GATE CS 2015 Set 2 Q.25 (CS Q.15): Pointers, Arrays and Strings

Consider the following function written in the C programming language.

void foo(char *a){
    if ( *a && *a != ' ' ){
        foo(a+1);
        putchar(*a);
    }
}

The output of the above function on input "ABCD EFGH" is

ABCD EFGH

ABCD

HGFE DCBA

DCBA

Official answer and solution

Official answer: D
From: Official GATE 2015 answer key

Solution

Trace the recursive execution of foo with the input string pointer "ABCD EFGH".

The function foo(char *a) checks:
if (*a && *a != ' '):
If true, it recursively calls foo(a+1), and after returning from recursion, calls putchar(*a).

Tracing execution on "ABCD EFGH":

foo("ABCD EFGH"): *a = 'A', calls foo("BCD EFGH")
foo("BCD EFGH"): *a = 'B', calls foo("CD EFGH")
foo("CD EFGH"): *a = 'C', calls foo("D EFGH")
foo("D EFGH"): *a = 'D', calls foo(" EFGH")
foo(" EFGH"): *a = ' ' (space), condition *a != ' ' is false; returns without recursive call or output.

Unwinding the call stack:

Level 4 prints *a = 'D'
Level 3 prints *a = 'C'
Level 2 prints *a = 'B'
Level 1 prints *a = 'A'

The characters printed are DCBA.

Final answer

Common trap

Assuming foo continues across the space to process the entire string, which would incorrectly yield HGFE DCBA.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2015 Computer Science and Information Technology (CS) Session 2
Question: 25 of 65, printed as CS Q.15
Section: Computer Science and Information Technology

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