How to Pointer Output in Programs?

Question

In a quiz today, we got this as our question2.

After seeing this, most of us believe that we will fail quite badly.

#include <stdio.h>

int main ( ) {
  char str[] = "StanfordIsGreat";

  char *ptr = str;

  printf("%s", ptr);
  printf("%s", ptr + 8);
  printf("%s", ptr + 'l' - 'b');
  printf("%s", ptr + 'k' - ptr[3]);
}

So can anyone guide me answer this question?

what i applied for 1st printf was that the actual string will be printed StanfordIsGreat

for 2nd printf, i think the pointer will shift 8 indexes to the right generating IsGreat

for 3rd printf, ptr + 'l' gave me StanfordIsGreatl, i didn't understand what - 'b' meant

for 4rth printf ptr + 'k' was same as above and ptr[3] stands for 'a' so i wrote StnfordisGretk

i am really confused about 3rd and 4rth, can someone guide me, so i can learn and not make mistakes like this for future quizes.

Martin Gardener · Accepted Answer · 2016-02-23 16:50:43Z

3

char str[] = "StanfordIsGreat";

Character array(string) which contains content StanfordIsGreat.

char *ptr = str;

Character pointer that points to the 1st letter of the string (S).

printf("%s", ptr);

Direct printing the string pointed from the pointer index till the end of the string hence
Output: StanfordIsGreat

printf("%s", ptr + 8);

Pointer is shifted 8 index, pointing to 'I' in StanfordIsGreat therefore printing from 'I' to end of string
Output: IsGreat

printf("%s", ptr + 'l' - 'b');

Easy if you know the ASCII values for 'l' and 'b', here you must subtract the ASCII values of 'l' and 'b' giving you 10, giving you equation ptr+10 which equals pointer pointing to 'G'.
Output: Great

printf("%s", ptr + 'k' - ptr[3]);

Same as above but tricking you with ptr with index, ptr[3] points to 'n' therefore now you must subtract 'k' and 'n', giving you a -ve value, an unexpected behaviour will rise due to ptr-value.
Output: Not Clear Since we don't have a memory pool to overview

Edited: thanks mch for clearing an error :)

Hope you understand :)

edited Feb 23, 2016 at 16:50

answered Feb 23, 2016 at 16:21

Martin Gardener

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2 Comments

user3185968 Over a year ago

Given that C does not mandate ASCII, the third and fourth printf()s could easily index out of bounds.

mch Over a year ago

ptr[3] is 'n', not 'a', so the resulting pointer in the last printf will point before the array and so it will invoke undefined behaviour.

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