Control Structures IF Statements

Control structures are fundamental building blocks in programming. They allow the programmer to dictate the flow of a program based on certain conditions. One of the most widely used control structures is the IF statement. IF statements are used to execute a block of code only when a specified condition is true.

1. Introduction to IF Statements

An IF statement is a conditional statement that evaluates a logical expression and executes the associated code block if the condition evaluates to true. This enables a program to make decisions and respond dynamically to different inputs or situations.

The basic syntax of an IF statement in many programming languages looks like this:

if (condition) then
! code to execute if condition is true
end if

In this structure, the condition is a logical expression that can be either true or false. If the condition is true, the code block inside the IF statement executes; otherwise, it is skipped.

2. Simple IF Statement

A simple IF statement checks a single condition and executes code only when the condition is true. This is the simplest form of decision-making in programming.

Example:

integer :: num
num = 7
if (num > 0) then
print *, "Positive number"
end if

Explanation:

• The variable num is assigned the value 7.
• The IF statement checks if num is greater than 0.
• Since the condition is true, the program prints “Positive number”.
• If num were less than or equal to 0, nothing would be printed.

The simple IF statement is useful for straightforward decision-making scenarios, such as validating input, checking flags, or ensuring specific conditions are met before executing code.

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3. IF…ELSE Statement

The IF…ELSE structure allows programmers to provide alternative actions when the IF condition is false. This enables more flexibility in controlling program flow.

Syntax:

if (condition) then
! code if condition is true
else
! code if condition is false
end if

Example:

integer :: num
num = -5
if (num > 0) then
print *, "Positive number"
else
print *, "Non-positive number"
end if

Explanation:

• num is -5, so the IF condition (num > 0) is false.
• The program executes the code inside the ELSE block.
• Output: “Non-positive number”

The IF…ELSE structure is essential for handling binary choices, such as success/failure, yes/no, or on/off situations.

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4. IF…ELSEIF…ELSE Statement

When multiple conditions need to be checked sequentially, the IF…ELSEIF…ELSE structure becomes useful. It allows multiple logical expressions to be evaluated in order until one is true.

Syntax:

if (condition1) then
! code if condition1 is true
elseif (condition2) then
! code if condition2 is true
else
! code if none of the conditions are true
end if

Example:

integer :: num
num = 0
if (num > 0) then
print *, "Positive number"
elseif (num < 0) then
print *, "Negative number"
else
print *, "Zero"
end if

Explanation:

• The program first checks if num is greater than 0. If false, it checks if num is less than 0.
• If both conditions are false, the ELSE block executes.
• For num = 0, the output will be “Zero”.

The IF…ELSEIF…ELSE structure is crucial for multi-way decision-making scenarios, such as categorizing numbers, grades, or types of input.

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5. Nested IF Statements

Sometimes, decisions are dependent on multiple levels of conditions. In such cases, nested IF statements are used, where one IF statement exists inside another.

Example:

integer :: num
num = 15
if (num > 0) then
if (mod(num,2) == 0) then
    print *, "Positive even number"
else
    print *, "Positive odd number"
end if
else
print *, "Non-positive number"
end if

Explanation:

• The outer IF checks if the number is positive.
• The inner IF further checks if the number is even or odd.
• For num = 15, output will be “Positive odd number”.

Nested IF statements provide a method to create complex decision-making paths and are commonly used in applications such as input validation, game logic, and financial calculations.

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6. Logical Operators in IF Statements

IF statements often involve logical operators to combine multiple conditions. These operators include:

• AND (.and. in Fortran) – True if both conditions are true
• OR (.or. in Fortran) – True if at least one condition is true
• NOT (.not. in Fortran) – True if the condition is false

Example:

integer :: num
num = 10
if (num > 0 .and. num < 20) then
print *, "Number is between 1 and 19"
end if

Explanation:

• The IF statement uses the AND operator to check two conditions simultaneously.
• The output will be “Number is between 1 and 19” because both conditions are true.

Logical operators make IF statements more powerful by allowing combinations of multiple conditions.

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7. Common Mistakes with IF Statements

When working with IF statements, programmers often make the following errors:

1. Incorrect condition syntax:
   Example: if num > 0 then (missing parentheses in Fortran)
2. Missing END IF:
   Every IF statement in Fortran must be closed with end if.
3. Overlapping conditions:
   Multiple conditions that are not mutually exclusive can cause unexpected behavior.
4. Improper use of logical operators:
   Using AND instead of OR, or vice versa, may lead to logical errors.

Correct understanding and careful coding can avoid these issues.

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8. IF Statements in Real-World Applications

IF statements are used in countless real-world applications:

1. Input Validation: Ensuring user input meets certain criteria.

integer :: age
read *, age
if (age >= 18) then
print *, "You are eligible to vote"
else
print *, "You are not eligible to vote"
end if

2. Grading System: Determining student grades based on scores.

integer :: score
score = 85
if (score >= 90) then
print *, "Grade A"
elseif (score >= 80) then
print *, "Grade B"
elseif (score >= 70) then
print *, "Grade C"
else
print *, "Grade F"
end if

3. Financial Applications: Applying discounts or penalties based on conditions.

real :: bill
bill = 120.0
if (bill > 100) then
print *, "Discount applied"
else
print *, "No discount"
end if

4. Game Development: Checking player status or health.

integer :: health
health = 0
if (health <= 0) then
print *, "Game Over"
else
print *, "Keep playing"
end if

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9. Tips for Writing Efficient IF Statements

1. Order Conditions by Probability: Place the most likely condition first to improve performance.
2. Avoid Deep Nesting: Too many nested IFs reduce code readability. Consider using ELSEIF or separate functions.
3. Use Logical Operators Wisely: Combine conditions when possible instead of writing separate IF statements.
4. Comment Complex Conditions: Helps maintain code and avoids logical mistakes.

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10. Summary

• IF statements allow conditional execution of code.
• Simple IF executes code only if the condition is true.
• IF…ELSE provides an alternative action when the condition is false.
• IF…ELSEIF…ELSE allows multiple conditions to be checked sequentially.
• Nested IF statements enable multi-level decision-making.
• Logical operators like AND, OR, and NOT enhance condition checks.
• Proper understanding of IF statements is essential for error-free, efficient programming.

IF statements form the backbone of decision-making in programming and are foundational for more complex structures like loops and switch-case statements. Mastering them is critical for any aspiring programmer.