About string length, terminating NUL, etc

Question

I\'m currently learning C and I\'m confused with differences between char array and string, as well as how they work.

Question 1:

Why is the

Answer 1

Question 1:

Why is there a difference in the outcomes of source code 1 and source code 2?

Source code 1:

#include 
#include 

int main()
{
    char c[2]="Hi";
    printf("%d", strlen(c));   //returns 3 (not 2!?)
    getchar();
}

Source code 2:

#include 
#include 

int main()
{
    char c[3]="Hi";
    printf("%d", strlen(c));   //returns 2 (not 3!?)
    getchar();
}

answer: Because in the first case, c[] is only holding "Hi". strlen looks for a zero at the end, and, depending on exactly what is behind c[] finds one sooner or later, or crashes. We can't say without knowing exactly what is in the memory behind the c[] array.

Question 2:

How is a string variable different from a char array? How to declare them with the minimum required index numbers allowing \0 to be stored if any (please read the codes below)?

char name[index] = "Mick";   //should index be 4 or 5?

char name[index] = {'M', 'i', 'c', 'k'};   //should index be 4 or 5?

answer Really depends on what you want to do. Probably 5 if you want to actually use the content as a string. But there's nothing saying you can't store "Mick" in a 4 character array - you just can't use strlen to find out how long it is, because strlen will continue to 5 and quite possibly (much) further to find the length, and if there is no zero in the next several memory locations, it could lead to a crash, because eventually, there won't be valid memory addresses to read.

#define name "Mick" //what is the size? Is there a \0?

This has absolutely no size at all, until you use name somwhere. #defines are not part of what the compiler sees - the pre-processor will replace name with "Mick" if you use name anywhere - and hopefully, that's in a place the compiler can make sense of. And then the same rules apply as in previous answer - it depends on how you want to use the array of characters. For correct operation with strlen, strpy, and nearly all other str... functions, you need a zero at the end.

Question 3:

Does the terminating null ONLY follow strings but not char arrays? So the actual value of the string "Hi" is [H][i][\0] and the actual value of the char array "Hi" is [H][i]?

Yes, no, maybe. It all depends on how you USE the "Hi" string literal (that's the technical name for 'something within double quotes'). If the compiler is "allowed", it will put a zero at the end. But if you initialize an array to a given size, it will stuff the bytes in there, and if there isn't room for a zero, that's your problem, not the compiler's.

Question 4:

Suppose c[2] is going to store "Hi" followed by a \0 (not sure how this is done, using gets(c) maybe?). So where is the \0 stored? Is it stored "somewhere" after c[2] to become [H][i]\0 or will c[2] be appended with a \0 to become c[3] which is [H][i][\0]?

In c[2], beyond the 'H', 'i', there is no telling what is stored [technically, it could well be "the end of the earth" - in computer terms, that's "memory that can't be read - in which case strlen on that WILL crash your program, because strlen reads beyond the end of the earth]. But if could also be a zero, a one, the letter 'a', the number 42, or any other 8-bit [1] value.

It is quiet confusing that sometimes there is a \0 following the string/char array and causes trouble when I compare two variables by if (c1==c2) as it most likely returns FALSE (0).

If c1 and c2 are char arrays, that will ALWAYS be false since c1 and c2 are never going to have the same address, and when using an array in C in that way, it becomes "the address in memory of the first element in the array". So no matter what teh contents of c1 and c2 is, their address can never be the same [because they are two different variables, and two variables can not have the same location in memory - that's like trying to park two cars in a parking space large enough only for one car - and no, crushing either car is not allowed in our thought experiment].

[1] Char isn't guaranteed to be 8 bits. But lets inore that for now.