C - Find out if one string occurs in another, ignoring case

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Description

Find out if one string occurs in another, ignoring case

Demo

#define __STDC_WANT_LIB_EXT1__ 1       // Make optional versions of functions available
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define TEXT_LEN 100                   // Maximum input text length
#define SUBSTR_LEN 40                  // Maximum substring length

int main(void)
{
  char text[TEXT_LEN];                 // Input buffer for string to be searched
  char substring[SUBSTR_LEN];          // Input buffer for string sought

  printf("Enter the string to be searched (less than %d characters):\n", TEXT_LEN);
  gets_s(text, TEXT_LEN);/*ww  w.  ja v a 2 s  .  c  o m*/

  printf("\nEnter the string sought (less than %d characters):\n", SUBSTR_LEN);
  gets_s(substring, SUBSTR_LEN);

  printf("\nFirst string entered:\n%s\n", text);
  printf("Second string entered:\n%s\n", substring);

  // Convert both strings to uppercase.
  for (int i = 0; (text[i] = (char)toupper(text[i])) != '\0'; ++i);
  for (int i = 0; (substring[i] = (char)toupper(substring[i])) != '\0'; ++i);

  printf("The second string %s found in the first.\n",
    ((strstr(text, substring) == NULL) ? "was not" : "was"));
  return 0;
}

Result

The program has three phases:

  • reading in the input strings,
  • converting both strings to uppercase,
  • searching the first string for an occurrence of the second.

The following code does the reading 

printf("Enter the string to be searched (less than %d characters):\n", TEXT_LEN);
gets_s(text, TEXT_LEN);

The following code shows the string just read. 

printf("\nFirst string entered:\n%s\n", text);
printf("Second string entered:\n%s\n", substring);

The conversion of both strings to uppercase is accomplished using the following statements: 

// Convert both strings to uppercase.
for(int i = 0 ; (text[i] = (char)toupper(text[i])) != '\0' ; ++i);
for(int i = 0 ; (substring[i] = (char)toupper(substring[i])) != '\0' ; ++i);

We use for loops to do the conversion, and the work is done entirely within the control expressions for the loops.

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