Conditional Statements in C

Conditional statements are fundamental building blocks in programming that allow your programs to make decisions based on specific conditions. They enable your code to execute different blocks based on whether certain conditions are true or false.

Why Do We Need Conditional Statements?

In real-world applications, programs need to make decisions based on different conditions. For instance:

• A banking application needs to check if a user has sufficient funds before processing a withdrawal
• A game needs to determine if a player has won, lost, or should continue playing
• A form validation system needs to verify if user input meets specific criteria
• A temperature control system needs to turn heating on or off based on current temperature readings

Conditional statements provide the logical structure that makes these decisions possible.

The if Statement

The if statement is the most fundamental conditional statement in C. It evaluates a condition and executes a block of code only if that condition is true.

Basic Syntax

1if (condition) {
2    // Code to execute if condition is true
3}

Simple Example

1#include <stdio.h>
2
3int main() {
4    int age = 20;
5    
6    if (age >= 18) {
7        printf("You are eligible to vote.\n");
8    }
9    
10    return 0;
11}

In this example:

1. We declare an integer variable age and assign it the value 20.
2. The condition age >= 18 checks if the age is greater than or equal to 18.
3. Since 20 is indeed greater than 18, the condition evaluates to true.
4. Therefore, the code block inside the if statement executes, printing "You are eligible to vote."

The if-else Statement

The if-else statement extends the if statement by providing an alternative code block to execute when the condition is false.

Basic Syntax

1if (condition) {
2    // Code to execute if condition is true
3} else {
4    // Code to execute if condition is false
5}

Example

1#include <stdio.h>
2
3int main() {
4    int number = 7;
5    
6    if (number % 2 == 0) {
7        printf("%d is an even number.\n", number);
8    } else {
9        printf("%d is an odd number.\n", number);
10    }
11    
12    return 0;
13}

Let's break down this example:

1. We declare an integer variable number and assign it the value 7.
2. The expression number % 2 == 0 checks if the remainder when number is divided by 2 equals 0 (which would mean it's even).
3. Since 7 divided by 2 has a remainder of 1, the condition evaluates to false.
4. Therefore, the code block inside the else statement executes, printing "7 is an odd number."

The else if Statement

When you need to check multiple conditions, you can use the else if construct. It allows you to test several conditions in sequence.

Basic Syntax

1if (condition1) {
2    // Code to execute if condition1 is true
3} else if (condition2) {
4    // Code to execute if condition1 is false and condition2 is true
5} else if (condition3) {
6    // Code to execute if condition1 and condition2 are false and condition3 is true
7} else {
8    // Code to execute if all conditions are false
9}

Example: Grading System

1#include <stdio.h>
2
3int main() {
4    int score = 85;
5    
6    if (score >= 90) {
7        printf("Grade: A\n");
8    } else if (score >= 80) {
9        printf("Grade: B\n");
10    } else if (score >= 70) {
11        printf("Grade: C\n");
12    } else if (score >= 60) {
13        printf("Grade: D\n");
14    } else {
15        printf("Grade: F\n");
16    }
17    
18    return 0;
19}

In this example:

1. We declare a variable score with the value 85.
2. The first condition score >= 90 evaluates to false (85 is not ≥ 90).
3. The second condition score >= 80 evaluates to true (85 is ≥ 80).
4. Since the second condition is true, "Grade: B" is printed.
5. The rest of the conditions are not evaluated since the program already found a true condition.

Nested if Statements

For more complex decision-making, you can nest if statements inside other if or else statements. This creates a hierarchical decision tree.

Example: Ticket Pricing

1#include <stdio.h>
2
3int main() {
4    int age = 25;
5    char student = 'Y';  // 'Y' for Yes, 'N' for No
6    
7    if (age < 18) {
8        printf("Child ticket: $8.00\n");
9    } else {
10        // Nested if for adults (age >= 18)
11        if (student == 'Y') {
12            printf("Student ticket: $10.00\n");
13        } else {
14            printf("Adult ticket: $12.00\n");
15        }
16    }
17    
18    return 0;
19}

Let's analyze this example step by step:

1. We have two variables: age (25) and student ('Y').
2. The outer if statement checks if age is less than 18.
3. Since 25 is not less than 18, the condition is false, so we move to the else block.
4. Inside the else block, there's a nested if that checks if student is 'Y'.
5. Since student is 'Y', the condition is true, so "Student ticket: $10.00" is printed.

This ticket pricing example could be rewritten more clearly using logical operators, which we'll explore next.

Logical Operators in Conditions

C provides logical operators that allow you to combine or negate conditions, making your code more concise and readable.

Rewriting the Ticket Pricing Example

1#include <stdio.h>
2
3int main() {
4    int age = 25;
5    char student = 'Y';  // 'Y' for Yes, 'N' for No
6    
7    if (age < 18) {
8        printf("Child ticket: $8.00\n");
9    } else if (age >= 18 && student == 'Y') {
10        printf("Student ticket: $10.00\n");
11    } else {
12        printf("Adult ticket: $12.00\n");
13    }
14    
15    return 0;
16}

This version is more readable and accomplishes the same task without nesting. The logical AND (&&) operator ensures that both conditions must be true for the "Student ticket" message to be printed.

