Here’s a list of 100 Important C language questions with their answers. I have categorized them into sections for better clarity.

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Basic Concepts

(100 Important C language questions with their answers)

1. What is C language?
   Answer: C is a general-purpose, procedural programming language developed by Dennis Ritchie in the 1970s at Bell Labs.
2. What are keywords in C?
   Answer: Keywords are reserved words in C, predefined by the language, such as int, return, if, while, etc.
3. What is the difference between int and float?
   Answer:
   • int is used for integer values.
   • float is used for decimal or floating-point values.
4. What is a variable?
   Answer: A variable is a storage location in memory with a name and a type to store data.
5. What is a constant in C?
   Answer: A constant is a value that cannot be changed during the execution of a program.

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Data Types and Operators

(100 Important C language questions with their answers)

6. List the basic data types in C.
   Answer: int, float, char, double.
7. What are derived data types in C?
   Answer: Arrays, pointers, structures, and unions.
8. What is the purpose of the sizeof() operator?
   Answer: It returns the size (in bytes) of a data type or variable.
9. What is typecasting?
   Answer: Typecasting is converting a variable from one data type to another.
10. What is the difference between = and ==?
    Answer:

• = is the assignment operator.
• == is the equality operator for comparison.

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Control Structures

(100 Important C language questions with their answers)

11. What is an if-else statement?
    Answer: It is a conditional statement that executes a block of code if the condition is true, and another block if it is false.
12. What is a switch statement?
    Answer: A multi-way decision-making statement to execute one block of code among many options.
13. What is the difference between while and do-while?
    Answer:

• while: Executes the block if the condition is true.
• do-while: Executes the block at least once, then checks the condition.

14. What is the syntax of a for loop?
    Answer:

for (initialization; condition; increment/decrement) {
    // Code block
}

15. What is a nested loop?
    Answer: A loop inside another loop.

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Functions

(100 Important C language questions with their answers)

16. What is a function in C?
    Answer: A block of code that performs a specific task and can be reused.
17. What is the difference between call by value and call by reference?
    Answer:

• Call by value: Copies the value of the argument.
• Call by reference: Passes the address of the argument.

18. What is the return type of main() in C?
    Answer: int.
19. What is a recursive function?
    Answer: A function that calls itself.
20. How do you declare a function in C?
    Answer:

return_type function_name(parameters);

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Pointers

(100 Important C language questions with their answers)

21. What is a pointer?
    Answer: A pointer is a variable that stores the address of another variable.
22. How do you declare a pointer?
    Answer: int *ptr;

23. What is the NULL pointer?
    Answer: A pointer that does not point to any memory location.
24. What is pointer arithmetic?
    Answer: Operations like addition or subtraction performed on pointers.
25. What is a void pointer?
    Answer: A pointer that can hold the address of any data type.

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Arrays and Strings

(100 Important C language questions with their answers)

26. What is an array in C?
    Answer: A collection of elements of the same data type stored in contiguous memory locations.
27. How do you declare an array?
    Answer:int arr[10];

28. What is a multidimensional array?
    Answer: An array with more than one dimension, such as a matrix.
29. How do you declare a string in C?
    Answer:char str[20];

30. What is the strlen() function?
    Answer: It returns the length of a string.

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Structures and Unions

(100 Important C language questions with their answers)

31. What is a structure in C?
    Answer: A user-defined data type to group different types of variables.
32. How do you declare a structure?
    Answer:

struct struct_name {
    int x;
    char y;
};

33. What is the difference between structure and union?
    Answer:

• Structure: Allocates memory for all members.
• Union: Allocates shared memory for all members.

34. What is the syntax for accessing structure members?
    Answer:

structure_variable.member_name;

35. How do you define a union?
    Answer:

union union_name {
    int x;
    char y;
};

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Advanced Topics

(100 Important C language questions with their answers)

36. What is dynamic memory allocation?
    Answer: Allocating memory at runtime using functions like malloc(), calloc(), realloc(), and free().
37. What is a file in C?
    Answer: A storage medium for saving data.
38. How do you open a file in C?
    Answer:

FILE *fp = fopen("filename.txt", "mode");

39. What is a preprocessor directive?
    Answer: Instructions processed before compilation, starting with #, like #include.
40. What is a macro?
    Answer: A preprocessor directive for defining constants or expressions using #define.

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Common Errors and Debugging

(100 Important C language questions with their answers)

41. What is a syntax error?
    Answer: An error in the code structure or grammar.
42. What is a segmentation fault?
    Answer: An error caused by accessing memory not allocated to the program.
43. What is an infinite loop?
    Answer: A loop that never terminates due to incorrect conditions.
44. What is the difference between a compiler and an interpreter?
    Answer:

• Compiler: Converts entire code into machine code at once.
• Interpreter: Executes code line by line.

45. What is debugging?
    Answer: The process of identifying and fixing errors in a program.

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    Dynamic Memory Allocation

    (100 Important C language questions with their answers)

    46. What is the malloc() function?
        Answer: malloc() allocates memory dynamically and returns a pointer to the allocated memory. Syntax:

    ptr = (type*)malloc(size);
    

    47. What is the difference between malloc() and calloc()?
        Answer:
    • malloc(): Allocates a single block of memory.
    • calloc(): Allocates multiple blocks of memory and initializes them to zero.
    48. What does free() do?
        Answer: free() deallocates previously allocated memory.
    49. What is realloc() used for?
        Answer: realloc() resizes the memory block allocated by malloc() or calloc().
    50. Why is dynamic memory allocation important?
        Answer: It allows efficient use of memory by allocating memory as needed during program execution.

