Thread: Comparing Strings

The task is following:

Write a function

Code:

int name(char *s1, char *s2)

that returns the position of the first occurrence of string s2 in string s1. The positions are numbered from 0 to strlen(s1)-1. If s1 has szero length or s2 has zero length or s2 cannot be found in s1 the function must return -1.

Write a main program that prompots for s1 and s2 and prints them along with the result of calling Locate with them.

Assume a maximal string length of 100 in the main program; in the function, the string lengths schould be limited by the maximal positive value of int only.

BTW: String-functions are not allowed to use.

1. How would you recommend to do fgets error-handling?

The task said, that the main program should be limited with a string-length of 100, so I used fgets, because it does not matter how much you put in, the string will always be stored with the count of characters you give to fgets(). The rest will stay in the buffer and excuted in the next round, but to prevent this I used fseek to let the File Pointer point to the end of the input buffer. I've also tried to use

Code:

while ((c = getchar()) != '\n' && c != EOF) {}

, but this will let you wait till a new input from the keyboard.

Which brings me to this question:
2. Can you actually use this while to clear the buffer without waiting for a new input? Is the solution fseek okay, because it only puts the file pointer at the end and do not delete the input buffer?

3. Are there even certain cases in which you only can use s[i] or *(s+i)? (when s is declared as a pointer)

4. What else do you think can I improve?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int Locate(char *s1, char *s2);

int StrCmp(char *s1, char *s2);
int StrLength(char *s);

int main(void)
{
  enum { MAX_LENGTH = 5 };

  char s1[MAX_LENGTH];
  char s2[MAX_LENGTH];

  int s1Len;
  int s2Len;

  printf("1. string (max. 100): ");

  while(fgets(s1, MAX_LENGTH, stdin) != NULL)
  {
    fseek(stdin, 0, SEEK_END);
    s1[s1Len=StrLength(s1) - 1] = '\0';    
      
    printf("2. string (max. 100): ");
    if (fgets(s2, MAX_LENGTH, stdin) == NULL)
    {
      printf("\nError, string s2 couldn't be read properly.\n");
      exit(EXIT_FAILURE);
    }

    fseek(stdin, 0, SEEK_END);
    s2[s2Len=StrLength(s2) - 1] = '\0';
    
    if (s1Len > 0 && s2Len > 0)
    {
      printf("\nPosition of the first occurence of ");
      printf("String 2: ");
      fputs(s2, stdout);

      printf(" in ");
      printf("String 1: ");
      fputs(s1, stdout);
      
      printf(" : %d\n", Locate(s1,s2));
    }
    else // empty input
    {
      printf("\nError, empty input.\n");
    }

    printf("\n1. string (max. 100): ");
  }

  printf("\nError, string s1 couldn't be read properly.\n");

  return 0;
}

/*looks for first occurence of s2 in s1*/
int Locate(char *s1, char *s2)
{
  int i=0;
  int s1Len;
  int s2Len;

  if ((s1Len=StrLength(s1)) <= INT_MAX && (s2Len=StrLength(s2)) <= INT_MAX)
  {
    if (s1Len < s2Len) return -1;

    while (s1[i] != '\0')
    {
      if (StrCmp(s1 + i, s2))
      {
        return i;
      }
      i++;
    }

    return -1;
  }
  else
  {
    printf("Error, input too long!");
    exit(EXIT_FAILURE);
  }
}

/*compares strings whereby their lengths are "length of s2
2*/
int StrCmp(char *s1, char *s2)
{
  int i = 0;
  int eq = 0;

  while (s2[i] != '\0')
  {
    if (s1[i] == s2[i])
    {
      eq++;
    }
    i++;
  } 
  
  if (i == eq) //check if s1 and s2 are identical (i (=eq) characters)
  {
    return 1; // identical
  }
  else 
  {
    return 0; // not identical
  }
}

/*counts string length of s*/
int StrLength(char *s)
{
  int cnt=0;

  while (s[cnt] != '\0')
  {
    cnt++;
  }

  return cnt;
}