C - Part 20

Basics of Pointers In C Programming Language

In today’s video tutorial lets learn basics of pointers in C programming language. Think of it as base of a building. If you can hold these basic concepts strong your building will be safer.

Topics Covered

1. What are pointers?
2. How to declare pointers.
3. How to access the value present at the address stored in pointer variable.
4. Using Address of operator.
5. Using Indirection operator.

What are pointers?

A pointer is a variable which refers to or points to an address in your computers memory.

Definition: A pointer variable is a variable which holds the address of another variable.

And since pointer variable is also a variable, it also has a unique address associated with it.

Operators

& is called “address of” operator.
* is called Indirection or “value at address” operator.

Video Tutorial: Basics of Pointers In C Programming Language

[youtube https://www.youtube.com/watch?v=1R6dBm3LN68]

YouTube Link: https://www.youtube.com/watch?v=1R6dBm3LN68 [Watch the Video In Full Screen.]

Source Code: Basics of Pointers In C Programming Language

#include<stdio.h>

int main()
{
    int num = 5;
    int *ptr; // pointer variable

    ptr = &num; // both hold address of num

    printf("Value of num = %d\n", num);
    printf("Address of num = %d\n", &num);
    printf("Address of ptr = %d\n", &ptr);
    printf("Value of ptr = %d\n", ptr);
    printf("Access value of num from ptr = %d\n", *ptr);
    printf("Access value of num from num = %d\n", *(&num));

    return 0;
}

Output:
Value of num = 5
Address of num = 6356732
Address of ptr = 6356728
Value of ptr = 6356732
Access value of num from ptr = 5
Access value of num from num = 5

Note: Whenever you declare and initialize a variable, for example, int num = 5; Your program does the following tasks:
1. Reserves space in your computers memory depending upon the data type.
2. Associates the name num with this memory location.
3. Stores the number 5 in this memory location.

Declaration of pointer variable

General Syntax
data_type* pointer_variable_name;

data_type *pointer_variable_name;

Example:

int *ptr1;

int* ptr2;

Both these declaration are valid.

Rules for constructing pointer variable are same as that of regular variables: Rules for Constructing Variable Names: C

Assigning value to pointer variable

General Syntax
pointer_variable_name = &another_variable;

Example:

ptr = &num;

This stores the address of variable num as value of pointer variable ptr.

Declaring and Assigning value to pointer variable

Example:

int num = 5;
int *ptr;  // Declaration of pointer variable

ptr = &num;  // Assigning value to pointer variable

Note: While assigning address to pointer variable do not include * in front of pointer variable.

Values and addresses

num will have 5.
&num will have address of num.
&ptr will have address of ptr.
ptr will have address of num.
*ptr will have value of num.

Call by Value and Call by reference

Call by Value Example: C Program To Swap Two Numbers using Function
Call by Reference Example: C Program To Swap Two Numbers using Pointers

Note: Address can’t be negative values. So we can use %u as format specifier, which means unsigned integer.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

C Program To Print Multiplication Table Using Function

Lets write a C program to print the multiplication table for a user input number, using function/method. The table should get displayed in the following format:

Example: Multiplication table of 5
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

Coding Challenge on Numbers!
Print the multiplication table which gives the following output:

multiplication table for number 37

Hint: Initialize count to 3. Increment the value of count by 3 for each iteration of for loop.

Answer: We’ve posted the source code to above coding challenge at the end of this blog post.

Video Tutorial: C Program To Print Multiplication Table Using Function

[youtube https://www.youtube.com/watch?v=5fafT5Pc1jw]

YouTube Link: https://www.youtube.com/watch?v=5fafT5Pc1jw [Watch the Video In Full Screen.]

Source Code: C Program To Print Multiplication Table Using Function

#include<stdio.h>

void tables(int);

int main()
{
    int num;

    printf("Enter a positive number\n");
    scanf("%d", &num);

    printf("\nMultiplication Table for %d is:\n", num);

    tables(num);

    return 0;
}

void tables(int num)
{
    int count;

    for(count = 1; count <= 10; count++)
    {
        printf("%d x %d = %d\n", num, count, num*count);
    }
}

Output 1:
Enter a positive number
5

Multiplication Table for 5 is:
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

Output 2:
Enter a positive number
2

Multiplication Table for 2 is:
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

Output 3:
Enter a positive number
7

Multiplication Table for 7 is:
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

Output 4:
Enter a positive number
9

Multiplication Table for 9 is:
9 x 1 = 9
9 x 2 = 18
9 x 3 = 27
9 x 4 = 36
9 x 5 = 45
9 x 6 = 54
9 x 7 = 63
9 x 8 = 72
9 x 9 = 81
9 x 10 = 90

Logic: Multiplication Table

User enters a positive number, we store it inside variable num. We pass this number entered by the user to a function called tables().

