Printing of string with %s format specifier yields no output

Question

So I came across a question in a C challenge, and I just want to make sure I understand why the answer is the way it is. The question is asking for the output of the following code:

char p[20];
char *s = "string";
int length = strlen(s);
for (int i = 0; i < length; i++)
p[i] = s[length - i];
printf("%s", p);

The correct answer is No output. At first I thought it was because '\0' was assigned to the first character, so maybe the characters after it weren't read when printing. I know C strings cannot be reassigned as a whole, but their elements can, similar to arrays of other data types. I know this because I could reassign and print out the other characters of the string individually. I changed the initialization of i in the for loop to 1, the condition to i <= length, and even added a '\0' after the last character in the array, but it still does not output anything when I use the %s format specifier. What's going on?

Answer 1

<length> gets 6 from strlen(). So in the for-loop - when you do this

p[i] = s[length - i];

And <i> value was zero, you copy s[6 - 0] (which is a string terminator) into p[0]. This means the first character in <p> is a null character.

%s normally reads characters from pointed address (first char address) forward, until string terminator was found. But in this case, because p[0] is null character, it stops there assuming the string (char array) was empty.

Answer 2

Ipang Yes, but I've also tried moving the initial value of i to 1. It still does not output anything.

Edit: Nevermind. It does work when i's initial value is 1, but for some reason, p[length] = '\0'; causes it to have no output again even when p[length] is after the other characters.

Answer 3

Put i=1 don't change p[0] which is '\0', If it is.
Try like
int length = strlen(s) - 1;
And loop condition as i<=length;
Or also
p[i] = s[length-i-1]; may works i think. (Not tested.)

Answer 4

Initializing <i> value by 1 wouldn't change the behaviour if your compiler initializes <p> with nulls. The first character in <p> will likely be a null char anyways. If your compiler doesn't initialize <p> with nulls, then you *may* see weird character printed; as the first character in <p> is not a null character.

Try to print each character casted into `int` to verify this ...

Answer 5

Thanks Ipang and Jayakrishna🇮🇳 . Both of these are correct. The reason why it still had no output after I made some changes on the for loop was the '\0' I added at the end of p while testing. Don't know why that happens though since the '\0' was placed after the other characters.