Why there is no output?!

Question

This question was in a C challenge

What is the output of this 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 answer was No output.. can anyone tell me why??

Answer 1

The length of string "string" is equal to 6
Every string in C ends with null-character ('\0'). It is implicit and it is appended by C for you.
The 6th character of "string" is null-character:
0 1 2 3 4 5 6
's' 't' 'r' 'i' 'n' 'g' '\0'

The first character you are copying in the loop is character with index 6, so the first character of p will be '\0'.

After executing of the for loop array p will contain the following chracters:
p = {'\0' , 'g', 'n', 'i', 'r', 't'};
So, p is an empty string (as first character is null-character) and you will get no output when you print the empty string. (The rest of characters in p will be ignored cause the end of a string is null character).

Answer 2

What AJ said is true , and if you wonder why you get no output it's because the off-by-one error copies the null-byte at end of the string (because string[6] is '\0') and places it at the beginning of p.

because p[0] is a null byte, printf thinks it reached the end of the string so it prints nothing.

Answer 3

Mostafa Ehab You should write

s[length - i - 1]

Because index start from 0 show when you write [length - i] then you will get index 6 (6 - 0) on i = 0 which is wrong but when you write [length - i - 1] you get index 5 on i = 0

Finally it will look like this:

p[0] = s[length - 0 - 1] = s[6 - 0 - 1] = s[5] = g
p[1] = s[length - 1 - 1] = s[6 - 1 - 1] = s[4] = n
p[2] = s[length - 2 - 1] = s[6 - 2 - 1] = s[3] = i
p[3] = s[length - 3 - 1] = s[6 - 3 - 1] = s[2] = r
p[4] = s[length - 4 - 1] = s[6 - 4 - 1] = s[1] = t
p[5] = s[length - 5 - 1] = s[6 - 5 - 1] = s[0] = s

Answer 4

Mostafa Ehab on index 6, program is giving exception and displaying no output.

In Java we get ArrayIndexOutOfBound.

But in C I am not aware much so I won't be able to tell

Answer 5

andriy kan

You mean to type: p = { '\0', 'g', 'n', 'i', 'r', 't' }, no?

My thinking is: i stops at 5 which is < length (length is 6) and i begins at 0. That would mean length - i start from 6 and end at 1 to skip first character at position 0?

Answer 6

Mostafa Ehab As I told you index start from 0 so when you reverse the string then index will look like this

g, n, i, r, t, s
5, 4, 3, 2, 1, 0

Answer 7

Gen2oo
Yes, you are absolutely correct, p will be
{'\0', 'g', 'n', 'i', 'r', 't'}
's' will not be copied.
I just tried to show the main idea and didn't look at the code when was typing the answer.

Answer 8

AJ #Infinity Love ok but why it’s not “nirts”

Answer 9

Also...'p' is not yet a string... it's just an array of characters...you need to append a NULL ('\0') to the end of it to make it a string.