Inside tables() function we write a for loop. We initialize the for loop counter variable count to 1 and iterate through this for loop until count is less than or equal to 10 – because we want to display multiplication tables from 1 to 10. For each iteration of for loop we increment the value of count by 1.

Inside for loop we display the multiplication table.

You can also watch C Program To Print Multiplication Table Using While Loop video tutorial.

Just For Some Number Fun

Here I’m initializing 3 to count variable. For loop executes until count is less than or equal to 27. For each iteration of for loop count value increments by 3.

We assign 37 to num variable. Check the output.

Source Code: Some numerology!

#include<stdio.h>

void tables(int);

int main()
{
    int num = 37;

    printf("\nMultiplication Table for %d is:\n", num);

    tables(num);

    return 0;
}

void tables(int num)
{
    int count;

    for(count = 3; count <= 27; count += 3)
    {
        printf("%d x %d = %d\n", num, count, num*count);
    }
}

Output:
Multiplication Table for 37 is:

37 x 3 = 111
37 x 6 = 222
37 x 9 = 333
37 x 12 = 444
37 x 15 = 555
37 x 18 = 666
37 x 21 = 777
37 x 24 = 888
37 x 27 = 999

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

C Program To Convert Year To Roman Equivalent

Write a general-purpose function to convert any given year into its Roman equivalent. Use these Roman equivalents for decimal numbers:

1 – i, 5 – v, 10 – x, 50 – l, 100 – c, 500 – d, 1000 – m.

Example:
Roman equivalent of 1988 is mdcccclxxxviii.
Roman equivalent of 1525 is mdxxv.

Chart for Roman equivalents of decimal numbers

Decimal	Roman Equivalent
1000	m
500	d
100	c
50	l
10	x
5	v
1	i

Video Tutorial: C Program To Convert Year To Roman Equivalent

[youtube https://www.youtube.com/watch?v=oocEj-8u4o8]

YouTube Link: https://www.youtube.com/watch?v=oocEj-8u4o8 [Watch the Video In Full Screen.]

Spurce Code: C Program To Convert Year To Roman Equivalent

#include<stdio.h>

void roman(int num)
{
    while(num)
    {
        if(num >= 1000)
        {
            printf("m");
            num = num - 1000;
        }
        else if(num >= 500)
        {
            printf("d");
            num = num - 500;
        }
        else if(num >= 100)
        {
            printf("c");
            num = num - 100;
        }
        else if(num >= 50)
        {
            printf("l");
            num = num - 50;
        }
        else if(num >= 10)
        {
            printf("x");
            num = num - 10;
        }
        else if(num >= 5)
        {
            printf("v");
            num = num - 5;
        }
        else if(num >= 1)
        {
            printf("i");
            num = num - 1;
        }
    }

    printf("\n");
}

int main()
{
    int year;

    printf("Input the year to get its Roman Equivalent\n");
    scanf("%d", &year);

    roman(year);

    return 0;
}

Output 1:
Input the year to get its Roman Equivalent
1988
mdcccclxxxviii

Output 2:
Input the year to get its Roman Equivalent
1525
mdxxv

Output 3:
Input the year to get its Roman Equivalent
2500
mmd

Output 4:
Input the year to get its Roman Equivalent
2020
mmxx

Output 5:
Input the year to get its Roman Equivalent
2021
mmxxi

Logic To Convert Year To Roman Equivalent

We ask the user to enter the year in decimal number format and store it inside variable year. Next we pass this user input year to function roman.

Inside function roman, we copy the value of year to a local variable num. We write a while loop and iterate until value of num is positive. Control exits while loop once num is equal to 0.

Inside while loop we check if the value of num is greater than or equal to 1000. If true, then we print roman equivalent of 1000, which is m. And then we decrement the value of num by 1000.

For each iteration of the while loop we check if num is greater than or equal to 1000 or 500 or 100 or 50 or 10 or 5 or 1. To whichever number the value of num matches we printout its corresponding representation in roman:

i.e., 1 – i, 5 – v, 10 – x, 50 – l, 100 – c, 500 – d, 1000 – m.

and also immediately decrement the value by its matched decimal equivalent. We keep doing this until num value is 0.

Example:

If user input year is 1525

Iteration	num	Match	Roman Equivalent	New Value of num
1	1525	1000	m	525
2	525	500	d	25
3	25	10	x	15
4	15	10	x	5
5	5	5	v	0

In above table, last value of num is 0, so control exits while loop. So roman equivalent of year 1525 is mdxxv.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

C Program To Count Prime Numbers Between Range

Lets write a C program to count prime numbers between user entered/input range of numbers, using function/method.

