IF-ELSE Statements in Fortran

Decision-making is a fundamental aspect of programming. Often, a program must perform certain actions depending on the value of a variable or the result of a condition. In Fortran, the IF-ELSE statement provides a structured way to execute alternative code blocks based on logical conditions.

This post explores IF-ELSE statements in depth, including syntax, examples, nested conditions, logical expressions, practical applications, and best practices.

1. Introduction to IF-ELSE Statements

The IF-ELSE statement allows a program to evaluate a condition and execute different blocks of code depending on whether the condition is true or false. It is one of the most common control flow structures in programming and is essential for making decisions in scientific computations, simulations, and data processing.

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2. Basic Syntax of IF-ELSE

The general syntax of an IF-ELSE statement in Fortran is:

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

• condition: A logical expression that evaluates to .true. or .false.
• then: Marks the beginning of the block executed when the condition is true
• else: (Optional) Block executed if the condition is false
• end if: Terminates the IF statement

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3. Example: Basic IF-ELSE Statement

program if_else_example
  integer :: num
  num = -3
  if (num > 0) then
  print *, "Positive"
  else
  print *, "Non-positive"
  end if
end program if_else_example

Output:

Non-positive

Explanation: The variable num is -3. The condition num > 0 is false, so the code in the else block executes.

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4. IF-ELSE with Logical Expressions

Conditions in IF statements can include relational operators (>, <, >=, <=, ==, /=) and logical operators (.and., .or., .not.).

4.1 Example: Checking Multiple Conditions

program if_else_logical
  integer :: x
  x = 7
  if (x > 0 .and. x < 10) then
  print *, "x is between 1 and 9"
  else
  print *, "x is outside the range"
  end if
end program if_else_logical

Output:

x is between 1 and 9

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5. Nested IF-ELSE Statements

Sometimes, multiple conditions must be checked sequentially. Nested IF-ELSE statements allow this.

5.1 Example: Nested IF-ELSE

program nested_if_else
  integer :: num
  num = 0
  if (num > 0) then
  print *, "Positive"
  else
  if (num == 0) then
      print *, "Zero"
  else
      print *, "Negative"
  end if
  end if
end program nested_if_else

Output:

Zero

Explanation: The outer IF checks if the number is positive. Since num is 0, it evaluates the inner IF.

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6. Using IF-ELSE for Multiple Conditions

For multiple alternatives, you can chain conditions using ELSE IF.

6.1 Example: ELSE IF Ladder

program elseif_example
  integer :: score
  score = 85

  if (score >= 90) then
  print *, "Grade: A"
  else if (score >= 75) then
  print *, "Grade: B"
  else if (score >= 50) then
  print *, "Grade: C"
  else
  print *, "Grade: F"
  end if
end program elseif_example

Output:

Grade: B

Explanation: The program evaluates each condition sequentially. score >= 75 is true, so “Grade: B” is printed.

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7. IF-ELSE with Real and Double Precision Variables

IF-ELSE statements can be applied to real and double precision variables as well.

7.1 Example: Checking Temperature

program temperature_check
  real :: temp
  temp = 36.5

  if (temp > 37.0) then
  print *, "Fever"
  else
  print *, "Normal"
  end if
end program temperature_check

Output:

Normal

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8. IF-ELSE with Logical Variables

Logical variables can directly control IF statements.

8.1 Example: Using Logical Variables

program logical_if
  logical :: is_raining
  is_raining = .true.

  if (is_raining) then
  print *, "Take an umbrella"
  else
  print *, "No need for an umbrella"
  end if
end program logical_if

Output:

Take an umbrella

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9. Practical Applications of IF-ELSE Statements

9.1 Age Checker

program age_checker
  integer :: age
  print *, "Enter your age:"
  read *, age

  if (age >= 18) then
  print *, "You are an adult."
  else
  print *, "You are a minor."
  end if
end program age_checker

9.2 Speed Limit Checker

program speed_check
  real :: speed
  print *, "Enter speed (km/h):"
  read *, speed

  if (speed > 100.0) then
  print *, "Speeding!"
  else
  print *, "Speed is within limit."
  end if
end program speed_check

9.3 Even or Odd Number

program even_odd
  integer :: n
  print *, "Enter a number:"
  read *, n

  if (mod(n,2) == 0) then
  print *, "Even number"
  else
  print *, "Odd number"
  end if
end program even_odd

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10. Best Practices for IF-ELSE Statements

1. Use clear and descriptive conditions: Conditions like x > 0 are self-explanatory.
2. Use ELSE IF for multiple alternatives instead of deeply nested IF statements.
3. Avoid redundant conditions: Check only necessary cases.
4. Indent code blocks properly: Improves readability.
5. Combine IF-ELSE with logical operators for complex decision-making.
6. Use comments to explain complex decision logic.

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11. IF-ELSE with Arrays

You can use IF-ELSE statements to check elements in arrays.

11.1 Example: Array Check

program array_check
  integer :: numbers(5) = (/1, -2, 3, 0, -5/)
  integer :: i

  do i = 1, 5
  if (numbers(i) > 0) then
      print *, "Element", i, "is positive"
  else if (numbers(i) == 0) then
      print *, "Element", i, "is zero"
  else
      print *, "Element", i, "is negative"
  end if
  end do
end program array_check

Output:

Element 1 is positive
Element 2 is negative
Element 3 is positive
Element 4 is zero
Element 5 is negative

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12. Combining IF-ELSE with User Input

IF-ELSE statements are commonly used to validate and respond to user input.

12.1 Example: Menu Selection

program menu_selection
  integer :: choice
  print *, "Select an option: 1) Start 2) Stop 3) Pause"
  read *, choice

  if (choice == 1) then
  print *, "Starting..."
  else if (choice == 2) then
  print *, "Stopping..."
  else if (choice == 3) then
  print *, "Pausing..."
  else
  print *, "Invalid selection"
  end if
end program menu_selection

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13. IF-ELSE in Scientific Calculations

13.1 Example: Quadratic Equation Solver

program quadratic_solver
  real :: a, b, c, discriminant, root1, root2
  a = 1.0
  b = -3.0
  c = 2.0

  discriminant = b**2 - 4*a*c

  if (discriminant > 0) then
  root1 = (-b + sqrt(discriminant)) / (2*a)
  root2 = (-b - sqrt(discriminant)) / (2*a)
  print *, "Two real roots:", root1, root2
  else if (discriminant == 0) then
  root1 = -b / (2*a)
  print *, "One real root:", root1
  else
  print *, "No real roots"
  end if
end program quadratic_solver

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14. Summary

The IF-ELSE statement is a critical control structure in Fortran for making decisions. It allows a program to:

• Execute different code blocks depending on conditions
• Handle multiple alternatives using ELSE IF
• Work with integers, reals, logical values, and arrays
• Validate input and implement decision-making in scientific and engineering applications