Reverse a given number-CodePudding

How to handle the test case if I enter the number 10000 and I get 1 as a result instead of getting 00001. How to handle this case?

    #include <stdio.h>
    int main() {
      int n, rev = 0, remainder;
      printf("Enter an integer: ");
      scanf("%d", &n);
      while (n != 0) {
         remainder = n % 10;
         rev = rev * 10   remainder;
         n /= 10;
     }
     printf("Reversed number = %d", rev);
     return 0;
    }

CodePudding user response：

Keep track of the number of digits in your number, and then print the number with a formatted length:

int numDigits = 0;
while (n != 0) {
    ...
    numDigits  ;
}

and then:

printf("Reversed number = %.*d", numDigits, rev);

CodePudding user response：

Use %n specifier to count the number of digits entered. Note the space before the first %n to skip any whitespace. Also this will fail if the input is not an integer in range.
Then use the number of digits to pad the output %0*d with leading zeros.

Try with input of 10000 and 00001.

#include <stdio.h>
int main ( void) {
    int n, rev = 0, remainder;
    int start = 0;
    int stop = 0;
    printf ( "Enter an integer: ");
    scanf ( " %n%d%n", &start, &n, &stop);
    int digits = stop - start;
    while ( n != 0 || digits) {
        --digits;
        remainder = n % 10;
        rev = rev * 10   remainder;
        n /= 10;
    }
    printf ( "Reversed number = %0*d", stop - start, rev);
    return 0;
}

outout:

Enter an integer: 00002
Reversed number = 20000
Enter an integer: 3000
Reversed number = 0003

CodePudding user response：

Read the input as a string and reverse it, if you use windows you have the string.h handy strrev():

char n[20];
printf("Enter an integer: ");
scanf("s", n);
printf ("%s", strrev(n));

Otherwise you have to make your own, here's a nice one using biwise XOR:

char *strrev(char *str)
{
      char *substr_lh, *substr_rh;

      if (! str || ! *str)
            return str;
      for (substr_lh = str, substr_rh = str   strlen(str) - 1; 
           substr_rh > substr_lh; 
             substr_lh, --substr_rh)
      {
            *substr_lh ^= *substr_rh;
            *substr_rh ^= *substr_lh;
            *substr_lh ^= *substr_rh;
      }
      return str;
}

Live sample

You can add validation for non digit characters which, I reckon , is trivial.

CodePudding user response：

In general the reversed number can not fit in an object of its type.

It is better to use the type unsigned int instead of the type int. Otherwise you can get an unexpected result.

What you need is just to output the reversed number in a field with the width equal to the number of digits in the entered number prefixed with zeroes if required.

Here is a demonstrative program.

#include <stdio.h>

int main(void) 
{
    const unsigned int Base = 10;
    
    unsigned int n = 0;
      
    printf( "Enter an integer: " );
    scanf( "%u", &n );
      
    unsigned int rev = 0;
    int count = 0;
      
    do
    {
        rev = rev * Base   n % Base;
          count;
    } while ( n /= Base );
    
    printf( "Reversed number = %0*u", count, rev );
    
    return 0;
}

The program output might look like

Enter an integer: 10000
Reversed number = 00001