文字列から特定文字列を検索

ある文字列内に別の文字列が含まれているか確認し、含まれている場合はその先頭が何文字目かを返す。含まれていない場合は -1 が返される。

int match_string(const char * s, const char * x){
    // check s includes x or not
    int base_str_length = strlen(s);
    int search_str_length = strlen(x);
    int value = -1;
    if( base_str_length > search_str_length ){
	int i, j;
	// search string index should be always reset, so j = 0
	for( i = 0, j = 0 ; i < base_str_length && value == -1 ; j = 0 ){
	    if( *(s + i) != *(x + j) ){
		i = i + 1; // Check next char
		continue;
	    }
	    value = i;
	    for(  ; j < search_str_length && value != -1 ; j++ ){
		if( *(s + i) == *(x + j) ){
		    i = i + 1; // Check next char
		    continue;
		}
		else {
		    // Entering this block means that s includes a part of x,
		    // but not whole same as x. For example, s is aabbcc, x is bbb.
		    // s has bb, this is a part of x, but bb is different from bbb,
		    // not whole same as x.
		    value = -1;
		    // Increment i value shouldn't be done at this block.
		}
	    }
	}
    }
    return value;
}

実行例は以下。

$ ./a.out 01234567890 34
3
$ ./a.out 01234567890 90
9
$ ./a.out 01234567890 2
2
$ ./a.out 01234567890 99
-1
$ ./a.out 01234567890 a
-1
$ ./a.out 01234567890 678
6
$ ./a.out 01234567890 45678
4
$ ./a.out 01234567890 00
-1
$ ./a.out 01234567890 01
0
$