Number Reverser – C Program

Problem Question

If a five-digit number is input through the keyboard, write a program to reverse the number.

Explanation of Problem

In this program the user enters a 5 digit number. The program should be able to reverse the number. For example if the user enters 12345, the output should be 54321.

Code

#include <stdio.h>
/*Number Reverser*
*@Language: ANSI C*
*@Compiler: GNU GCC*
*@IDE: CodeBlocks 12.11*
*@Author: Toxifier*
*@URL: http://letsplaycoding.blogspot.com/*
*@Date: 01-04-2014**/
int main()
{
    printf("\n\nEnter a five digit number: ");
    int number, reverseNumber = 0, digit1, digit2, digit3, digit4, digit5;
    scanf("%d", &number);
    printf("\nCalculating reverse of number......");
    digit1 = (number % 100000) / 10000;
    digit2 = (number % 10000) / 1000;
    digit3 = (number % 1000) / 100;
    digit4 = (number % 100) / 10;
    digit5 = (number % 10);
    reverseNumber = (digit5 * 10000) + (digit4 * 1000) + (digit3 * 100) + (digit2 * 10) + digit1;
    printf("\nThe reverse of number is: %d\n", reverseNumber);
    system("pause");
    return 0;
}

Explanation of Code

#include <stdio.h> -> This is the step which occurs before compilation starts. The compiler calls the C Preprocessor to include the STDIO(Standard Input Output) header file into the program, thus letting the use of the standard input/output functions like printf() and scanf() which come from STDIO.H

int main() -> The entry point of the program where the execution starts. This function has to named main. As per the ANSI specification, the return type has to be int. If you use the traditional C, you may use void as the return type. Since the return type is specified as int in my program, I have to use a return statement at the end of my code. So I use return 0 since zero returned from a function, by convention, implies a correct execution of the program. The return values are used to debug the program.

printf() -> This is a standard output function used to print something on the screen. We have to pass a string to this function which will be displayed on user's terminal.

scanf() -> This is the scanf() function which waits for the user to enter certain value using his/her keyboard. We store the user input at the location in memory which is pointed to by the variable whose address is passed to this function.

digit1 = (number % 100000) / 10000;
digit2 = (number % 10000) / 1000;
digit3 = (number % 1000) / 100;
digit4 = (number % 100) / 10;
digit5 = (number % 10);

The above piece of code reflects the part of the code in which we extract all the digits of the 5-digit number. ‘%’ operator (called the modulus operator) finds the remainder of the division of the number on left with the number on right of the operator. What I did here is, first I applied the modulus operator between the number and 100000. On dividing, the remainder found, with 10000, I found the digit at the ten-thousand position of the number. This is possible because I am using the ‘int’ data type, which thus rejects the digits after the decimal. Similarly, the other digits are found. You can try it on paper to justify my statement to yourself.

reverseNumber = (digit5 * 10000) + (digit4 * 1000) + (digit3 * 100) + (digit2 * 10) + digit1; -> In this statement I calculate the final number we required. So I just did the necessary multiplications which are quite self-explanatory.

system("pause") -> This statement is used to pause the program, until user presses a key. This function is not necessary in your program, I use it to see my outputs paused. If you use cmd to run your programs, you might not need this. If you use linux/unix you might not need this. Depending on your compiler, this function may or may not work. Moreover, removing this line of code from this program, doesn't affect the functionality of the program.

Output(s)