如何使用带指针的 For 循环消除最后一个零？

Question

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

int main(){
    char num[6];
    char pass[6];
    int digits=4, i;
    char *p;
    memset(pass, '\0', sizeof(pass));
    memset(num, '\0', sizeof(num));
    strncpy(num, "000104",6);
    p = (char *)&num;
    //printf("p has this [%6.6s]\n",p);

    for(i = 0; i < 5, *p == 0; i++, p++){
    //printf("for [%d]\n",i);
    //printf("pointer [%c]\n",*p);
    strncpy(pass, num+(6-digits), digits);
    //printf("pass: [%6.6s]\n", pass);
    }
 
}

This is my code. I want to delete the last zero in the room variable using that pointer in the if condition. I'm new using pointers. This is the last output I'm getting from the variable:

pass: [ 0104] (it should be 104).

Can someone explain me how that pointer condition is working? Why is not detecting and deleting the last zero character in the string?

Answer 1

Your question title is confusing. To me, deleting the "last zero" in your string would mean the zero between 1 and 4 . What you're actually trying to do is remove leading zeroes . That is very simple:

while (*p == '0') ++p;

Now, p points to the first character that is not '0' .

It's not clear what you're trying to do in your loop, which is why I haven't attempted to fix it. I will say that copying an entire string every time around the loop is generally bad practice.

Answer 2

As @paddy noticed, the question is rather unclear, but there are a couple of things that catch the eye and are worth pointing out.
I've highlighted them in the multiline comments /**/

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

int main()
{
    char  num[6];
    char  pass[6];
    int   digits = 3;
    int   i;
    char *p;
    
    memset(pass, '\0', sizeof(pass));
    memset(num, '\0', sizeof(num));
    strncpy(num, "000104", 6);

/**
 * Casting &num to (char *) silences the compiler, but it was highlighting a mistake!
 * Also, the cast was not needed at all,
 * because num is a char[] and it can be assigned to a char * 
 */ 
    p = (char *)&num; // p = num; is the right way
    // printf("p has this [%6.6s]\n", p);

/**
 * Only the first 5 elements would be accessed,
 * if they were accessed at all, because *p == '0', then *p != 0,
 * therefore this loop is never executed
 */
    for(i = 0; i < 5, *p == 0; i++, p++)
    {
        // printf("for [%d]\n",i);
        // printf("pointer [%c]\n",*p);
    /**
     * Here strncpy() always copies the last 4 bytes of num,
     * which is { '0', '0', '0', '1', '0', '4' },
     * that's why pass is always "0104" instead instead of "104"
     */
        strncpy(pass, num+(6-digits), digits);
        // printf("pass: [%6.6s]\n", pass);
    }
}