C questions in placement tests and university exams are rarely about syntax alone. Most of what you will find here is output prediction, pointer arithmetic, arrays and strings, structures and unions, storage classes, recursion, and file handling. The explanations walk through memory behaviour step by step, which is usually where the confusion sits rather than in the code itself.
Which of the following is NOT a preprocessor directive in C?
Answer: D
#declare is not a valid preprocessor directive. Valid directives include #define, #include, #pragma, #ifdef, #endif, #if, #else, etc.
Q.2Easy
What is the output of the following code? #define MAX 5 int arr[MAX];
Answer: B
The preprocessor replaces MAX with 5, creating an array of size 5. This is a valid use of macro expansion.
Q.3Easy
Which preprocessor directive is used to include standard library files?
Answer: C
#include <filename> is used for standard library files (searched in standard paths). #include "filename" is used for user-defined header files (searched in current directory first).
Q.4Easy
What does the following preprocessor directive do? #define SQUARE(x) ((x)*(x))
Answer: C
This is a function-like macro that takes parameter x and expands to ((x)*(x)). The parentheses around x prevent operator precedence issues.
Q.5Easy
What is the output? #define MAX(a,b) (a>b?a:b) printf("%d", MAX(10, 5));
Answer: A
The macro expands to (10>5?10:5) which evaluates to 10 since the condition is true.
Q.6Easy
What is the purpose of the #undef directive?
Answer: A
#undef is used to remove the definition of a macro so it cannot be used in subsequent code.
Q.7Easy
What is the primary role of the C preprocessor in the compilation process?
Answer: A
The preprocessor is a separate program that processes source code before the compiler. It handles directives like #include, #define, and #ifdef.
Q.8Easy
Which of the following is NOT a preprocessor directive?
Answer: D
#allocate is not a valid preprocessor directive. Valid directives include #include, #define, #pragma, #ifdef, #ifndef, #if, #else, #elif, #endif, #error, #undef.
Q.9Easy
Which preprocessor directive is used to conditionally compile code based on whether a macro is defined?
Answer: D
#ifdef checks if a macro is defined, #ifndef checks if it's not defined. Both are used for conditional compilation based on macro definition status.
Q.10Easy
Which statement about preprocessor is TRUE?
Answer: B
Preprocessor directives (starting with #) are processed before the actual compilation by a separate preprocessor tool. They perform text substitution and conditional compilation.
Q.11Easy
Which preprocessor directive should be used to check if a macro is NOT defined?
Answer: D
Both #ifndef and #if !defined(MACRO) are equivalent and check if a macro is not defined. #ifndef is simpler syntax for this specific case.
Q.12Easy
Which header file is included using angle brackets in standard C library?
Answer: B
Standard library headers like stdio.h, stdlib.h, string.h are included using angle brackets #include <filename>.
Q.13Easy
Which preprocessor directive is used to define a constant that cannot be changed during program execution?
Answer: A
#define is used to create macro definitions and constants at compile time. #const, #constant, and #fixed are not valid preprocessor directives in C.
Q.14Easy
Which of the following is a difference between #include <file.h> and #include "file.h"?
Answer: D
The angle bracket version searches in standard system directories first, while quoted version searches in the current/local directory first. Both statements B and C correctly describe this distinction.
Q.15Easy
Consider the following code: #define PI 3.14 #undef PI #define PI 3.14159 What is the value of PI after execution?
Answer: B
#undef removes the previous definition of PI. The subsequent #define redefines PI as 3.14159. This is valid C syntax and the final value of PI is 3.14159.
Q.16Easy
Which header file must be included to use the NULL macro?
Answer: D
NULL is defined in multiple standard headers including <stdio.h>, <stdlib.h>, <stddef.h>, <string.h>, and others. Any of these can be included to use NULL.
Q.17Easy
Which of the following correctly demonstrates the use of conditional compilation?
Answer: A
Option A uses #ifdef to check if DEBUG is defined, which is correct for conditional compilation. Option B would check if DEBUG has a non-zero value (expression-based). Options C and D have invalid syntax.
Q.18Easy
What is the output of the following code? #define PI 3.14 int main() { printf("%f", PI); return 0; }
Answer: C
PI is replaced by 3.14 during preprocessing. When printed with %f format specifier, it displays as 3.140000 (default 6 decimal places).
Q.19Easy
What does the ## operator in preprocessor do?
Answer: B
The ## operator concatenates two tokens into one. For example, #define CONCAT(a,b) a##b creates CONCAT(hello, world) → helloworld.
Q.20Easy
Which predefined macro gives the line number in the source file?
Answer: B
__LINE__ expands to the current line number. __FILE__ gives filename, __DATE__ gives compilation date, __STDC__ indicates C standard compliance.