Prime Number: is a natural number greater than 1, which has no positive divisors other than 1 and itself.

Video Tutorial: C Program To Count Prime Numbers Between Range

[youtube https://www.youtube.com/watch?v=vY6HFMblBJY]

YouTube Link: https://www.youtube.com/watch?v=vY6HFMblBJY [Watch the Video In Full Screen.]

Source Code: C Program To Count Prime Numbers Between Range

#include<stdio.h>
#include<math.h>

int isPrime(int num)
{
    int inum = sqrt(num), prime = 1, count;

    for(count = 2; count <= inum; count++)
    {
         if(num % count == 0)
         {
            prime = 0;
            break;
         }
    }

    return(prime);
}

int main()
{
    int start, end, temp, num, slno = 0, on_off;

    printf("Enter start and end value\n");
    scanf("%d%d", &start, &end);

    printf("Do you want to print prime numbers? (yes = 1, no = 0)\n");
    scanf("%d", &on_off);

    if(start > end)
    {
        temp  = start;
        start = end;
        end   = temp;
    }

    if(on_off)
        printf("\nPrime numbers between %d and %d are:\n\n", start, end);

    for(num = start; num <= end; num++)
    {
        if(num == 1)
        {
            continue;
        }

        if( isPrime(num) )
        {
            slno++;

            if(on_off)
            {
                printf("%d. %d is prime number.\n", slno, num);
            }
         }
    }

    printf("\nThere are %d prime numbers between %d and %d.\n", 
             slno, start, end);

    return 0;
}

Output 1:
Enter start and end value
10
30
Do you want to print prime numbers? (yes = 1, no = 0)
1

Prime numbers between 10 and 30 are:

1. 11 is prime number.
2. 13 is prime number.
3. 17 is prime number.
4. 19 is prime number.
5. 23 is prime number.
6. 29 is prime number.

There are 6 prime numbers between 10 and 30.

Output 2:
Enter start and end value
10
30
Do you want to print prime numbers? (yes = 1, no = 0)
0

There are 6 prime numbers between 10 and 30.

Output 3:
Enter start and end value
1
10
Do you want to print prime numbers? (yes = 1, no = 0)
1

Prime numbers between 1 and 10 are:

1. 2 is prime number.
2. 3 is prime number.
3. 5 is prime number.
4. 7 is prime number.

There are 4 prime numbers between 1 and 10.

Output 4:
Enter start and end value
1
300
Do you want to print prime numbers? (yes = 1, no = 0)
0

There are 62 prime numbers between 1 and 300.

Output 5:
Enter start and end value
1
1000
Do you want to print prime numbers? (yes = 1, no = 0)
0

There are 168 prime numbers between 1 and 1000.

Logic To Count Prime Numbers Between Range

First we ask the user to enter the range and store it inside address of variables start and end. We make sure that start value is less than end value. If start value is greater than end value, we use a temporary variable to swap the values of start and end.

Swap 2 Numbers Using a Temporary Variable: C

Next we ask the user if he / she wants to display the prime numbers between the range or just want to know the count of prime numbers between the entered range. We store the user answer in a variable called on_off.

We start the for loop: We initialize the loop counter variable num to start and iterate through the loop until num is less than or equal to end. For each iteration of the for loop we increment the value of num by 1. This for loop selects number one by one from start to end. And this selected number, which is present inside variable num is checked for prime or not. If its prime we display it to the console window and keep track of the count of prime numbers, if not we simply ignore that non-prime number.

We use a function to check if the selected number present in variable num is a prime number or not. We call the function/method isPrime() inside if condition and pass the value of num to it. isPrime() returns 1 or 0 value. 1 means true and 0 means false. So if isPrime() returns 1, then the if condition becomes true and we increment the value of slno by 1 and optionally printout the prime number. If isPrime() returns 0, then we simply ignore it and go to the next number.

isPrime() function has the logic to determine if the given number is prime or not. We have explained the complete logic of this in a separate video tutorial, link to which is present below.

C Program To Find Prime Number or Not using For Loop

Note:
1. We are including math.h header file or library file since we are using sqrt() builtin method. sqrt() method is present inside math.h file.

2. We are also using continue and break keywords in our program and we’ve explained about it in detail in separate videos. Please watch them for more clarity about the topic.

Continue Statement In C Programming Language
break Statement In C Programming Language

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For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

C Program To Print N Co-Prime Numbers

Lets write a C program to print N co-prime or relative prime numbers. N being the user entered limit for printing the co-prime number pairs.

Co-Prime numbers / Relative Prime Numbers: Two numbers are said to be co-prime or relative prime numbers if they do not have a common factor other than 1.

