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Control Structures: Loops

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In this section, we will explore the concept of loops in Fortran. Loops are fundamental control structures that allow you to execute a block of code multiple times. Fortran provides several types of loops, each suited for different scenarios.

Types of Loops in Fortran

  1. DO Loop: The most common loop in Fortran, used for a fixed number of iterations.
  2. DO WHILE Loop: Executes as long as a specified condition is true.
  3. INFINITE Loop: A loop that runs indefinitely until explicitly terminated.

DO Loop

The DO loop is used when you know in advance how many times you want to execute a block of code. The syntax is as follows:

DO variable = start, end, step
    ! Code to be executed
END DO
  • variable: The loop control variable.
  • start: The initial value of the loop control variable.
  • end: The final value of the loop control variable.
  • step: The increment (or decrement) for each iteration (optional, default is 1).

Example

PROGRAM DoLoopExample
    INTEGER :: i

    DO i = 1, 10
        PRINT *, "Iteration: ", i
    END DO
END PROGRAM DoLoopExample

Explanation: This program prints the numbers from 1 to 10. The loop control variable i starts at 1 and increments by 1 until it reaches 10.

DO WHILE Loop

The DO WHILE loop continues to execute as long as a specified condition is true. The syntax is:

DO WHILE (condition)
    ! Code to be executed
END DO
  • condition: A logical expression that is evaluated before each iteration.

Example

PROGRAM DoWhileExample
    INTEGER :: i
    i = 1

    DO WHILE (i <= 10)
        PRINT *, "Iteration: ", i
        i = i + 1
    END DO
END PROGRAM DoWhileExample

Explanation: This program also prints the numbers from 1 to 10. The loop continues as long as i is less than or equal to 10.

INFINITE Loop

An infinite loop runs indefinitely until it is explicitly terminated using an EXIT statement. The syntax is:

DO
    ! Code to be executed
    IF (condition) EXIT
END DO
  • condition: A logical expression that, when true, causes the loop to terminate.

Example

PROGRAM InfiniteLoopExample
    INTEGER :: i
    i = 1

    DO
        PRINT *, "Iteration: ", i
        i = i + 1
        IF (i > 10) EXIT
    END DO
END PROGRAM InfiniteLoopExample

Explanation: This program prints the numbers from 1 to 10. The loop runs indefinitely but exits when i becomes greater than 10.

Practical Exercises

Exercise 1: Sum of First N Natural Numbers

Write a Fortran program to calculate the sum of the first N natural numbers using a DO loop.

Solution

PROGRAM SumOfNaturalNumbers
    INTEGER :: i, N, sum
    sum = 0

    PRINT *, "Enter the value of N: "
    READ *, N

    DO i = 1, N
        sum = sum + i
    END DO

    PRINT *, "The sum of the first ", N, " natural numbers is: ", sum
END PROGRAM SumOfNaturalNumbers

Exercise 2: Factorial of a Number

Write a Fortran program to calculate the factorial of a given number using a DO WHILE loop.

Solution

PROGRAM Factorial
    INTEGER :: i, N, factorial
    factorial = 1

    PRINT *, "Enter a number: "
    READ *, N

    i = 1
    DO WHILE (i <= N)
        factorial = factorial * i
        i = i + 1
    END DO

    PRINT *, "The factorial of ", N, " is: ", factorial
END PROGRAM Factorial

Exercise 3: Infinite Loop with User Input

Write a Fortran program that continuously asks the user to enter a number and prints it. The loop should terminate when the user enters a negative number.

Solution

PROGRAM InfiniteLoopWithExit
    INTEGER :: number

    DO
        PRINT *, "Enter a number (negative to exit): "
        READ *, number
        IF (number < 0) EXIT
        PRINT *, "You entered: ", number
    END DO

    PRINT *, "Program terminated."
END PROGRAM InfiniteLoopWithExit

Common Mistakes and Tips

  • Off-by-One Errors: Ensure that your loop boundaries are correctly set to avoid iterating one time too many or too few.
  • Infinite Loops: Be cautious with conditions in DO WHILE loops to avoid creating unintended infinite loops.
  • Variable Initialization: Always initialize your loop control variables to avoid unexpected behavior.

Conclusion

In this section, we covered the different types of loops in Fortran, including DO, DO WHILE, and infinite loops. We also provided practical examples and exercises to help you understand how to use loops effectively. Mastering loops is essential for writing efficient and effective Fortran programs. In the next section, we will delve into arrays and strings, which are crucial for handling collections of data.

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