Question

Consider the following C program: \begin{verbatim} #include stdio.h void fX(); int main(){ fX(); return 0; } ____________________________ void fX(){ char a; if((a = getchar()) != '\n') fX(); if(a != '\n') putchar(a); } \end{verbatim} Assume that the input to the program from the command line is 1234 followed by a newline character. Which one of the following statements is CORRECT?

• A. The program will not terminate.
• B. The program will terminate with no output.
• C. The program will terminate with 4321 as output.
• D. The program will terminate with 1234 as output.

Hint:

In recursive functions with delayed output, the stack unwinding order determines the sequence of printed characters.

Answer 1

The Correct Option is C

Step 1: Understand the recursive function.
The function \texttt{fX()} reads a character using \texttt{getchar()}.
If the character is not a newline (\texttt{"/n"}), \texttt{fX()} is called recursively.
After returning from the recursive call, the character is printed using \texttt{putchar()} if it is not a newline. Step 2: Analyze the flow of recursion.
When \texttt{fX()} is first called, it reads the first character, '1', and calls itself recursively.
The second call reads '2', and again calls itself recursively.
This continues for '3' and '4'. When \texttt{getchar()} reads the newline (\texttt{'/n'}), the recursion stops and begins to unwind. Step 3: Output generation during unwinding.
As the recursion unwinds, each character ('4', '3', '2', '1') is printed in reverse order of their input due to the stack behavior of recursion. Final Answer: \[ \boxed{\text{The program will terminate with 4321 as output.}} \]