find one string in another string using c - c

Question

I have written following code to find str1 is present in str2 or not. But it doesn't work all scenarios.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
int i,j,flag=1;
char str1[]="goa",str2[]="gogoa";
if (strlen(str1)>strlen(str2))
{
printf("not found");
return;
}
for ( i = 0; str2[i]; i++)
{
if (str1[0]==str2[i])
{
for ( j = 0; str1[j]; j++)
{
if (str1[j]!=str2[i+j])
{
printf("not found");
flag=0;
}
}
break;
}
}
if (flag==1)
{
printf("found at index %d ",i);
}
getchar();
}
its not working when str1 starting character is present multiple times in str2,otherwise it works fine.
How can I optimize this to make it work in all scenarios?

Answer 1

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(){
int i, j, flag=0, len1, len2;
char str1[]="goa",str2[]="gogoa";
len1 = strlen(str1);
len2 = strlen(str2);
if (len1 > len2){
printf("not found");
return 0;
}
for (i = 0; i <= len2 - len1; ++i){
if (str1[0]==str2[i]){
flag = 1;
for (j = 1; str1[j]; ++j){
if (str1[j]!=str2[i+j]){
flag=0;
break;
}
}
if(flag==1)
break;
}
}
if (flag==1){
printf("found at index %d ",i);
} else {
printf("not found");
}
return 0;
}

Answer 2

try this:
int i,j,flag=0, match=0;
char str1[]="goa",str2[]="gogoa";
if (strlen(str1)>strlen(str2))
{
printf("not found");
return;
}
for ( i = 0; str2[i]; i++)
{
if (str1[0]==str2[i])
{
match=1;
for ( j = 0; str1[j]; j++)
{
if (str1[j]!=str2[i+j])
match=0;
}
if(match == 1)
{
flag = 1;
break;
}
}
}
if (flag==1)
printf("found at index %d ",i);
else
printf("not found");

Answer 3

I agree to comments that you should show what you have done. Please do that next time.
For now, I would like to suggest two approaches to debug out of sticky problems: use printfs and understand the flow OR use gdb. I have commented your program w.r.t. the printf approach. Have not tested all cases...but this should hopefully give you a direction to your debugging. Hope it helps. Remember to do this step before you post a question next time :-)
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
main()
{
int i,j,flag=1;
char str1[]="goa",str2[]="gogoa";
if (strlen(str1)>strlen(str2)) {
printf("not found");
return;
}
for ( i = 0; str2[i]; i++) {
/* If starting letter does not match, forget it */
if (str1[0]==str2[i]) {
flag = 1;
// Iterate thro' str1
for ( j = 0; str1[j]; j++) {
if (str1[j] != str2[i+j]) {
printf("%s not found at index %d of %s\n", str1, i, str2);
flag=0;
break; // out of inner for loop
}
}
if (j == strlen(str1)) {
// found the entire string.
break; // out of outer for loop
}
}
}
if (flag==1) {
printf("%s found at index %d of %s\n", str1, i, str2);
}
getchar();
}

Answer 4

I have rewritten the code with slight modifications in it and it's working for me,
check all your necessary scenarios, and let me know.
public static bool findString(string searchText, string fullText)
{
bool result = false;
if (searchText.Length > fullText.Length)
{
return false;
}
for (int i = 0; i <= (fullText.Length - searchText.Length); i++)
{
if (searchText[0] == fullText[i])
{
for (int j=0; j<searchText.Length ;j++)
{
if ((i+j)< fullText.Length && searchText[j] == fullText[i+j])
result = true;
else
{
result = false;
break;
}
}
}
if (result)
{
Console.WriteLine("found at index {0}", i);
break;
}
}
return result;
}
Sorry about the syntax, I only had c# available. you'll have to use strlen(string) for lengths and '%d' for '{0}', printf for Console.Writeline(" ");

Answer 5

#include <string.h>
if(strstr(str2, str1) != NULL) {
/* ... */
}
Please checkout the doc of strstr function.

Answer 6

//Declaration: char *strstr(const char *str1, const char *str2);
//Return: It returns a null pointer if no match is found.
#include <string.h>
#include <stdio.h>
int main(void)
{
char *p;
p = strstr("goa", "gogoa");
printf(p);
return 0;
}