GATE CSE 2008 | Question: 61

Question

Choose the correct option to fill $?1$ and $?2$ so that the program below prints an input string in reverse order. Assume that the input string is terminated by a new line character.

void reverse(void) 
{ 
    int c;
    if(?1) reverse();
    ?2 
} 
main() 
{ 
    printf("Enter text"); 
    printf("\n"); 
    reverse(); 
    printf("\n"); 
}

• A. $?1$ is $(getchar() != ’\setminus n’)$

  $?2$ is $getchar(c);$

• B. $?1$ is $((c = getchar() ) !=’\setminus n’)$

  $?2$ is $getchar(c);$

• C. $?1$ is $(c != ’\setminus n’)$

  $?2$ is $putchar(c);$

• D. $?1$ is $((c = getchar() ) != ’\setminus n’)$

  $?2$ is $putchar(c);$

Comments

"Since assignment is an operator in C just like +,the act of assignment produces a result,just as adding two numbers produces a result."

"The result of an assignment $v=e$ is the value of v after the assignment"

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Similar question: https://gateoverflow.in/3703/gate2004-it-60

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the input string is terminated by a ‘\n’ character.

Input : abc\n

output : \ncba

So,the above program will print newline first and then the reversed string.

Answer 1

$getchar()$ - reads a single character at a time from the $stdin$.

$putchar(c)$ - writes a character specified by the argument to $stdout$.

As getchar() and putchar() both are needed to read the string and print its reverse and only option D contains both the function. D is the answer. 

Now coming to the code.

$reverse(void)$ is calling itself recursively. when \n is encountered $putchar()$ gets executed and prints the last character and then the function returns to its previous call and prints last 2nd character and so on.