Unit 2: Operators and Expressions

Simple Computer Programs

A "simple computer program" in C typically follows a basic structure. It's the skeleton into which you place your logic.

The most basic C program ("Hello, World!") is:

// 1. Preprocessor Directive: Includes the Standard I/O library #include <stdio.h> // 2. The main function: Entry point of every C program int main() { // 3. Program Logic: A function call to print to screen printf("Hello, World!\n"); // 4. Return Statement: Informs the OS that the program finished successfully return 0; } 

All programs in this course will be built inside this main() { ... } block.

Relational Operators

Relational operators are used to **compare** two values. They are the foundation of decision-making in C. The result of a comparison is always a Boolean value: 1 (for true) or 0 (for false).

Logical Operators

Logical operators are used to combine or invert the results of relational expressions. They are used to build complex logical statements.

Logical Expression and Statements

A logical expression is any expression that evaluates to either true (1) or false (0). We create them by combining relational and logical operators.

A logical statement is a programming construct that uses a logical expression to make a decision. The most common is the if statement (covered in Unit 3).

Examples of Logical Expressions:

1. Check if age is between 18 and 65:

(age >= 18) && (age <= 65)

2. Check if grade is 'A', 'B', or 'C':

(grade == 'A') || (grade == 'B') || (grade == 'C')

3. Check if number is NOT zero:

number != 0 (This is equivalent to !(number == 0))

Short-Circuit Evaluation:
C is efficient. When it evaluates a logical expression, it stops as soon as it knows the answer.

• For expr1 && expr2: If expr1 is false, C doesn't even look at expr2, because the whole expression must be false.
• For expr1 || expr2: If expr1 is true, C doesn't even look at expr2, because the whole expression must be true.

Example Program (Unit 2)

This program demonstrates relational and logical operators by checking if a number is positive and even.

#include <stdio.h> int main() { int num; int is_positive, is_even; printf("Enter an integer: "); scanf("%d", &num); // Relational operators  is_positive = (num > 0); // Will be 1 (true) or 0 (false) is_even = (num % 2 == 0); // Will be 1 (true) or 0 (false) printf("Number is positive: %d\n", is_positive); printf("Number is even: %d\n", is_even); // Logical expression  if (is_positive && is_even) { printf("Result: The number is positive AND even.\n"); } else if (is_positive || is_even) { printf("Result: The number is positive OR even (or both).\n"); } else { printf("Result: The number is NOT positive and NOT even.\n"); } // Logical NOT  if (!(num == 0)) { printf("The number is not zero.\n"); } return 0; } 

Unit 2: Exam Quick Tips