    scanf("%d", &marks);
    if(marks >= 40)
        printf("Pass");
    else
        printf("Fail");
    getch();
}

56

Enter marks: Pass

If the two numbers are equal, neither is greater, so the program says so.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d%d", &a, &b);
    if(a > b)
        printf("%d is greater", a);
    else if(b > a)
        printf("%d is greater", b);
    else
        printf("Both numbers are equal");
    getch();
}

8 15

Enter two numbers: 15 is greater

#include<stdio.h>
#include<conio.h>
void main()
{
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n % 5 == 0)
        printf("Divisible by 5");
    else
        printf("Not divisible by 5");
    getch();
}

35

Enter a number: Divisible by 5

Both conditions must be true, so the logical AND (&&) is used.

#include<stdio.h>
#include<conio.h>
void main()
{
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n % 5 == 0 && n % 7 == 0)
        printf("Divisible by both 5 and 7");
    else
        printf("Not divisible by both");
    getch();
}

70

Enter a number: Divisible by both 5 and 7

Use == to compare. A single = is the assignment sign.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d%d", &a, &b);
    if(a == b)
        printf("The numbers are equal");
    else
        printf("The numbers are not equal");
    getch();
}

9 9

Enter two numbers: The numbers are equal

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d%d", &a, &b);
    if(a < b)
        printf("Smaller = %d", a);
    else
        printf("Smaller = %d", b);
    getch();
}

23 17

Enter two numbers: Smaller = 17

#include<stdio.h>
#include<conio.h>
void main()
{
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n >= 100)
        printf("The number is 100 or more");
    else
        printf("The number is less than 100");
    getch();
}

100

Enter a number: The number is 100 or more

#include<stdio.h>
#include<conio.h>
void main()
{
    float sales, comm;
    printf("Enter monthly sales: ");
    scanf("%f", &sales);
    if(sales >= 1000)
        comm = sales * 10 / 100;
    else
        comm = sales * 5 / 100;
    printf("Commission = %.2f", comm);
    getch();
}

2500

Enter monthly sales: Commission = 250.00

#include<stdio.h>
#include<conio.h>
void main()
{
    int qty;
    float rate, total, disc, net;
    printf("Enter number of books and rate: ");
    scanf("%d%f", &qty, &rate);
    total = qty * rate;
    if(total >= 1000)
        disc = total * 10 / 100;
    else
        disc = total * 5 / 100;
    net = total - disc;
    printf("Total = %.2f\nDiscount = %.2f\nNet = %.2f", total, disc, net);
    getch();
}

12 150

Enter number of books and rate: Total = 1800.00
Discount = 180.00
Net = 1620.00

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b, c;
    printf("Enter three numbers: ");
    scanf("%d%d%d", &a, &b, &c);
    if(a >= b && a >= c)
        printf("Biggest = %d", a);
    else if(b >= c)
        printf("Biggest = %d", b);
    else
        printf("Biggest = %d", c);
    getch();
}

4 19 11

Enter three numbers: Biggest = 19

#include<stdio.h>
#include<conio.h>
void main()
{
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n > 0)
        printf("Positive");
    else if(n < 0)
        printf("Negative");
    else
        printf("Zero");
    getch();
}

-6

Enter a number: Negative

If sp is equal to cp there is neither gain nor loss, so a third case is added.

#include<stdio.h>
#include<conio.h>
void main()
{
    int sp, cp;
    printf("Enter selling price and cost price: ");
    scanf("%d%d", &sp, &cp);
    if(sp > cp)
        printf("Gain = %d", sp - cp);
    else if(cp > sp)
        printf("Loss = %d", cp - sp);
    else
        printf("No gain, no loss");
    getch();
}

850 1000

Enter selling price and cost price: Loss = 150

#include<stdio.h>
#include<conio.h>
void main()
{
    float sales, comm;
    printf("Enter daily sales: ");
    scanf("%f", &sales);
    if(sales <= 5000)
        comm = sales * 0.05;
    else if(sales <= 10000)
        comm = sales * 0.10;
    else
        comm = sales * 0.15;
    printf("Commission = %.2f", comm);
    getch();
}