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    Files in C

    (100 Important C language questions with their answers)

    51. What are the different modes to open a file?
        Answer:
    • "r": Read
    • "w": Write
    • "a": Append
    • "r+": Read and write
    52. How do you close a file?
        Answer:

    fclose(file_pointer);
    

    53. What is the purpose of fscanf() and fprintf()?
        Answer:
    • fscanf(): Reads data from a file.
    • fprintf(): Writes data to a file.
    54. What is the difference between fgetc() and fgets()?
        Answer:
    • fgetc(): Reads a single character from a file.
    • fgets(): Reads a string from a file.
    55. What is the use of feof()?
        Answer: Checks if the end of the file has been reached.

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    Storage Classes

    (100 Important C language questions with their answers)

    56. What are storage classes in C?
        Answer: Storage classes define the scope, visibility, and lifetime of variables.
    57. List the types of storage classes in C.
        Answer:
    • auto
    • register
    • static
    • extern
    58. What is the difference between auto and register?
        Answer:
    • auto: Default storage class for local variables.
    • register: Suggests storing the variable in a CPU register for faster access.
    59. What is the purpose of the static keyword?
        Answer: The static keyword retains a variable’s value between function calls.
    60. What is the extern keyword?
        Answer: Declares a global variable or function in another file.

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    Preprocessor Directives

    (100 Important C language questions with their answers)

    61. What is the use of #include?
        Answer: Includes the contents of a file (usually header files) in the program.
    62. What does #define do?
        Answer: Defines a macro or constant.
    63. What is the purpose of #undef?
        Answer: Undefines a macro.
    64. What is a conditional compilation?
        Answer: Compilation based on conditions using directives like #ifdef, #ifndef, etc.
    65. What is the difference between #ifdef and #ifndef?
        Answer:
    • #ifdef: Compiles the code if the macro is defined.
    • #ifndef: Compiles the code if the macro is not defined.

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    Functions

    (100 Important C language questions with their answers)

    66. What is the default return type of a function in C?
        Answer: int.
    67. Can a function return multiple values in C?
        Answer: No, but you can use pointers or structures to achieve this.
    68. What is the scope of a local variable?
        Answer: Limited to the block or function where it is declared.
    69. What is a global variable?
        Answer: A variable declared outside all functions, accessible throughout the program.
    70. What is function overloading?
        Answer: C does not support function overloading; it’s a feature of C++ or other languages.

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    Bitwise Operators

    (100 Important C language questions with their answers)

    71. What is the purpose of the & operator?
        Answer: Bitwise AND.
    72. What is the purpose of the | operator?
        Answer: Bitwise OR.
    73. What is the purpose of the ^ operator?
        Answer: Bitwise XOR.
    74. What is the purpose of the ~ operator?
        Answer: Bitwise NOT.
    75. What is a bitwise shift operator?
        Answer:
    • <<: Left shift.
    • >>: Right shift.

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    Error Handling

    (100 Important C language questions with their answers)

    76. What is an error in C?
        Answer: A mistake in the program that prevents it from executing correctly.
    77. What are the types of errors in C?
        Answer:
    • Syntax errors
    • Runtime errors
    • Logical errors
    78. What is the purpose of perror()?
        Answer: Displays a descriptive error message.
    79. What is the errno variable?
        Answer: A global variable used to indicate error codes.
    80. How do you handle errors in C?
        Answer: Using error-checking mechanisms like if conditions and errno.

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    Miscellaneous (100 Important C language questions with their answers)

    81. What is a typedef in C?
        Answer: Used to create a new name for an existing type.
    82. What is the purpose of the const keyword?
        Answer: Declares variables that cannot be modified.
    83. What is the difference between break and continue?
        Answer:
    • break: Exits a loop or switch.
    • continue: Skips the current iteration of a loop.
    84. What is the purpose of goto?
        Answer: Transfers control to a labeled statement.
    85. What is a command-line argument?
        Answer: Arguments passed to the program at the time of execution.

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    Programming Questions

    (100 Important C language questions with their answers)

    86. Write a program to check if a number is even or odd.

    #include <stdio.h>
    int main() {
        int num;
        printf("Enter a number: ");
        scanf("%d", &num);
        if (num % 2 == 0)
            printf("Even\n");
        else
            printf("Odd\n");
        return 0;
    }
    

    87. Write a program to calculate the sum of digits of a number.

    #include <stdio.h>
    int main() {
        int num, sum = 0, digit;
        printf("Enter a number: ");
        scanf("%d", &num);
        while (num > 0) {
            digit = num % 10;
            sum += digit;
            num /= 10;
        }
        printf("Sum of digits: %d\n", sum);
        return 0;
    }
    

    88. Write a program to reverse a string.

    #include <stdio.h>
    #include <string.h>
    int main() {
        char str[100], temp;
        int i, len;
        printf("Enter a string: ");
        gets(str);
        len = strlen(str);
        for (i = 0; i < len / 2; i++) {
            temp = str[i];
            str[i] = str[len - i - 1];
            str[len - i - 1] = temp;
        }
        printf("Reversed string: %s\n", str);
        return 0;
    }
    

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    Key Concepts

    (100 Important C language questions with their answers)

    89. What is the difference between char and string?
        Answer:
    • char: Stores a single character.
    • string: Stores a sequence of characters (array of char).
    90. What is a dangling pointer?
        Answer: A pointer pointing to memory that has already been freed.
    91. What is the difference between exit() and return?
        Answer:
    • exit(): Terminates the program immediately.
    • return: Exits a function and returns a value.
    92. What is the stack in C?
        Answer: A memory segment for local variables and function calls.
    93. What is a heap in C?
        Answer: A memory segment for dynamic memory allocation.

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