I have the answer, I'd like to know why.

[mpatters]

Hey Guys,

I'm trying to understand this function 'f'. The code for function f was provided in the book. I wrote the main() function that calls f. The question is "What is the value of f("abcd", "babc")? Here's the code I wrote to try and figure this out:

#include <stdio.h>
// http://knking.com/books/c2/answers/c13.html


int f(char *s, char*t);


int main(void){
    int dang;
    dang = f("abcd", "babc");
    printf("dang=%s\n", dang);
    return 0;
}


int f(char *s, char*t){
    //printf("s=%s, t=%s\n",s,t);
    char *p1, *p2;
    for(p1=s; *p1; p1++){


        for(p2=t; *p2; p2++){
            if(*p1==*p2) break;
        }
        if(*p2== '\0') break;
    }
    printf("p1=%s, s=%s\n",p1,s); //prints p1=d, s=abcd 
    return p1-s;
}

The Answer: The value of f is 3. The reason: The length of the longest prefix of the string s that consists entirely of characters from the string t. Or, equivalently, the position of the first character in s that is not also in t.

How do I print out the answer '3'? In the final printf statement, p1=d, s=abcd. d-abcd = 3? I just don't get it.

In the 2nd question, the value of f("abcd", "bcd") is 0 which I also don't understand. Can someone please help me understand what is happening.

[christop]

The function f() returns an int and you assign the return value to an int (dang), so you also need to print dang as an int in main() (hint: use the %d format specifier).

To answer your second question, what is the first position in the string "abcd" that contains a character that is not also in the string "bcd"? It's the first position, right? That's position 0 (indices are zero-based in C).

By the way, this is the same thing the strspn() function in the standard library does.

[john.c]

p1 is a pointer. It's value is the address of 'd', not 'd' itself. Same with s. It is a pointer and it's value is the address of 'a' in the string "abcd". Subtracting s from p1 gives the "distance" between the pointers, which is the answer.

About f("abcd", "bcd"), the answer is 0 since the very first letter of s is not in t, so the scan stops at that point.

#include <stdio.h>
 
int f(const char *s, const char *t) {
    const char *p = s;
    for (const char *q; *p; ++p) {
        for (q = t; *q && *p != *q; ++q) ;
        if (!*q) break;
    }
    return p - s;
}
 
int main() {
    struct {
        const char *s, *t;
        int n;
    } tests[] = {
        {"abcd",       "babc", 3},
        {"aaaa",       "x",    0},
        {"xxxxxxtxxx", "qx",   6},
        {"xxxx",       "x",    4},
        {"abcd",       "bcd",  0}
    };
    int size = sizeof tests / sizeof tests[0];
 
    for (int i = 0; i < size; ++i) {
        printf("[%s] [%s]\n", tests[i].s, tests[i].t);
        int n = f(tests[i].s, tests[i].t);
        printf("%d", n);
        if (n != tests[i].n)
            printf("  WRONG: should be %d", tests[i].n);
        putchar('\n');
    }
 
    return 0;
}

[mpatters]

Hey christop, thank you so much. I can't believe so many of my errors are with silly things like %s vs %d. OMG, so embarrassing. I will look at your reasoning in a bit. I see John.c also made a comment. I'm working on another exercise at the moment and will come back to this.

[mpatters]

Hey John, thank you for the explanation. I got it now thanks to you and Christop. "It's value is the address of 'd', not 'd' itself. Same with s."

I've read the chapter 3 times and it never covered the content to answer this question. Must have been a prior chapter...