12000

Enter daily sales: Commission = 1800.00

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b, c;
    printf("Enter three numbers: ");
    scanf("%d%d%d", &a, &b, &c);
    if(a <= b && a <= c)
        printf("Smallest = %d", a);
    else if(b <= c)
        printf("Smallest = %d", b);
    else
        printf("Smallest = %d", c);
    getch();
}

14 6 9

Enter three numbers: Smallest = 6

A number is in the middle when it lies between the other two. The sign >= and <= take care of equal numbers.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a, b, c;
    printf("Enter three numbers: ");
    scanf("%d%d%d", &a, &b, &c);
    if((a >= b && a <= c) || (a <= b && a >= c))
        printf("Middle = %d", a);
    else if((b >= a && b <= c) || (b <= a && b >= c))
        printf("Middle = %d", b);
    else
        printf("Middle = %d", c);
    getch();
}

14 6 9

Enter three numbers: Middle = 9

The conditions are checked from the biggest amount to the smallest, so the first true condition gives the right rate.

#include<stdio.h>
#include<conio.h>
void main()
{
    int qty;
    float rate, total, disc, net;
    printf("Enter number of books and rate: ");
    scanf("%d%f", &qty, &rate);
    total = qty * rate;
    if(total >= 10000)
        disc = total * 0.20;
    else if(total >= 5000)
        disc = total * 0.10;
    else if(total >= 1000)
        disc = total * 0.05;
    else
        disc = 0;
    net = total - disc;
    printf("Total = %.2f\nDiscount = %.2f\nNet = %.2f", total, disc, net);
    getch();
}

50 160

Enter number of books and rate: Total = 8000.00
Discount = 800.00
Net = 7200.00

#include<stdio.h>
#include<conio.h>
void main()
{
    int m;
    printf("Enter marks in English: ");
    scanf("%d", &m);
    if(m >= 90)
        printf("A+");
    else if(m >= 80)
        printf("A");
    else if(m >= 70)
        printf("B+");
    else if(m >= 60)
        printf("B");
    else if(m >= 50)
        printf("C+");
    else if(m >= 40)
        printf("C");
    else if(m >= 35)
        printf("D");
    else
        printf("NG");
    getch();
}

74

Enter marks in English: B+

Looping (iteration) means running a block of statements again and again until a condition becomes false. C has the while, do-while and for loops.

The even numbers are 2, 4, 6, … The i-th even number is 2 × i. So the loop runs from i = 1 to n and adds 2 × i each time.

#include<stdio.h>
#include<conio.h>
void main()
{
    int n, i, sum = 0;
    printf("Enter n: ");
    scanf("%d", &n);
    for(i = 1; i <= n; i++)
    {
        sum = sum + 2 * i;
    }
    printf("Sum of first %d even numbers = %d", n, sum);
    getch();
}

5

Enter n: Sum of first 5 even numbers = 30

Check: 2 + 4 + 6 + 8 + 10 = 30.

Algorithm

Step 1: Start
Step 2: Set count = 0 and i = 1
Step 3: Read salary
Step 4: If salary >= 5000 and salary <= 10000 then count = count + 1
Step 5: i = i + 1
Step 6: If i <= 200 go to Step 3
Step 7: Print count
Step 8: Stop

Program

#include<stdio.h>
#include<conio.h>
void main()
{
    int i, count = 0;
    float salary;
    for(i = 1; i <= 200; i++)
    {
        printf("Enter salary of employee %d: ", i);
        scanf("%f", &salary);
        if(salary >= 5000 && salary <= 10000)
            count++;
    }
    printf("Employees with salary from 5000 to 10000 = %d", count);
    getch();
}

Here is the same program tried for 5 employees, so that the output can be shown in a short space.

