Search a String in a string (strings are input by user)

Question

I need to write a programm where two strings are entered through the keyboard and the 1st string is searched for the 2nd and then the point where it first appeared is given out.

#include <stdio.h>
#include <string.h>

int main() {

char in1[10000];
char in2[10000];


fgets(in1, sizeof(in1), stdin);
fgets(in2, sizeof(in2), stdin);
printf("%d", strstr(in1, in2) - in1+1);

return 0; 
}

I did 3 tests

in1=11121
in2=121
result=3 correct

in1=11121121
in2=121
result=6 wrong

in1=11121211
in2=121
result=-2348863 obviously wrong

in1=111211211211
in2=121
result=-2348879

I don't know why it gives out the 2nd time the sequence occurs if it occurs twice and a big negative number if it occurs thrice.

What did I do wrong?

I bet that the large negative number is -in1+1, when strstr returns null after not finding anything. Please post a version of your program that actually produces the outputs you show above. — Mad Physicist
– Mad Physicist, Commented Jan 4, 2014 at 4:00
The program I posted does produce those results. And it looks like you are right. But I don't understand why it returns null if the sequence does appear in the string. — user3004619
– user3004619, Commented Jan 4, 2014 at 4:08

Victor Maia Aldecôa · Accepted Answer · 2014-01-04 04:21:35Z

2

In this kind of situation it's better to use scanf because it truncates newlines from input while fgets does not. Also, you should use %ld since you're doing an operation with pointers (or make a cast to int).

The code below works for all your examples:

#include <stdio.h>
#include <string.h>

int main()
{
     char in1[10000];
     char in2[10000];
     int i;

     scanf("%s", in1);
     scanf("%s", in2);

     char* result = strstr(in1, in2);
     if (result != NULL) printf("%ld\n", strstr(in1, in2) - in1 + 1);
     else printf("No match\n");

     return 0;
}

edited Jan 4, 2014 at 4:21

answered Jan 4, 2014 at 4:09

Victor Maia Aldecôa

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2 Comments

user3072164 Over a year ago

This is correct, the reason being, that fgets will include the newline read in the string, while scanf doesn't. Obviously the comparison in strstr always fails (except at the end) because of the newline character.

mafso Over a year ago

scanf("%s",... is like gets... (or maybe even worse)

Digital_Reality · Accepted Answer · 2014-01-04 04:50:32Z

0

because function: char *fgets(char *buf, int bufsize, FILE *stream) will get the '\n'; so in your case,your 2nd test is "11121121'\n'\0\"and "121'\n''0'", you can use "debug" to see the values of in1 and in2

edited Jan 4, 2014 at 4:50

Digital_Reality

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answered Jan 4, 2014 at 4:25

zhangzhang

1

Comments

mafso · Accepted Answer · 2014-01-04 05:55:07Z

The problem is, that you don't discard the newline characters. So, what you really searched for in your first input example was "121\n" and this matches the end of "11121\n". And the first match of "121\n" in "11121121\n" is at 6-th position.

What happens with "11121211\n" and "121\n" is: There is no match and strstr returns 0.

int main() {
    char in1[10000];
    char in2[10000];
    fgets(in1, sizeof(in1), stdin);
    { /* Delete the newline character if there is one. */
        size_t len = strlen(in1);
        if(in1[len-1]=='\n') in1[len-1] = 0;
    }
    fgets(in2, sizeof(in2), stdin);
    {
        size_t len = strlen(in2);
        if(in2[len-1]=='\n') in2[len-1] = 0;
    }
    char *match = strstr(in1, in2);
    if(match) {
        printf("%td\n", match-in1+1);
        // t is the conversion specifier for ptrdiff_t
    }
    return 0;
}

Also, don't forget to check the return values of fgets. Also note, that len-1 is an invalid offset in case len is 0. So you must handle this either.

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