Constants and literals - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant in C programming?

A constant is a value that cannot be changed during the execution of a program. It is fixed and used to represent data that should remain the same.

beginner

What is a literal in C?

A literal is a fixed value written directly in the code, like numbers (e.g., 5), characters (e.g., 'a'), or strings (e.g., "hello").

beginner

How do you declare a constant integer named MAX in C?

Use the keyword <code>const</code>: <br><code>const int MAX = 100;</code> This means MAX cannot be changed later.

intermediate

What is the difference between a constant and a literal?

A literal is a fixed value written directly in the code. A constant is a named value that cannot change and is usually defined using const or #define.

intermediate

What does the #define directive do in C?

It defines a constant value or macro before compilation. For example, #define PI 3.14 replaces all occurrences of PI with 3.14 in the code.

Which keyword is used to declare a constant variable in C?

A let

B const

C static

D var

The const keyword declares a variable whose value cannot be changed.

What is the type of the literal 'A' in C?

A int

B float

C string

D char

Single quotes denote a character literal of type char.

What will happen if you try to change a const variable's value?

A The compiler will give an error

B The value will change at runtime

C The program will compile and run normally

D The program will crash

Trying to modify a const variable causes a compile-time error.

Which of these is a valid integer literal in C?

A 123

B "123"

C '123'

D 12.3

The number 123 without quotes is an integer literal.

What does this line do? #define SIZE 10

A Creates a function named SIZE

B Declares a variable SIZE with value 10

C Defines a constant SIZE with value 10

D Allocates memory of size 10

#define creates a constant or macro named SIZE with value 10.

Explain the difference between a constant and a literal in C.

Think about how you use names versus direct values in code.

How do you declare a constant integer and why would you use it?

Consider protecting important values from accidental changes.

Practice

1. What is a constant in C programming?

easy

A. A function that returns a value

B. A fixed value that cannot be changed during program execution

C. A variable that can change its value anytime

D. A type of loop in C

Solution

1. Step 1: Understand the meaning of constant

   A constant is a value that remains the same throughout the program and cannot be modified.

2. Step 2: Compare options with definition

   Only A fixed value that cannot be changed during program execution describes a fixed value that does not change, which matches the definition of a constant.

3. Final Answer:

   A fixed value that cannot be changed during program execution -> Option B

4. Quick Check:

   Constant = fixed value [OK]

Hint: Constants never change value once set [OK]

Common Mistakes:

• Confusing constants with variables
• Thinking constants are functions
• Mixing constants with loops

2. Which of the following is the correct way to define a constant integer named MAX with value 100 in C?

easy

A. All of the above

B. #define MAX 100

C. const int MAX = 100;

D. int const MAX = 100;

Solution

1. Step 1: Review ways to define constants in C

   Constants can be defined using #define or using const keyword with variable declaration.

2. Step 2: Check each option

   #define MAX 100 uses #define, which is valid. int const MAX = 100; uses int const, which is valid. const int MAX = 100; uses const int, also valid. So all are correct.

3. Final Answer:

   All of the above -> Option A

4. Quick Check:

   All methods define constants [OK]

Hint: Use #define or const keyword for constants [OK]

Common Mistakes:

• Using variable without const
• Missing semicolon in const declaration
• Confusing #define with variable

3. What will be the output of the following C code?

#include <stdio.h>
#define PI 3.14
int main() {
    const int radius = 5;
    float area = PI * radius * radius;
    printf("%.2f", area);
    return 0;
}

medium

A. Syntax error

B. 31.40

C. 78.50

D. 25.00

Solution

1. Step 1: Calculate area using given formula

   Area = PI * radius * radius = 3.14 * 5 * 5 = 3.14 * 25 = 78.5

2. Step 2: Check printf formatting

   Using %.2f prints the float with 2 decimal places, so output is 78.50

3. Final Answer:

   78.50 -> Option C

4. Quick Check:

   Area = 3.14 * 25 = 78.50 [OK]

Hint: Calculate formula carefully and check decimal format [OK]

Common Mistakes:

• Using wrong formula
• Ignoring decimal places
• Confusing radius with diameter

4. Identify the error in the following C code snippet:

const int x;
x = 10;
printf("%d", x);

medium

A. Cannot assign value to const variable after declaration

B. Missing semicolon after declaration

C. printf format specifier is wrong

D. No error, code runs fine

Solution

1. Step 1: Understand const variable rules

   A const variable must be initialized when declared and cannot be assigned a value later.

2. Step 2: Analyze the code

   Here, const int x; declares x without initialization, then tries to assign 10 later, which is not allowed.

3. Final Answer:

   Cannot assign value to const variable after declaration -> Option A

4. Quick Check:

   const variables must be initialized once [OK]

Hint: Initialize const variables when declared [OK]

Common Mistakes:

• Assigning value after declaration
• Not initializing const variables
• Ignoring const rules

5. You want to create a constant array of 3 integers in C that cannot be modified after initialization. Which of the following declarations achieves this correctly?

hard

A. int arr[3] = {1, 2, 3};

B. int const *arr = {1, 2, 3};

C. #define arr {1, 2, 3}

D. const int arr[3] = {1, 2, 3};

Solution

1. Step 1: Understand const array declaration

   Declaring an array with const int arr[3] means the elements cannot be changed after initialization.

2. Step 2: Check each option

   int arr[3] = {1, 2, 3}; declares a normal array, elements can be changed. const int arr[3] = {1, 2, 3}; declares a constant array correctly. #define arr {1, 2, 3} uses #define incorrectly for array. int const *arr = {1, 2, 3}; declares a pointer to const int, but initialization is invalid.

3. Final Answer:

   const int arr[3] = {1, 2, 3}; -> Option D

4. Quick Check:

   const array elements cannot change [OK]

Hint: Use const before type for constant arrays [OK]

Common Mistakes:

• Using #define for arrays
• Declaring non-const arrays
• Incorrect pointer initialization