#include<stdio.h>
#include<conio.h>
void main()
{
    int i, count = 0;
    float salary;
    for(i = 1; i <= 5; i++)
    {
        scanf("%f", &salary);
        if(salary >= 5000 && salary <= 10000)
            count++;
    }
    printf("Employees with salary from 5000 to 10000 = %d", count);
    getch();
}

4500 5000 7800 10000 12000

Employees with salary from 5000 to 10000 = 3

for(i = 0; i < 6; i++)
{
    if(i == 3)
        break;
    printf("%d ", i);
}

0 1 2

for(i = 0; i < 6; i++)
{
    if(i == 3)
        continue;
    printf("%d ", i);
}

0 1 2 4 5

Program to print the first 10 terms of the series 1, 2, 3, …

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 1; i <= 10; i++)
    {
        printf("%d\t", i);
    }
    getch();
}

1       2       3       4       5       6       7       8       9       10

The last digit is taken with % 10. It is added to the reverse after multiplying the reverse by 10. Then the number is divided by 10 to remove the last digit. This is repeated four times, so a number like 1200 gives 0021 correctly.

#include<stdio.h>
#include<conio.h>
void main()
{
    int n, i, d;
    printf("Enter a four-digit number: ");
    scanf("%d", &n);
    printf("Reverse = ");
    for(i = 1; i <= 4; i++)
    {
        d = n % 10;
        printf("%d", d);
        n = n / 10;
    }
    getch();
}

1250

Enter a four-digit number: Reverse = 0521

Printing each digit as it is taken keeps the zeros of numbers like 1250, which gives 0521.

A matrix is stored in a two dimensional array. int a[2][2] has 2 rows and 2 columns, and the element in row i and column j is a[i][j]. Matrices of the same order are added or subtracted element by element: c[i][j] = a[i][j] + b[i][j] (or minus).

Matrix addition

#include<stdio.h>
#include<conio.h>
void main()
{
    int a[2][2], b[2][2], c[2][2], i, j;
    printf("Enter elements of first matrix: ");
    for(i = 0; i < 2; i++)
        for(j = 0; j < 2; j++)
            scanf("%d", &a[i][j]);
    printf("Enter elements of second matrix: ");
    for(i = 0; i < 2; i++)
        for(j = 0; j < 2; j++)
            scanf("%d", &b[i][j]);
    for(i = 0; i < 2; i++)
        for(j = 0; j < 2; j++)
            c[i][j] = a[i][j] + b[i][j];
    printf("\nSum of the matrices:\n");
    for(i = 0; i < 2; i++)
    {
        for(j = 0; j < 2; j++)
            printf("%d\t", c[i][j]);
        printf("\n");
    }
    getch();
}

1 2 3 4   (first matrix)
5 6 7 8   (second matrix)

Enter elements of first matrix: Enter elements of second matrix: 
Sum of the matrices:
6	8	
10	12	

Matrix subtraction is the same program with c[i][j] = a[i][j] - b[i][j];. With the same input the output is:

Difference of the matrices:
-4      -4
-4      -4

#include<stdio.h>
#include<conio.h>
void main()
{
    int age, i, count = 0;
    for(i = 1; i <= 40; i++)
    {
        printf("Enter age of student %d: ", i);
        scanf("%d", &age);
        if(age >= 15 && age <= 22)
            count++;
    }
    printf("Students with age from 15 to 22 = %d", count);
    getch();
}

The same program tried for 6 students:

#include<stdio.h>
#include<conio.h>
void main()
{
    int age, i, count = 0;
    for(i = 1; i <= 6; i++)
    {
        scanf("%d", &age);
        if(age >= 15 && age <= 22)
            count++;
    }
    printf("Students with age from 15 to 22 = %d", count);
    getch();
}

14 15 18 22 23 17

Students with age from 15 to 22 = 4

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 1; i <= 100; i++)
        printf("%d ", i);
    getch();
}

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 

#include<stdio.h>
#include<conio.h>
void main()
{
    int i, sum = 0;
    for(i = 1; i <= 100; i++)
    {
        printf("%d ", i);
        sum = sum + i;
    }
    printf("\nSum = %d", sum);
    getch();
}

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 
Sum = 5050

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 2; i <= 100; i = i + 2)
        printf("%d ", i);
    getch();
}

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 1; i <= 20; i++)
        printf("%d ", 2 * i);
    getch();
}