Two numbers whose Greatest Common Divisor(GCD) is 1 are known as Co-Prime or Relative Prime Numbers.

Note: User entered numbers(to check for co-prime) do not require to be prime numbers.

Factors of a number: All the numbers which perfectly divide a given number are called as Factors of that number.

Expected Result

If user inputs limit = 5
Output
1. ( 2, 3 ) are co-prime numbers.
2. ( 3, 4 ) are co-prime numbers.
3. ( 2, 5 ) are co-prime numbers.
4. ( 3, 5 ) are co-prime numbers.
5. ( 4, 5 ) are co-prime numbers.

Logic To Find If Two Numbers are Co-Prime or Not

Entire logic is present in our previous video tutorial. Please visit the link and watch the video without fail before going further: C Program To Find Two Numbers are Co-Prime or Not

Video Tutorial: C Program To Print N Co-Prime Numbers

[youtube https://www.youtube.com/watch?v=EnXhsLgFlAQ]

YouTube Link: https://www.youtube.com/watch?v=EnXhsLgFlAQ [Watch the Video In Full Screen.]

Source Code: C Program To Print N Co-Prime Numbers

#include<stdio.h>

int main()
{
    int limit, num1, num2 = 3, count, flag, slno = 1;

    printf("How many co-prime numbers you want to print?\n");
    scanf("%d", &limit);

    while(limit)
    {

        for(num1 = 2; num1 <= num2; num1++)
        {
            for(count = 2; count <= num1; count++)
            {
                flag = 1;
                if(num1 % count == 0 && num2 % count == 0)
                {
                    flag = 0;
                    break;
                }
            }

            if(flag)
            {
                printf("%d. (%d, %d) are co-prime numbers.\n", 
                      slno++, num1, num2);
                limit--;

                if(limit == 0)
                {
                    num1 = num2 + 10;
                }
            }

        }

        num2++;
    }

    return 0;
}

Output 1:
How many co-prime numbers you want to print?
10
1. ( 2, 3 ) are co-prime numbers.
2. ( 3, 4 ) are co-prime numbers.
3. ( 2, 5 ) are co-prime numbers.
4. ( 3, 5 ) are co-prime numbers.
5. ( 4, 5 ) are co-prime numbers.
6. ( 5, 6 ) are co-prime numbers.
7. ( 2, 7 ) are co-prime numbers.
8. ( 3, 7 ) are co-prime numbers.
9. ( 4, 7 ) are co-prime numbers.
10. ( 5, 7 ) are co-prime numbers.

Output 2:
How many co-prime numbers you want to print?
14
1. ( 2, 3 ) are co-prime numbers.
2. ( 3, 4 ) are co-prime numbers.
3. ( 2, 5 ) are co-prime numbers.
4. ( 3, 5 ) are co-prime numbers.
5. ( 4, 5 ) are co-prime numbers.
6. ( 5, 6 ) are co-prime numbers.
7. ( 2, 7 ) are co-prime numbers.
8. ( 3, 7 ) are co-prime numbers.
9. ( 4, 7 ) are co-prime numbers.
10. ( 5, 7 ) are co-prime numbers.
11. ( 6, 7 ) are co-prime numbers.
12. ( 3, 8 ) are co-prime numbers.
13. ( 5, 8 ) are co-prime numbers.
14. ( 7, 8 ) are co-prime numbers.

Logic To Print N Co-Prime Numbers

User enters limit value i.e., how many co-prime numbers he or she wants to print. We put that limit inside while loop condition. So while loop code executes repeatedly until limit is not equal to 0. We decrement the value of limit by 1 as and when we print the co-prime numbers.

Inside while loop we write nested for loop. The outer for loop selects the values for num1 and num2. We know that the first co-prime or relative prime number is 2 and 3. So we initialize num1 = 2 and num2 = 3. We keep incrementing the value of num2 by one outside the outer for loop, and iterate the for loop until num1 is less than or equal to num2.

If num2 is 3, then for loop checks for (num1 always starts from 2, till num1 <= num2) – (2, 2), (3, 2).

Next num2 will be 4. So for loop checks for – (2, 4), (3, 4), (4, 4)

Next num2 will be 5. So for loop checks for – (2, 5), (3, 5), (3, 5), (4, 5) ..and so on.

Inside inner for loop we check if the selected numbers present in num1 and num2 are co-prime or not. That logic is present at C Program To Find Two Numbers are Co-Prime or Not.

Note: Since limit– is inside a for loop, even though limit becomes 0 the while loop won’t exit immediately, until the for loop completes its iterations. It avoid that, we make sure the value of num1 is assigned number greater than num2. That way control exits for loop. Since limit is 0, the control even exits while loop.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert