GATE IT 2008 | Question: 49

Question

What is the output printed by the following C code?

# include <stdio.h>
int main ()
{
    char a [6] = "world";
    int i, j;
    for (i = 0, j = 5; i < j; a [i++] = a [j--]);
    printf ("%s\n", a);
}

• A. dlrow
• B. Null string
• C. dlrld
• D. worow

Comments

for (i = 0, j = 5; i < j; a [i++] = a [j--]);

I want to ask since "," comma  has associativity from left -> right therefore is this line

equivalent to 

for (j = 5; i < j; a [i++] = a [j--]);

so we are taking i=0 because default value of i is zero 

or we are actually executing the line  i = 0, j = 5 first time when we are starting the iteration of for loop

and from next iteration the for loop i actually:

for (j = 5; i < j; a [i++] = a [j--]);

---

 

c[ ]=

W

O

R

L

D

\0

1. • Initialization: i = 0, j = 5
     • i starts at 0, and j starts at 5.
   • Condition: i < j
     • The loop continues as long as i < j.
   • Iteration: a[i++] = a[j--]
     • After each iteration, i is incremented, and j is decremented.

   Let’s look at how the loop modifies the array:

    
   • Step 1:
     • i = 0, j = 5
     • a[i++] = a[j--] → a[0] = a[5] → a[0] = '\0' (null character).
     • Now, i = 1, j = 4.
   • Step 2:
     • i = 1, j = 4
     • a[i++] = a[j--] → a[1] = a[4] → a[1] = 'd'.
     • Now, i = 2, j = 3.
   • Step 3:
     • i = 2, j = 3
     • a[i++] = a[j--] → a[2] = a[3] → a[2] = 'l'.
     • Now, i = 3, j = 2.
   • The condition i < j is now false, so the loop terminates.

2. Modified Array: After the loop, the array a looks like this:

   a[0] = '\0' a[1] = 'd' a[2] = 'l' a[3] = 'l' a[4] = 'd' a[5] = '\0'

3. Printing the Array:

   printf("%s\n", a);

   • The %s format specifier prints the string starting from a[0] until a null character (\0) is encountered.
   • Since a[0] is \0, the output is an empty string.

Final Output:

(empty string)

---

Loop is terminated in the first semicolon which is add after loop which means not print null string.

Answer 1

Char $a[6] = \begin{array}{|l|l|l|l|l|l|} \hline \text{w} &  \text{o} &  \text{r} &  \text{l} &  \text{d} &  \text{\0}\\\hline \end{array}$ 

After the loop executes for the first time, 

$a[0] = a[5]$

$a[0] =$`\0`

Next two more iterations of the loop till $i < j$ condition becomes false, are not important for the output as the first position is '\0';

printf(‘’%s’’, $a$);

printf function for format specifier '%s' prints the characters from the corresponding parameter (which should be an address) until "\0" occurs. Here, first character at $a$ is "\0" and hence it will print NOTHING. 

So, option (B).                  

Comments

@admin you pasted the wrong code in Ideone that is why it was giving an error.

the printf is outside the for loop and you placed it inside 

try this https://ideone.com/Si6VP9 its working fine
 

---

@admin

It runs fine.

https://ideone.com/uuogzo

---

me

Answer 2

$char$ $a[6] = $ " $\text{WORLD}$ "  which be stored like this 

\[
\begin{array}{|c|c|c|c|c|c|}
\hline
W & O & R & L & D & \text{\0} \\
\hline
\end{array}
\]
Key of this question : there is semicolon $(;)$ after the for loop  , so printf statement will only execute when control will come out of the loop or loop condition will be false.

Iteration 1 : $ i = 0 , j=5 ; 0<5$ (condition is true ) ;

                 $ a[0++] = j[5--] $

\[
\begin{array}{|c|c|c|c|c|c|}
\hline
\0 & o & r & l & d & \texttt{\0} \\
\hline
\end{array}
\]
 ( here , R.H.S will execute first and $j[5--](=null)$ will be assigned to $a[0++]$ .and then $j=5$ will be decremented once, bcz post decrement here , same with $a[0++] $. $i$ will be incremented by one after this statement . )

iteration 2: $i=1,j=4 ; 1<4 $( condition is true )

                  $ a[1++] = j[4--]$

\[
\begin{array}{|c|c|c|c|c|c|}
\hline
\texttt{\o} & d & r & l & d & \texttt{\ o} \\
\hline
\end{array}
\]
iteration 3 :  $i=2,j=3; 2<3$ (condition is true)

                     $ a[2++]=j[3--]$

\[
\begin{array}{|c|c|c|c|c|c|}
\hline
\texttt{\ o} & d & l & l & d & \texttt{\ o} \\
\hline
\end{array}
\]
iteration 4 : $i=3,j=2; 3<2$ (condition false ) 

 so control will go to the printf statement , and the command inside printf statement is saying print the string present on this address of $a$  (base address of array) upto $null$.

but , here first string is $null$ at $a[0] $. so control will not move further and the output will be $null $.

So, Correct Answer : $B)$ $\text{Null string}$

Comments

thank you for good explaination bharti

---

@bharti  thanks for this line

Key of this question : there is semicolon (;) after the for loop  , so printf statement will only execute when control will come out of the loop or loop condition will be false.

---

for (i = 0, j = 5; i < j; a [i++] = a [j--]);

I want to ask since "," comma  has associativity from left -> right therefore is this line

equivalent to 

for (j = 5; i < j; a [i++] = a [j--]);

so we are taking i=0 because default value of i is zero 

or we are actually executing the line  i = 0, j = 5 first time when we are starting the iteration of for loop

and from next iteration the for loop i actually:

for (j = 5; i < j; a [i++] = a [j--]);

Answer 3

$a[6]=w|o|r|l|d|\setminus0$

$int\ i,j;$

$for(i=0,j=5;i<j;a[i++]=a[j--])$ $;$

$0<5 //condition\ True$

Because of the semi-colon it will not execute the statement instead it will do increment/decrement.

$a[0]=a[5]$

$i=1,j=4$

$1<4 //condition\ True$

$a[1]=a[4]$

$i=2,j=3$

$2<3 //condition\ True$

$a[2]=a[3]$

$i=3,j=2$

$3<2 //condition\ False$

$printf("\%s\setminus n",a)$

Correct Answer (B): Null string

Answer 4

We can consider like this

for(i=0, j=5 ; i<j ; i++, j--)

{

a[i] = a[j];

}

Therefore a[0] = null char which means end of string. Hence Ans will be Null String.

Thank you for reading.