C program to swap first and last digit of a number

Write a C program to input a number from user and swap first and last digit of the given number. How to swap first and last digits of a number in C programming. Logic to swap first and last digit of a number in C program.

Input any number: 12345

Number after swapping first and last digit: 52341

Required knowledge

Basic C programming, Basic Mathematics

Must know – Program to find first and last digit

Logic to swap first and last digit of a number

 Logic to swap first and last digit of a number Begin: read(num) lastDigit ← num % 10; digits ← log10(num); firstDigit ← num / pow(10, digits); swappedNum ← lastDigit * pow(10, digits); swappedNum ← swappedNum + num % pow(10, digits); swappedNum ← swappedNum - lastDigit; swappedNum ← swappedNum + firstDigit; End

Let us do a dry run of the algorithm to get grip on the logic.

Suppose num = 12345 -------------------- lastDigit = 12345 % 10 => 5 digits = log10(12345) => 4 firstDigit = 12345 / pow (10, 4) => 12345 / 10000 => 1 swappedNum = 5 * pow(10, 4) => 5 * 10000 => 50000 swappedNum = 50000 + (12345 % 10000) => 50000 + 2345 => 52345 swappedNum = 52345 - 5 => 52340 swappedNum = 52340 + 1 => 52341

Program to swap first and last digit of a number

/** * C program to swap first and last digit of a number */ #include <stdio.h> #include <math.h> int main() { int num, swappedNum; int firstDigit, lastDigit, digits; /* Input number from user */ printf("Enter any number: "); scanf("%d", &num); /* Find last digit */ lastDigit = num % 10; /* Find total number of digit - 1 */ digits = (int)log10(num); /* Find first digit */ firstDigit = (int)(num / pow(10, digits)); swappedNum = lastDigit; swappedNum *= (int) pow(10, digits); swappedNum += num % ((int) pow(10, digits)); swappedNum -= lastDigit; swappedNum += firstDigit; printf("Original number = %d", num); printf("Number after swapping first and last digit: %d", swappedNum); return 0; }

Note: In some compiler the above program may not produce valid results for some inputs. You may use the below program with similar logic with a little change in the code.

Program to swap first and last digit of a number

/** * C program to swap first and last digit of a number */ #include <stdio.h> #include <math.h> int main() { int num, swappedNum; int firstDigit, lastDigit, digits; /* Input a number from user */ printf("Enter any number: "); scanf("%d", &num); /* Find last digit */ lastDigit = num % 10; /* Total number of digit - 1 */ digits = (int) log10(num); /* Find first digit */ firstDigit = (int) (num / pow(10, digits)); swappedNum = lastDigit; swappedNum *= (int) round(pow(10, digits)); swappedNum += num % ((int)round(pow(10, digits))); swappedNum -= lastDigit; swappedNum += firstDigit; printf("Original number = %d", num); printf("Number after swapping first and last digit: %d", swappedNum); return 0; }

Note: In the above program I have used three mathematical function pow(), log10() and round().

• pow() is used to find power of a number.
• log10() is used to find log base 10 value of the passed parameter.
• round() function is used to round a number to nearest integer.

Enter any number: 1234 Original number = 1234 Number after swapping first and last digit: 4231

Happy coding 😉