GATE IT 2004 | Question: 60

Question

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

#include <stdio.h>
void wrt_it (void);
int main (void)
{
    printf("Enter Text"); 
    printf ("\n");
    wrt_it();
    printf ("\n");
    return 0;
}
void wrt_it (void)
{
    int c;
    if (?1)
        wrt_it();
    ?2
}

• A. $?1$ is  $getchar() ! =$ '\n'
  $?2$ is  $getchar(c);$
• B. $?1$ is  $(c = getchar()); ! =$ '\n'
  $?2$ is  $getchar(c);$
• C. $?1$ is  $c! =$ '\n'
  $?2$ is  $putchar(c);$
• D. $?1$ is  $(c = getchar()) ! =$ '\n'
  $?2$ is  $putchar(c);$

Comments

Whats the problem in that? The question asks about the entered string only – not the newline.

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Sir, it is printing newline also and no one is talking about it that’s why i asked but now i am realizing it is not much relevant. it just shifts the output in new line :)

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simple recursion. keep calling till the newline character (Enter) is not pressed and once newline character is entered as input then just empty the call stack one by one together printing the entered text in reverse order.

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.  :P

Now coming to the code.

$wrt\_it (void)$ 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. 

Comments

Yes, '\n' this will be printed, Which is new line So, before the actual output DCBA(if input is ABCD) there will be a newline above this.

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coz getchar() returns the ASCII value of characters taken from STDIN . Same is applicable for putchar() as well , it too takes the ASCII value of characters and prints the actual character into STDOUT

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the output would be \nELPPA  if input is APPLLE\n

so option D 

Answer 2

it should be option D

?1 is  (c = getchar()) ! = '\n'
?2 is  putchar(c);

Comments

Why the code showing error?

                        #include <stdio.h>
                        void wrt_it (void);
                        int main (void)
                        {
                        	char a[5]="ABCDE";
                            printf("%s",a); 
                            printf ("\n");
                            wrt_it();
                            printf ("\n");
                            return 0;
                        }
                        void wrt_it (void)
                        {
                            int c;
                            if ((c = getchar())!='\n')
                                wrt_it();
                            putchar(c);
                        }

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@shreya .. I found no error check here : http://ideone.com/1nHHtI 

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implementation

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@Kabir5454 @Arjun  sir

Why char and int both are working fine.

i.e. c is of type int and taking char as well as integer.

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@GateOverflow04 Because every character is appropriately converted to int and then back to char.

Answer 3

getchar() is used to get the input character from the user and putchar() to print the entered character, but before printing reverse is called again and again until ‘\n’ is entered. When ‘\n’ is entered the functions from the function stack run putchar() statements one by one. Therefore, last entered character is printed first.
You can try running below program