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 

#include<stdio.h>
#include<conio.h>
void main()
{
    int i, n;
    printf("Enter the number of terms: ");
    scanf("%d", &n);
    for(i = 1; i <= n; i++)
        printf("%d ", 2 * i);
    getch();
}

8

Enter the number of terms: 2 4 6 8 10 12 14 16 

#include<stdio.h>
#include<conio.h>
void main()
{
    int i, sum = 0;
    for(i = 99; i >= 1; i = i - 2)
    {
        printf("%d ", i);
        sum = sum + i;
    }
    printf("\nSum = %d", sum);
    getch();
}

99 97 95 93 91 89 87 85 83 81 79 77 75 73 71 69 67 65 63 61 59 57 55 53 51 49 47 45 43 41 39 37 35 33 31 29 27 25 23 21 19 17 15 13 11 9 7 5 3 1 
Sum = 2500

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 1; i * i <= 100; i++)
        printf("%d ", i * i);
    getch();
}

1 4 9 16 25 36 49 64 81 100 

Every term is the cube of its term number: 1³, 2³, 3³ and so on.

#include<stdio.h>
#include<conio.h>
void main()
{
    int i;
    for(i = 1; i <= 10; i++)
        printf("%d ", i * i * i);
    getch();
}

1 8 27 64 125 216 343 512 729 1000 

An array is a collection of data of the same type, kept in consecutive memory locations under one variable name. Each item is called an element and is reached by its index. The index starts from 0.

One dimensional array has one index, like a row of boxes. Declaration: int marks[5]; The elements are marks[0] to marks[4]. It can be given values at the time of declaration:

int marks[5] = {70, 65, 80, 55, 90};

Two dimensional array has two indexes, row and column, like a table or a matrix. Declaration: int m[2][3]; This has 2 rows and 3 columns, so 6 elements, from m[0][0] to m[1][2].

int m[2][3] = {{1, 2, 3}, {4, 5, 6}};

#include<stdio.h>
#include<conio.h>
void main()
{
    int m[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int i, j;
    for(i = 0; i < 2; i++)
    {
        for(j = 0; j < 3; j++)
            printf("%d ", m[i][j]);
        printf("\n");
    }
    getch();
}

1 2 3 
4 5 6 

A string is a group of characters ended by the null character \0. In C it is stored in a character array, for example char name[20] = "Nepal"; The string functions are in the header file string.h.

#include<stdio.h>
#include<string.h>
void main()
{
    char a[20] = "Nepal", b[20];
    printf("Length = %d\n", strlen(a));
    strcpy(b, a);
    strrev(b);
    printf("%s", b);
}

Length = 5
lapeN

An operator is a symbol that tells the computer to do an operation on the data (operands). The types of operators in C are as follows.

Program that uses these operators

#include<stdio.h>
#include<conio.h>
void main()
{
    int a = 17, b = 5, max;
    printf("a + b = %d\n", a + b);
    printf("a / b = %d\n", a / b);
    printf("a %% b = %d\n", a % b);
    printf("a > b gives %d\n", a > b);
    printf("a > 10 && b > 10 gives %d\n", a > 10 && b > 10);
    max = (a > b) ? a : b;
    printf("Maximum = %d\n", max);
    a++;
    printf("After a++, a = %d\n", a);
    printf("5 << 1 = %d, 5 & 3 = %d", 5 << 1, 5 & 3);
    getch();
}

a + b = 22
a / b = 3
a % b = 2
a > b gives 1
a > 10 && b > 10 gives 0
Maximum = 17
After a++, a = 18
5 << 1 = 10, 5 & 3 = 1

Algorithm

Step 1: Start
Step 2: Read the selling price (sp) and cost price (cp)
Step 3: If sp > cp then
            Print "Gain"
        Else if sp < cp then
            Print "Loss"
        Else
            Print "No gain, no loss"
Step 4: Stop

Program

#include<stdio.h>
#include<conio.h>
void main()
{
    int sp, cp;
    printf("Enter the selling price and cost price: ");
    scanf("%d%d", &sp, &cp);
    if(sp > cp)
        printf("Gain");
    else if(sp < cp)
        printf("Loss");
    else
        printf("No gain, no loss");
    getch();
}

1200 1000

Enter the selling price and cost price: Gain

First the marks of all 7 subjects are checked. If any one is below 35, the student fails. Otherwise the percentage is found by dividing the total by 7 (each subject is out of 100) and the division is decided. The program uses per >= 45 in place of the range 45 to 59, so a percentage such as 59.5 is not missed.

