functions with char pointer

Hi i need help please,
the main program below uses a ”helper” function substr() to determine and report if the secondstring that the user enters is ”contained in” the first string. I need to use a nested for loop for this. So far, the program reports certain characters from the second string as false even if they are contained. I do not know what to add or modify to get the program to accurately report all given characters.

Code:

---

#include <stdio.h>
#include <string.h>
// returns the starting index of the first such occurrence
// where sub_str was found within str - if one exists
// otherwise returns -1
int substr(const char str[], const char sub_str[]);
int main()
{
        char str[255];
        printf("Enter a string: "); fflush(stdout);
        scanf("%s", str);
        char sub_str[255];
        do
        {
                printf("Enter a string to search for (\"quit\" to quit): "); fflush(stdout);
                scanf("%s", sub_str);
                if ( strcmp(sub_str, "quit") )
                {
                        int ind = substr(str, sub_str);
                        if ( ind == -1 )
                        {
                                printf("Not found: %s\n", sub_str);
                        }
                        else
                        {
                                printf("\"%s\" found at starting index %d\n", sub_str, ind);
                        }
                }
        } while ( strcmp(sub_str, "quit") );
        printf("Done!\n");
        return 0;
}
int substr(const char str[], const char sub_str[])
{
        int i, j, big = strlen(str), small = strlen(sub_str), flag = 0;


        for(i=0; i<(big-small); i++)
        {
                for(j=0; j<small; j++)
                {
                        if(str[i+j]!=sub_str[j])
                        {
                                i+=1;
                        }
                        else
                        {
                                flag = 1;
                        }


                }
                        if (flag)


                                return i;
                        else
                        {
                                i++;
                        }
        }
                        return -1;
}

---

So what's an example of a test case that you think should pass, but actually fails?

For example if the first string is 'burger' and the second string is 'b' it should report 0, which is the index of the first occurrence in the first string. But 'u' reports not found instead of 1 . And it does the same with some of the other characters.

Well you could step through the code with a debugger.

Like I did.

Code:

---

$ gcc -g foo.c
$ gdb -q ./a.out
Reading symbols from ./a.out...done.
(gdb) b substr
Breakpoint 1 at 0x4008b9: file foo.c, line 33.
(gdb) run
Starting program: ./a.out
Enter a string: burger
Enter a string to search for ("quit" to quit): u

Breakpoint 1, substr (str=0x7fffffffdc50 "burger", sub_str=0x7fffffffdd50 "u") at foo.c:33
33          int i, j, big = strlen(str), small = strlen(sub_str), flag = 0;
(gdb) n
35          for (i = 0; i < (big - small); i++) {
(gdb)
36            for (j = 0; j < small; j++) {
(gdb)
37              if (str[i + j] != sub_str[j]) {
(gdb)
38                i += 1;
(gdb)
36            for (j = 0; j < small; j++) {
(gdb)
43            if (flag)
(gdb)
46              i++;
(gdb)
35          for (i = 0; i < (big - small); i++) {
(gdb) p i
$1 = 2

---

At which point, you might wonder why i is now 2 rather than 1.
Because by skipping straight to 2, there is no chance to match 'u' at position 1.