Segmentation fault at writing the telephone number program - c

Question

I am writing a C program that prompts the user to enter a telephone number in the form (xxx)xxx-xxxx and then displays the number in the format xxx.xxx.xxxx. Here is an example:
Enter a phone number [(xxx) xxx-xxxx]: [phone]
You entered the data 404.817.6200
I created two strings, one for storing the phone number with parenthesis and '-' signs and the other, empty. I want to add each character to the empty string, changing the ')' and '-' to '.'. Here is my code:
#include <stdio.h>
int main(void){
char num[15];
char phone[13];
int i = 1;
int j = 0;
printf("Please, enter your phone number in the format (xxx)xxx-xxxx:");
scanf("\n%s", num);
while(num != '\0'){
if(num[i] == ')' || num[i] == '-'){
phone[j] = '.';
}else{
phone[j] = num[i];
}
i++;
j++;
}
printf("\n%s",phone);
}
When I run the program, it gives me the error message that says:
Segmentation fault
Can somebody explain why it is happening and how to prevent it in the future?

Answer 1

I see three issues in your program:
(1) while(num != '\0') should be while(num[i] != '\0'. num is an array (and will never compare equal '\0', so you get an endless loop and exceed array bounds.
(2) you need at least 14 bytes for phone (just one less than num, not two less)
(3) you need to write a string termination character into phone; othwerwise printf("\n%s",phone); will again exceed the bounds of phone; e.g:
}
phone[j] = '\0';
printf("\n%s",phone);`

Answer 2

When you reference the name of an array variable in C, without appending the [] (index) operator, you are (effectively) referring to the address of that array's first element. This will never be zero (or NULL) for an array declared as a local variable, so the comparison of num to zero ('\0') in your while loop will never be true, and the loop will run on unstopped, until you try to read or write to an invalid address, at which point the program will crash.
With compiler warnings enabled, you should see something like the following (generated by clang-cl):
warning : comparing a pointer to a null character constant; did you
mean to compare to NULL? [-Wpointer-compare] warning : comparison
of array 'num' not equal to a null pointer is always true
[-Wtautological-pointer-compare]
What you should do, instead, is to compare the 'current' element (at index i) to the nul character ('\0') to check for the end of the string:
while (num[i] != '\0') {// Check character at position "i"
// rest of your loop ...
You should also ensure that your phone string is properly nul-terminated (although some compilers will initialize the array to zero, don't rely on this). You can do this either by adding an initializer to the declaration of phone:
char phone[13] = { 0 }; // Will set all elements to zero
or by adding the nul terminator immediately after the end of your while loop:
// body of while loop
// ...
}
phone[j] = '\0'; // Append the terminator