Short-Circuit Evaluation

C uses short-circuit evaluation for logical operators:

• For &&: If the first expression is false, the second is not evaluated (since the result will be false regardless).
• For ||: If the first expression is true, the second is not evaluated (since the result will be true regardless).

This behavior can be used to your advantage, as shown in the following example:

1#include <stdio.h>
2
3int main() {
4    int x = 5;
5    
6    // The second part is evaluated only if x > 0
7    if (x > 0 && printf("x is positive\n")) {
8        // The body still executes because printf returns a non-zero value
9        printf("This will also print\n");
10    }
11    
12    x = 0;
13    
14    // The second part is NOT evaluated because x <= 0
15    if (x > 0 && printf("This won't print\n")) {
16        printf("This won't print either\n");
17    }
18    
19    return 0;
20}

The Ternary Operator

The ternary operator (? :) is a concise way to express simple conditional expressions. It's essentially a one-line if-else statement.

Basic Syntax

1condition ? expression_if_true : expression_if_false

Example

1#include <stdio.h>
2
3int main() {
4    int age = 20;
5    
6    // Using if-else
7    if (age >= 18) {
8        printf("Using if-else: Adult\n");
9    } else {
10        printf("Using if-else: Minor\n");
11    }
12    
13    // Using ternary operator
14    printf("Using ternary: %s\n", age >= 18 ? "Adult" : "Minor");
15    
16    // Assigning value based on condition
17    int ticketPrice = (age < 12) ? 5 : (age < 65 ? 10 : 7);
18    printf("Ticket price: $%d\n", ticketPrice);
19    
20    return 0;
21}

In this example:

1. The first printf uses a traditional if-else structure.
2. The second printf uses the ternary operator to achieve the same result more concisely.
3. The third example shows how to assign different values to a variable based on a condition.
4. The fourth example shows a nested ternary expression (though these can quickly become hard to read).

Common Pitfalls and Best Practices

1// INCORRECT - Assignment instead of comparison
2if (x = 5) {
3    // This will always execute because x is assigned 5 (non-zero is true)
4}
5
6// CORRECT - Comparison
7if (x == 5) {
8    // Executes only if x equals 5
9}

1// Misleading indentation
2if (condition)
3    printf("First statement\n");
4    printf("This always executes regardless of condition\n"); // Not part of the if block!
5
6// Correct usage
7if (condition) {
8    printf("First statement\n");
9    printf("Second statement, also part of the if block\n");
10}

1// PROBLEMATIC - Direct equality comparison with floating-point
2float a = 0.1 + 0.2;
3if (a == 0.3) {
4    printf("Equal\n"); // This might not execute due to floating-point precision!
5}
6
7// BETTER - Use a small epsilon value
8float epsilon = 0.0001;
9if (fabs(a - 0.3) < epsilon) {
10    printf("Approximately equal\n");
11}

Real-World Examples

Let's look at some practical examples of conditional statements in real-world applications:

Form Input Validation

1#include <stdio.h>
2#include <string.h>
3
4int main() {
5    char email[100];
6    int containsAtSymbol = 0;
7    
8    printf("Enter your email address: ");
9    scanf("%99s", email);
10    
11    // Check if email contains @ symbol
12    int length = strlen(email);
13    for (int i = 0; i < length; i++) {
14        if (email[i] == '@') {
15            containsAtSymbol = 1;
16            break;
17        }
18    }
19    
20    // Validate email format
21    if (length < 5) {
22        printf("Error: Email is too short\n");
23    } else if (!containsAtSymbol) {
24        printf("Error: Email must contain an @ symbol\n");
25    } else {
26        printf("Email format is valid!\n");
27    }
28    
29    return 0;
30}

Simple Login System

1#include <stdio.h>
2#include <string.h>
3
4int main() {
5    // Predefined credentials (in a real system, these would be securely stored)
6    const char correctUsername[] = "admin";
7    const char correctPassword[] = "secure123";
8    
9    char username[50];
10    char password[50];
11    
12    printf("Username: ");
13    scanf("%49s", username);
14    
15    printf("Password: ");
16    scanf("%49s", password);
17    
18    if (strcmp(username, correctUsername) == 0) {
19        if (strcmp(password, correctPassword) == 0) {
20            printf("Login successful!\n");
21        } else {
22            printf("Incorrect password.\n");
23        }
24    } else {
25        printf("Username not found.\n");
26    }
27    
28    return 0;
29}

Practice Exercises

Summary

Conditional statements are essential tools in programming that allow your code to make decisions based on specific conditions. In C, you have several options for implementing conditional logic:

• Use if statements for simple conditions
• Add an else clause for alternative actions
• Use else if statements for multiple conditions
• Combine conditions with logical operators (&&, ||, !)
• Use the ternary operator ?: for simple conditional expressions

By mastering conditional statements, you'll be able to create more dynamic and responsive programs that can adapt to different inputs and situations. The next tutorial will cover switch statements, which provide an elegant way to handle multiple conditions for a single variable.