To process many students, the whole work is put in a loop. Here the loop runs for the number of students entered by the user.

#include<stdio.h>
#include<conio.h>
void main()
{
    int s[7], i, k, n, fail;
    float total, per;
    printf("How many students? ");
    scanf("%d", &n);
    for(k = 1; k <= n; k++)
    {
        total = 0;
        fail = 0;
        printf("\nEnter the marks of 7 subjects of student %d: ", k);
        for(i = 0; i < 7; i++)
        {
            scanf("%d", &s[i]);
            total = total + s[i];
            if(s[i] < 35)
                fail = 1;
        }
        per = total / 7;
        printf("Percentage = %.2f\n", per);
        if(fail == 1)
            printf("Result: Fail");
        else if(per >= 60)
            printf("Result: First Division");
        else if(per >= 45)
            printf("Result: Second Division");
        else
            printf("Result: Third Division");
    }
    getch();
}

2
70 65 80 55 90 60 72
40 38 30 50 45 60 42

How many students? 
Enter the marks of 7 subjects of student 1: Percentage = 70.29
Result: First Division
Enter the marks of 7 subjects of student 2: Percentage = 43.57
Result: Fail

Two loops are used. The outer loop takes each number from 1 to n, and the inner loop goes from 1 to 10 and prints the table of that number.

#include<stdio.h>
#include<conio.h>
void main()
{
    int n, i, j;
    printf("Enter n: ");
    scanf("%d", &n);
    for(i = 1; i <= n; i++)
    {
        printf("\nTable of %d\n", i);
        for(j = 1; j <= 10; j++)
            printf("%d x %d = %d\n", i, j, i * j);
    }
    getch();
}

3

Enter n: 
Table of 1
1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
1 x 4 = 4
1 x 5 = 5
1 x 6 = 6
1 x 7 = 7
1 x 8 = 8
1 x 9 = 9
1 x 10 = 10

Table of 2
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
2 x 4 = 8
2 x 5 = 10
2 x 6 = 12
2 x 7 = 14
2 x 8 = 16
2 x 9 = 18
2 x 10 = 20

Table of 3
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9
3 x 4 = 12
3 x 5 = 15
3 x 6 = 18
3 x 7 = 21
3 x 8 = 24
3 x 9 = 27
3 x 10 = 30

A prime number is a number greater than 1 that is divisible only by 1 and itself. The program divides the number by 2, 3, 4 … up to n/2. If any division leaves no remainder, the number is not prime. Numbers 0 and 1 are not prime, so they are checked first.

#include<stdio.h>
#include<conio.h>
void main()
{
    int n, i, flag = 1;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n < 2)
        flag = 0;
    for(i = 2; i <= n / 2; i++)
    {
        if(n % i == 0)
        {
            flag = 0;
            break;
        }
    }
    if(flag == 1)
        printf("%d is a prime number", n);
    else
        printf("%d is not a prime number", n);
    getch();
}

29

Enter a number: 29 is a prime number

The continue statement skips the counting when the age is outside 50 to 60, and the loop moves to the next person.

