C String: substring Function for ANSI C

Introduction

This article describe how to create a function that get a sub string from source string by start index and end index (similar to Java's String.substring) in ANSI C.

The Program

substring.c

A custom substring function and test program in this file.

#include <stdio.h>
#include <string.h>
/**
 * 
 * Tested with Dev-C++ 4.9.9.2
 *  
 * Practice of string copy and concat.
 *
 * char * strncpy ( char * destination, const char * source, size_t num );
 *         copy first n (the specified num) char
 *         from source to destination
 * void* malloc (size_t size);
 *         Allocate memory block
 *         Allocates a block of size bytes of memory,
 *         returning a pointer to the beginning of the block.
 *         
 *         The content of the newly allocated block of memory is not initialized,
 *         remaining with indeterminate values.
 *         
 *         If size is zero, the return value depends on the particular
 *         library implementation (it may or may not be a null pointer),
 *         but the returned pointer shall not be dereferenced.
 * void free (void* ptr);
 *         A block of memory previously allocated by a call to malloc,
 *         calloc or realloc is deallocated,
 *         making it available again for further allocations.
 *         
 *         If ptr does not point to a block of memory allocated with the
 *         above functions, it causes undefined behavior.
 *         
 *         If ptr is a null pointer, the function does nothing.
 *         Notice that this function does not change the value of ptr itself,
 *         hence it still points to the same (now invalid) location.
 * void * memset ( void * ptr, int value, size_t num );
 *         Fill block of memory
 *         Sets the first num bytes of the block of memory pointed by ptr to
 *         the specified value (interpreted as an unsigned char).
 * 
 * char * substring ( const char * source, int startIndex, int endIndex );
 *         custom function for getting a substring of source string
 *         source: the source string
 *         startIndex: start index (inclusive) of sub string in source string
 *         endIndex: end index (exclusive) of sub string in source string
 *         return: pointer of the sub string, or null if any error occured
 *
 *         NOTE: substring will return malloced pointer,
 *               remember to free it as needed.
 *
 * References:
 * http://www.cplusplus.com/reference/cstring/strncpy/
 * http://www.cplusplus.com/reference/cstdlib/malloc/
 * http://www.cplusplus.com/reference/cstdlib/free/
 * http://www.cplusplus.com/reference/cstring/memset/
 * 
 */
char * substring ( const char * source, int startIndex, int endIndex );
int main () {
    char* chArr = "this is substring";

    // test negitive startIndex
    char* subString = substring(chArr, -1, 8);
    printf("%s\n\n", subString);
    free(subString);

    // test endIndex smaller than or equal to startIndex
    subString = substring(chArr, 17, 8);
    printf("%s\n\n", subString);
    free(subString);

    subString = substring(chArr, 8, 8);
    printf("%s\n\n", subString);
    free(subString);

    // test startIndex out of bound
    subString = substring(chArr, 177, 178);
    printf("%s\n\n", subString);
    free(subString);

    // test endIndex out of bound
    subString = substring(chArr, 17, 178);
    printf("%s\n\n", subString);
    free(subString);

    // get sub string and show it
    subString = substring(chArr, 0, 1);
    printf("%s\n\n", subString);
    free(subString);

    subString = substring(chArr, strlen(chArr)-1, strlen(chArr));
    printf("%s\n\n", subString);
    free(subString);

    subString = substring(chArr, 8, 17);
    printf("%s\n\n", subString);
    free(subString);

    subString = substring(chArr, 14, 17);
    printf("%s\n\n", subString);
    free(subString);

    system("PAUSE");
}
/**
 * custom function for getting a substring of source string
 *         source: the source string
 *         startIndex: start index (inclusive) of sub string in source string
 *         endIndex: end index (exclusive) of sub string in source string
 *         return: pointer of the sub string, or null if any error occured
 *
 *         NOTE: substring will return malloced pointer,
 *               remember to free it as needed.
 */
char * substring ( const char * source, int startIndex, int endIndex ) {
    char* result = NULL;
    if (startIndex < 0) {
        printf("startIndex should be a positive value\n");
    }
    else if (endIndex <= startIndex) {
        printf("endIdnex should larger than startIndex\n");
    } else if (startIndex > (strlen(source))) {
        printf("startIdnex should smaller than source length\n");
    } else if (endIndex > (strlen(source)+1)) {
        printf("endIdnex should smaller than or equal to source length\n");
    } else {
        int len = endIndex - startIndex;
        result = (char*)malloc(sizeof(char)*len+1);
        memset (result, '\0', len+1);
        strncpy (result, source+startIndex, len);
    }
    return result;
}

The Result

Reference

strncpy
http://www.cplusplus.com/reference/cstring/strncpy/

malloc
http://www.cplusplus.com/reference/cstdlib/malloc/

free
http://www.cplusplus.com/reference/cstdlib/free/

memset
http://www.cplusplus.com/reference/cstring/memset/

Get a substring of a char* at stackoverflow
http://stackoverflow.com/questions/4214314/get-a-substring-of-a-char

Download

substring.c
https://github.com/benbai123/C_Cplusplus_Practice/blob/master/C_StringProcessing/utils/substring.c