GATE CSE 2015 Set 3 | Question: 7

Question

Consider the following C program segment.

# include <stdio.h>
int main()
{
    char s1[7] = "1234", *p;
    p = s1 + 2;
    *p = '0';
    printf("%s", s1);
}

What will be printed by the program?

• A. $12$
• B. $120400$
• C. $1204$
• D. $1034$

Comments

One more key learning here.

If it would have been char *s= "1234", then it will be stored as a string constant in the data segment(read-only). we cannot change the content of string using another pointer. It will give an error.

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1204 is answer.

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in simple terms initially p was pointing to 1 later it was pointing to 3 and then was asked to change 3 → 0 via this line *p = '0';  after this modification, when you print s1, it will display "1204".

Answer 1

p = s1 + 2;

Type of s1 is char[7] and sizeof *s1 is sizeof (char) = 1. So, s1 + 2 will return address in s1 + 2 *sizeof(char) = address in s1 + 2. So,  $p$ now points to the third element in s1.

*p = '0';

The third element in s1 is made $0$. So, $1234$ becomes $1204$. C choice. 

Comments

https://gateoverflow.in/16618/doubt-regarding-string-constant

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Great Explanation. Helped a lot in cracking the confusion.

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I still didn't get it when we declared the array of characters as s1[7], we wasted the memory because we used only 4 characters and a null at last.

so they did it purposefully in the question?

Answer 2

Answer is C.
For the confused people,   
*p = '0';    So answer is 1204
If *p = 0;   Here answer will be 12

0 means Ascii 0  which is Null character.

'0'  means Ascii 48 which is character '0'
 

Comments

If *p = 0, then will there be an error? As integer value is assigned to char variable.

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No, a char can hold upto 255.

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@Ahwan

NULL has nothing to do with strings, it is a macro defined in stddef.h

'\0' has ascii value of 0 which is used at the end of each string

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Is it fine now?

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Yes.

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@Ahwan  your answerer confused me

what is the difference between *p = '0'  &  *p = 0. I can't understand.

please reply

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*p = '\0' will also print  12

this assumption  is wrong because there is nothing like character '\0' in and may lead to error as we assigning 2 char in one location.

Am I right  @Arjun Sir?

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No. \0 is a valid character as \ is an escape character and it's ASCII code is 0.

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Anything which can be stored in 1Byte is called as char. Computer understands this only. Although we are able to see that 0 is integer. But computer checks whether the thing can be stored in 1 byte or not..

Answer 3

Here s1 is an array, So s1 points to base address.

so , p=s1+2 will point to 3rd element of s1.

and *p='0'

value at p (Which is the third element of s1) , so 1234 becomes 1204.

Answer 4

1204 will be correct answer.