Create a C program to find the sum of digits in a mixed string of letters and digits (eg co45p263) entered by the user?

Question

/*C program to get a string with letters and numbers from the
  user then find the sum of all the numbers in the string */


#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

int main()

{
   char ch;
   int j;
   char str1[9];
   int sum = 0;

   printf("\n\n  Please enter a string of letters and numbers: ");
   gets(str1);
   printf("\n\n");
   printf("  Printing the numbers in the string\n");
   printf("\n\n  Numbers found in string   ");
   printf("\n\n");
   for (j = 0; j < strlen(str1); j++) {
     if(isdigit(str1[j]))
       printf("   %c  ", str1[j] );
  }   
  printf("\n\n  Calculating the sum of the numbers in the string\n");
  for (int j = 0; j < strlen(str1); j++){
     if(isdigit(sum += j))
     printf("  %d  ",str1[j]);
  }  

     scanf("%c%c", &ch, &ch);
   return 0;
}      

Answer 1

This application of isdigit doesn't look right:

if(isdigit(sum += j))
   ...

Compare that to how it is used in the previous for loop, where you are just searching for digits. The if condition should look the same here, and then sum should be incremented in the if's body.