C++ Preparation SJVN

Questions

Here’s a set of practice questions for C/C++ that cover common competitive/programming exam topics:

Expression Evaluation & Operators

1. What is the value of int a = 5; int b = a++ + ++a;?
2. Evaluate: (3 && 0) || (2 || 0)
3. What is the output of printf("%d", 10 & 3 | 2);?

Variables & Scope
4. Difference between auto, register, static, and extern variables.
5. What will be printed if a local static variable is incremented in a function called multiple times?

Constants & Literals
6. Which of the following is a valid octal constant: 075, 089, 0x1F?
7. How do you define a long double constant?
8. Difference between const int x = 10; and #define X 10.

Arrays & Pointers
9. What is the difference between char *p and char arr[]?
10. What is printed by printf("%d", sizeof(arr)/sizeof(arr[0]));?
11. Can you increment a pointer to a multi-dimensional array?

Strings & I/O
12. Difference between gets(), fgets(), and scanf("%s").
13. What happens if you forget \0 at the end of a string?
14. Explain strlen, strcpy, strcmp with examples.

Functions & Recursion
15. Difference between call by value and call by reference.
16. Write a recursive function to calculate factorial.
17. What is a function pointer and how is it used?

Control Statements
18. Difference between while and do-while.
19. What is the output of nested loops with break and continue?
20. Explain switch-case fallthrough with an example.

Preprocessor & Macros
21. Difference between #include <file> and #include "file".
22. What is the output of using #define SQR(x) x*x in SQR(1+2)?

Miscellaneous
23. Difference between NULL, nullptr, and 0 in C++.
24. What is the result of sizeof(char*), sizeof(int*) on a 64-bit machine?
25. Explain short-circuit evaluation with && and ||.

Answers

Expression Evaluation & Operators

1. int a = 5; int b = a++ + ++a;
   • Answer: b = 12
   • Explanation: a++ uses 5, then a becomes 6; ++a increments to 7 then uses it → 5 + 6? Actually compute step by step:
     • a = 5
     • a++ = 5, a = 6
     • ++a = 7, a = 7
     • b = 5 + 7 = 12 ✅ Correction: b = 12
2. (3 && 0) || (2 || 0)
   • Answer: 1
   • Explanation:
     • 3 && 0 → 0 (false)
     • 2 || 0 → 1 (true)
     • 0 || 1 → 1
3. printf("%d", 10 & 3 | 2);
   • Answer: 2
   • Explanation:
     • 10 & 3 → 1010 & 0011 = 0010 = 2 ⭐
     • 2 | 2 → 2

Variables & Scope

4. auto, register, static, extern

   • Answer / Explanation:
     • auto → default local variable
     • register → hint to store in CPU register
     • static → retains value, scope limited (local static) or file-level (global static)
     • extern → global variable declared in another file

5. Local static variable incremented multiple times

   void fun() {
       static int x = 0;
       x++;
       printf("%d ", x);
   }

   • Answer: 1 2 3 ... on repeated calls
   • Explanation: static retains value across function calls
   • Even if you write static int x = 0; inside the function, the initialization happens only once, when the program first encounters the function. On subsequent calls, x keeps its previous value; it is not re-initialized.

Constants & Literals 6. Valid octal constant: 075 ✅, 089 ❌, 0x1F ❌ (hexadecimal)

7. Long double constant:

   long double x = 3.14159L;

8. Difference:

   • const int x = 10; → type-checked, read-only
   • #define X 10 → preprocessor text substitution, no type checking

Arrays & Pointers 9. char *p vs char arr[] - char *p → pointer, can point elsewhere - char arr[] → actual array in memory, cannot be reassigned

10. sizeof(arr)/sizeof(arr[0]) → number of elements in array
11. Pointer increment in multi-dimensional array: yes, pointer arithmetic depends on row/column layout

Strings & I/O

12. gets() → reads until newline, unsafe
    fgets() → reads max n chars, includes newline, safe
    scanf("%s") → reads until whitespace
13. Forget \0 → undefined behavior when using string functions
14. strlen → length of string
    strcpy → copy string
    strcmp → compare strings

Functions & Recursion

15. Call by value → copies value, original unchanged
    Call by reference → modifies original variable
16. Factorial (recursive):

int fact(int n){ return (n<=1)?1:n*fact(n-1); }

17. Function pointer: stores address of function ⭐

int (*fp)(int,int) = &add; fp(2,3);

Control Statements 18. while → check condition before execution
do-while → executes at least once

19. Nested loops: break exits loop, continue skips current iteration
20. Switch-case fallthrough: no break → executes next case

Preprocessor & Macros

21. #include <file> → system headers
    #include "file" → user-defined files
22. #define SQR(x) x*x; SQR(1+2) → expands to 1+2*1+2 = 5 ❌ (wrong math if not parenthesized) ⭐⭐

Miscellaneous

23. NULL → 0 in C, nullptr → C++11 null pointer, 0 → integer literal usable as null
24. sizeof(char*) = 8, sizeof(int*) = 8 (on 64-bit machine)
25. Short-circuit: && stops if first is false, || stops if first is true