Needing a function that would check me for the existence of a substring and give me the location I created this function, I wanted to know if something similar already exists in the C headers and how it works.
This own function give the position where start the substring Hello world! if i search world give 6If the string is not found then give -1
#include <stdio.h>#include <string.h>int findfstr(const char mainstring[], const char substring[]){// int = findstring(mainstring,substring) give position if found and -1 if notint main_length = strlen(mainstring); // Read the mainstring lengthint subs_length = strlen(substring); // Read the substring lengthint where = 0; // Set to 0 the var of start positionint steps = (main_length - subs_length); //Retrive the numbers of the chars without substringint cicle = 0; // Set to 0 the var used for increment stepschar found_string[subs_length]; // Set the Array to the substring lengthif ( subs_length <= main_length){ // If substring is bigger tha mainstring make errorwhile (where == 0){ //loop until var "where are equal to 0"//Stop loop if and when cicle is bigger than stepsif (cicle >= steps && where == 0){ where = -1;} //retrive the substring and store in found_stringstrncpy(found_string, mainstring+cicle, subs_length); found_string[subs_length] = '\0'; //Add terminator char to end string//If retrived string are equal to substring then set where with clicle valueif ((strcmp(found_string, substring) == 0 )) { where = cicle;}cicle++; //add +1 to cicle}}else{ printf("\n substring is to big \n"); } //error messagereturn where; }int main(){int fs = 0;// This is how use the functionfs = findfstr("Hello world!","world");if ( fs > 0 ){ printf("\n String found and start in: %d", fs);}if ( fs < 0 ){ printf("\n String not found value: %d", fs);}return 0;}Output:
String found and start in: 6Best Answer
I wanted to know if something similar already exists in the C
Yes, there is.
strstr is what you are looking for. See https://man7.org/linux/man-pages/man3/strstr.3.html
If you want the offset from the start of the string just use pointer arithmetic.
Something like:
#include <stdio.h>#include <string.h>int main(void){char* str ="Hello";char* needle = strstr(str, "ell"); // Search for "ell" in strif (needle){int offset = needle - str; // Calculate the offsetprintf("offset is %d\n", offset);}else{// not found...}return 0;}Output:
offset is 1There is the standard C string function strstr declared like
char *strstr(const char *s1, const char *s2);that does in fact the same task. Only its return type differs from the return type of your function.
As for your function then it is too complicated and as any too complicated function has at least a bug that can invoke undefined behavior.
For example you declared a variable length array in this declaration
char found_string[subs_length];that itself is a bad idea because there can be not enough memory to allocate such an array for big strings.
So the valid range of indices for this array is [0, subs_length). However you are using the value subs_length as an index to access memory beyond the array
found_string[subs_length] = '\0';Also the type int used as the return type can be not large enough to store a position in a big character array. You have to use the type size_t instead.
Here is a demonstrative program that shows how the function can be defined simpler without defining an intermediate variable length array that requires program resources.
#include <stdio.h>#include <string.h>#include <inttypes.h>char * findfstr( const char *s1, const char *s2 ){const char *p = NULL;size_t n1 = strlen( s1 );size_t n2 = strlen( s2 );for ( ; p == NULL && !( n1 < n2 ); --n1 ){if ( strncmp( s1, s2, n2 ) == 0 ){p = s1;}else{++s1;}}return ( char * )p;}int main(void) {char s1[] = "Hello World";const char *s2 = "Hello";char *p = findfstr( s1, s2 );if ( p != NULL ) printf( "%" PRIuPTR "\n", p - s1 );s2 = "World";p = findfstr( s1, s2 );if ( p != NULL ) printf( "%" PRIuPTR "\n", p - s1 );return 0;}The program output is
06To output a difference between two pointers use for example the macro PRIdPTR or PRIuPTR declared in the header <inttypes.h>. To declare a variable that keeps a difference between two pointers use the name ptrdiff_t declared in the header <stddef.h>.
For example
#include <stddef.h>//...ptrdiff_t diff = p - s1;