Reading a string using fgets, couldn't get output of second string

Question

• Objective of Program : Get the number of strings to be entered, read the string, reverse the string and print the string. Proceed with the next string

  #include <stdio.h>
  
  int main() {
      int num_tc, index_tc, char_idx, str_len = 0;
      char S[31];
  
      scanf("%d\n", &num_tc);
  
      for (index_tc = 1; index_tc <= num_tc; index_tc++) {
          fgets(S, sizeof(S), stdin);
  
          /* To compute the string length */
          for (char_idx = 0; S[char_idx] != NULL; char_idx++)
              str_len++;
  
          /* Reverse string S  */
          for (char_idx = 0; char_idx < str_len / 2; char_idx++) {
              S[char_idx] ^= S[str_len - char_idx - 1];
              S[str_len - char_idx - 1] ^= S[char_idx];
              S[char_idx] ^= S[str_len - char_idx - 1];           
          }
          puts(S);
      }
      return 0;
  }
  

• Input to the program

       2<\n>
       ab<\n>
       aba<\n>
  

  Output

      ba
  

• Kindly let me know why the second string is not taken for string reversal.

• If I remove string reversal logic, I could see both strings output

Answer 1

You do not reset str_len to 0 in the body of the loop. The length of the second string is incorrect, hence the second string is not properly reversed. Change the loop to:

for (str_len = 0; S[str_len] != '\0'; str_len++)
    continue;

Note that you should strip the trailing '\n' before reversing the string. You can do this with S[strcspn(S, "\n")] = '\0'; before computing str_len.

Here is a simplified version using scanf(), that reverses individual words:

#include <stdio.h>

int main(void) {
    int num_tc, tc, len, left, right;
    char buf[31];

    if (scanf("%d\n", &num_tc) != 1)
        return 1;

    for (tc = 0; tc < num_tc; tc++) {
        if (scanf("%30s", buf) != 1)
            break;

        /* Compute the string length */
        for (len = 0; buf[len] != '\0'; len++)
            continue;

        /* Reverse string in buf */
        for (left = 0, right = len - 1; left < right; left++, right--) {
            buf[left] ^= buf[right];
            buf[right] ^= buf[left];
            buf[left] ^= buf[right];           
        }
        puts(buf);
    }
    return 0;
}