#include<stdio.h>
#include<conio.h>
void main()
{
    int age, i, count = 0;
    for(i = 1; i <= 100; i++)
    {
        printf("Enter age of person %d: ", i);
        scanf("%d", &age);
        if(age < 50 || age > 60)
            continue;
        count++;
    }
    printf("Persons between 50 and 60 = %d", count);
    getch();
}

The same program tried for 6 persons:

#include<stdio.h>
#include<conio.h>
void main()
{
    int age, i, count = 0;
    for(i = 1; i <= 6; i++)
    {
        scanf("%d", &age);
        if(age < 50 || age > 60)
            continue;
        count++;
    }
    printf("Persons between 50 and 60 = %d", count);
    getch();
}

45 50 58 60 61 72

Persons between 50 and 60 = 3

Flowchart

Program

#include<stdio.h>
#include<conio.h>
void main()
{
    int n, i;
    printf("Enter a positive integer less than 20: ");
    scanf("%d", &n);
    if(n > 0 && n < 20)
    {
        for(i = 1; i <= 10; i++)
            printf("%d x %d = %d\n", n, i, n * i);
    }
    else
        printf("The number must be positive and less than 20");
    getch();
}

7

Enter a positive integer less than 20: 7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

while loop: print the numbers 1 to 10

#include<stdio.h>
#include<conio.h>
void main()
{
    int a = 1;
    while(a <= 10)
    {
        printf("%d ", a);
        a = a + 1;
    }
    getch();
}

1 2 3 4 5 6 7 8 9 10

do-while loop: print the numbers 1 to 10

#include<stdio.h>
#include<conio.h>
void main()
{
    int a = 1;
    do
    {
        printf("%d ", a);
        a = a + 1;
    } while(a <= 10);
    getch();
}

1 2 3 4 5 6 7 8 9 10

The real difference is seen when the condition is false from the start

#include<stdio.h>
#include<conio.h>
void main()
{
    int a = 20;
    while(a <= 10)
        printf("while loop ran\n");
    do
    {
        printf("do-while loop ran once\n");
    } while(a <= 10);
    getch();
}

do-while loop ran once

An array is a collection of homogeneous (same type) data, stored in consecutive memory locations under a common variable name.

The program compares each element with all the elements after it. If the first one is smaller, they are swapped. After one round, the biggest remaining value is in place.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a[5], i, j, t;
    printf("Enter 5 integers: ");
    for(i = 0; i < 5; i++)
        scanf("%d", &a[i]);
    for(i = 0; i < 5; i++)
    {
        for(j = i + 1; j < 5; j++)
        {
            if(a[i] < a[j])
            {
                t = a[i];
                a[i] = a[j];
                a[j] = t;
            }
        }
    }
    printf("Descending order: ");
    for(i = 0; i < 5; i++)
        printf("%d ", a[i]);
    getch();
}

25 8 91 14 60

Enter 5 integers: Descending order: 91 60 25 14 8 

Importance of an array

Program: the greatest number is found by taking the first element as the greatest, and then replacing it whenever a bigger element is found.

#include<stdio.h>
#include<conio.h>
void main()
{
    int a[10] = {34, 78, 12, 99, 45, 67, 23, 88, 5, 61};
    int gre, i;
    gre = a[0];
    for(i = 1; i < 10; i++)
    {
        if(a[i] > gre)
            gre = a[i];
    }
    printf("Greatest = %d", gre);
    getch();
}

Greatest = 99

Each character of the text is checked. If it is one of a, e, i, o, u (small or capital), it is a vowel. Any other letter is a consonant. Spaces, digits and symbols are not counted.

#include<stdio.h>
#include<conio.h>
void main()
{
    char a[100];
    int i, v = 0, c = 0;
    printf("Enter a text: ");
    gets(a);
    for(i = 0; a[i] != '\0'; i++)
    {
        if(a[i] == 'a' || a[i] == 'e' || a[i] == 'i' || a[i] == 'o' || a[i] == 'u' ||
           a[i] == 'A' || a[i] == 'E' || a[i] == 'I' || a[i] == 'O' || a[i] == 'U')
            v++;
        else if((a[i] >= 'a' && a[i] <= 'z') || (a[i] >= 'A' && a[i] <= 'Z'))
            c++;
    }
    printf("Vowels = %d\nConsonants = %d", v, c);
    getch();
}

Nepal is beautiful

Enter a text: Vowels = 8
Consonants = 8

A structure keeps the name and marks of one student together. An array of 10 such structures keeps all the students. While sorting by marks, the whole record (name and marks) is swapped, so a name always stays with its own marks.

