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Wait for the Water to Boil (While Loops)

In the last section, we used a for loop to go through every item on our recipe card. A for loop is perfect when you know exactly how many times you need to repeat something—once for each ingredient in a list, for example.

But what if you don't know how many times you need to repeat? What if you need to repeat an action until a certain condition is met?

This is where the while loop comes in. It's like waiting for a pot of water to boil. You don't say, "I'm going to check the temperature 10 times." You say, "While the water is not yet boiling, I will keep waiting."

A while loop continuously executes a block of code as long as a given condition remains true.

The Danger of a Never-Ending Boil

A while loop needs one crucial thing: something inside the loop must eventually change the condition to false. If not, the loop will run forever, creating an infinite loop.

Infinite Loop Warning!

An infinite loop is a common bug where a loop's condition never becomes false. This will cause your program to get stuck and become unresponsive. It’s like a chef who turns on a mixer and forgets to turn it off—it will just keep running until you pull the plug! Always make sure your loop is making progress toward its end condition.

Let's simulate our boiling water example. We'll start with a low temperature and keep "heating" it (increasing the value) inside the loop until it reaches the boiling point.

water_temp = 20 # The starting temperature in Celsius

print("Putting the pot on the stove...")

# The loop will continue as long as this condition is True.
while water_temp < 100:
  print(f"Water is at {water_temp}°C. Still simmering...")
  # This is the crucial part: we change the value we are checking.
  # Each time the loop runs, the temperature gets closer to 100.
  water_temp += 10

# Once water_temp is 100 or more, the condition becomes False,
# and the loop stops. This line runs next.
print(f"Ding! The water is at {water_temp}°C. Time to add the pasta!")

let waterTemp = 20; // The starting temperature in Celsius

console.log("Putting the pot on the stove...");

// The loop will continue as long as this condition is true.
while (waterTemp < 100) {
  console.log(`Water is at ${waterTemp}°C. Still simmering...`);
  // This is the crucial part: we change the value we are checking.
  // Each time the loop runs, the temperature gets closer to 100.
  waterTemp += 10;
}

// Once waterTemp is 100 or more, the condition becomes false,
// and the loop stops. This line runs next.
console.log(`Ding! The water is at ${waterTemp}°C. Time to add the pasta!`);

The while loop is essential for situations where the number of iterations isn't known beforehand, like waiting for a user to type "quit", processing data until a file ends, or, in our final project, giving a player guesses until they win or run out of tries.

Mini-Exercise 💡

  1. Create a variable called countdown and set it to 10.
  2. Write a while loop that continues as long as countdown is greater than 0.
  3. Inside the loop, print the value of countdown.
  4. After printing, decrease the value of countdown by 1.
  5. After the loop finishes, print the message "Blast off! 🚀".