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Loops and Iteration

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In this section, we will explore loops and iteration in ALGOL. Loops are fundamental control structures that allow you to execute a block of code multiple times. Understanding how to use loops effectively is crucial for writing efficient and concise programs.

Key Concepts

  1. Types of Loops in ALGOL:

    • For Loop: Executes a block of code a specific number of times.
    • While Loop: Repeats a block of code as long as a specified condition is true.
    • Repeat-Until Loop: Similar to the while loop but checks the condition after executing the block of code.

  2. Loop Control Statements:

    • Break: Exits the loop immediately.
    • Continue: Skips the current iteration and proceeds to the next iteration of the loop.

For Loop

The for loop in ALGOL is used to iterate over a range of values. Here is the basic syntax:

for i := start to end do
begin
    // Code to be executed
end;

Example

begin
    integer i;
    for i := 1 to 5 do
    begin
        print(i);
    end;
end;

Explanation:

  • The loop starts with i equal to 1 and increments i by 1 after each iteration.
  • The loop continues until i is greater than 5.
  • The print(i) statement outputs the value of i in each iteration.

Exercise 1

Write a for loop that prints the first 10 even numbers.

Solution:

begin
    integer i;
    for i := 2 to 20 by 2 do
    begin
        print(i);
    end;
end;

While Loop

The while loop repeats a block of code as long as a specified condition is true. Here is the basic syntax:

while condition do
begin
    // Code to be executed
end;

Example

begin
    integer i;
    i := 1;
    while i <= 5 do
    begin
        print(i);
        i := i + 1;
    end;
end;

Explanation:

  • The loop starts with i equal to 1.
  • The loop continues as long as i is less than or equal to 5.
  • The print(i) statement outputs the value of i in each iteration.
  • The i := i + 1 statement increments i by 1 after each iteration.

Exercise 2

Write a while loop that prints the numbers from 10 to 1 in descending order.

Solution:

begin
    integer i;
    i := 10;
    while i >= 1 do
    begin
        print(i);
        i := i - 1;
    end;
end;

Repeat-Until Loop

The repeat-until loop is similar to the while loop but checks the condition after executing the block of code. Here is the basic syntax:

repeat
    // Code to be executed
until condition;

Example

begin
    integer i;
    i := 1;
    repeat
        print(i);
        i := i + 1;
    until i > 5;
end;

Explanation:

  • The loop starts with i equal to 1.
  • The print(i) statement outputs the value of i in each iteration.
  • The i := i + 1 statement increments i by 1 after each iteration.
  • The loop continues until i is greater than 5.

Exercise 3

Write a repeat-until loop that prints the first 5 odd numbers.

Solution:

begin
    integer i;
    i := 1;
    repeat
        print(i);
        i := i + 2;
    until i > 9;
end;

Common Mistakes and Tips

  • Infinite Loops: Ensure that the loop condition will eventually become false. Otherwise, the loop will run indefinitely.
  • Off-by-One Errors: Be careful with the loop boundaries to avoid executing the loop one time too many or too few.
  • Loop Control Statements: Use break and continue judiciously to manage the flow of loops effectively.

Summary

In this section, we covered the three main types of loops in ALGOL: for, while, and repeat-until. We also discussed loop control statements like break and continue. By practicing the provided exercises, you should now have a solid understanding of how to use loops to control the flow of your programs.

Next, we will explore Switch Case Statements to handle multiple conditions more efficiently.

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