#include<stdio.h>
#include<string.h>
#include<conio.h>
void main()
{
    int i, j, t;
    char temp[15];
    struct school
    {
        char name[15];
        int marks;
    } a[10];
    printf("Enter name and marks of 10 students:\n");
    for(i = 0; i < 10; i++)
        scanf("%s%d", a[i].name, &a[i].marks);
    for(i = 0; i < 10; i++)
    {
        for(j = i + 1; j < 10; j++)
        {
            if(a[i].marks < a[j].marks)
            {
                strcpy(temp, a[i].name);
                strcpy(a[i].name, a[j].name);
                strcpy(a[j].name, temp);
                t = a[i].marks;
                a[i].marks = a[j].marks;
                a[j].marks = t;
            }
        }
    }
    printf("\nAfter sorting\n");
    for(i = 0; i < 10; i++)
        printf("%s\t%d\n", a[i].name, a[i].marks);
    getch();
}

Ram 56
Sita 82
Hari 67
Gita 91
Mohan 45
Rita 73
Bibek 60
Anita 88
Suman 39
Pooja 77

Enter name and marks of 10 students:

After sorting
Gita	91
Anita	88
Sita	82
Pooja	77
Rita	73
Hari	67
Bibek	60
Ram	56
Mohan	45
Suman	39

The names are compared with strcmp(). It gives a value greater than 0 when the first name comes after the second name in the dictionary. In that case the two records are swapped, and the address moves along with its name.

#include<stdio.h>
#include<string.h>
#include<conio.h>
void main()
{
    int i, j;
    char temp[15];
    struct school
    {
        char name[15];
        char address[15];
    } a[5];
    printf("Enter name and address of 5 persons:\n");
    for(i = 0; i < 5; i++)
        scanf("%s%s", a[i].name, a[i].address);
    for(i = 0; i < 5; i++)
    {
        for(j = i + 1; j < 5; j++)
        {
            if(strcmp(a[i].name, a[j].name) > 0)
            {
                strcpy(temp, a[i].name);
                strcpy(a[i].name, a[j].name);
                strcpy(a[j].name, temp);
                strcpy(temp, a[i].address);
                strcpy(a[i].address, a[j].address);
                strcpy(a[j].address, temp);
            }
        }
    }
    printf("\nIn alphabetical order\n");
    for(i = 0; i < 5; i++)
        printf("%s\t%s\n", a[i].name, a[i].address);
    getch();
}

Ram Pokhara
Sita Butwal
Hari Dharan
Bina Kathmandu
Anil Biratnagar

Enter name and address of 5 persons:

In alphabetical order
Anil	Biratnagar
Bina	Kathmandu
Hari	Dharan
Ram	Pokhara
Sita	Butwal

#include<stdio.h>
#include<conio.h>
void main()
{
    int a[50], n, i, j, t;
    printf("How many numbers? ");
    scanf("%d", &n);
    printf("Enter the numbers: ");
    for(i = 0; i < n; i++)
        scanf("%d", &a[i]);
    for(i = 0; i < n; i++)
    {
        for(j = i + 1; j < n; j++)
        {
            if(a[i] > a[j])
            {
                t = a[i];
                a[i] = a[j];
                a[j] = t;
            }
        }
    }
    printf("Ascending order: ");
    for(i = 0; i < n; i++)
        printf("%d ", a[i]);
    getch();
}

6
42 7 19 3 88 15

How many numbers? Enter the numbers: Ascending order: 3 7 15 19 42 88 

The switch statement matches the entered number with the case labels. The break statement ends each case, and default handles a number outside 1 to 7.

#include<stdio.h>
#include<conio.h>
void main()
{
    int d;
    printf("Enter a number from 1 to 7: ");
    scanf("%d", &d);
    switch(d)
    {
        case 1: printf("Sunday"); break;
        case 2: printf("Monday"); break;
        case 3: printf("Tuesday"); break;
        case 4: printf("Wednesday"); break;
        case 5: printf("Thursday"); break;
        case 6: printf("Friday"); break;
        case 7: printf("Saturday"); break;
        default: printf("Invalid number");
    }
    getch();
}

3

Enter a number from 1 to 7: Tuesday