Ruby while Loop

Ruby while Loop

The while loop repeatedly executes a block of code as long as the specified condition is true.

The syntax of the while loop is:

while condition
  # Body of loop
end

Here,

1. The while loop first evaluates the condition.
2. If condition evaluates to true, the loop body is executed.
3. Then, condition is evaluated again.
4. This process continues as long as condition evaluates to true.
5. If condition evaluates to false, the loop stops.

Flowchart of while Loop

Example 1: Display Numbers From 1 to 3

# Initialize variable i
i = 1

# Loop runs until i is less than 4
while i < 4
  puts i

  # Increase value of i by 1
  i += 1
end

Output

1
2
3

Here's how the program works in each iteration of the loop:

To learn more about loop conditions, visit Ruby Comparison and Logical Operators.

Example 2: Sum of Only Positive Numbers

sum = 0

# Loop as long as input number is positive
while true
  print "Enter a number: "
  user_input = gets.chomp

  # Convert string to integer
  input_number = user_input.to_i

  # Exit loop if negative
  break if input_number < 0

  # Add positive number
  sum += input_number
end

puts "The sum is #{sum}"

Output

Enter a number: 2
Enter a number: 4
Enter a number: -3
The sum is 6

In the above program,

• We take input from the user using gets.
• We convert the input string to an integer using .to_i.
• If the number is negative, the loop stops using break.
• Otherwise, the number is added to sum.

Note: In Ruby, inputs are always read as strings. So you must convert them using .to_i for math operations.

Ruby until Loop

The until loop executes a block of code repeatedly until the specified condition becomes true.

It is basically the opposite of the while loop.

The syntax of the until loop is:

until condition
  # Body of loop
end

Here,

1. The until loop evaluates the condition.
2. If condition evaluates to false, the code inside the loop runs.
3. After each iteration, condition is checked again.
4. This process continues as long as condition evaluates to true.
5. If condition evaluates to true, the loop terminates.

Flowchart of until Loop

Example 3: Display Numbers from 3 to 1

i = 3

# Loop runs until i becomes 0
until i == 0
  puts i

  # Decrease value of i by 1
  i -= 1
end

Output

3
2
1

Here, the initial value of i is 3. Then, we used the until loop to iterate over the values of i. Here's how the loop works in each iteration:

i = 5

# False condition
# Body not executed
while i > 10
  print "#{i}, "
  i += 1
end

# No output

i = 5

# False condition
# Body is executed
until i > 10
  print "#{i}, "
  i += 1
end

5, 6, 7, 8, 9, 10, 

Example 4: Sum of Positive Numbers

sum = 0
input_number = 0

# Run the loop at least once
# Keep adding numbers until the user enters a negative value
until input_number < 0
  # Add the current number to the sum
  sum += input_number

  # Ask the user to enter a number
  print "Enter a number: "
  user_input = gets.chomp

  # Convert the input string to an integer
  input_number = user_input.to_i
end

# Finally, display the sum
puts "The sum is #{sum}"

Output

Enter a number: 2
Enter a number: 4
Enter a number: -3
The sum is 6

In the above program, the until loop prompts the user to enter a number.

As long as we enter positive numbers, the loop adds them and asks for more input.

If we enter a negative number, the loop terminates without adding the negative number.

i = 1

# Condition is always true
while i < 5
  puts i
end

i = 1

# Condition never becomes true
until i > 5
  puts i
end

# Display "hi" 3 times
for i in 1..3
  puts "hi"
end

puts "bye"

hi
hi
hi
bye

# Boolean variable to use as loop condition
is_display = true

# Variable to store user input
user_choice = nil

# Display "hi" as long as user wants
# Basically, run the loop as long as is_display is true
while is_display
  puts "hi"

  # Get user input
  print "Print hi again? y for yes: "
  user_choice = gets.chomp

  # Set is_display to false if user_choice is not "y"
  is_display = false if user_choice != "y"
end

puts "bye"

hi
print hi again? y for yes: y
hi
print hi again? y for yes: y
hi
print hi again? y for yes: n
bye