Stir Until Combined (For Loops)

In the last section, we created our recipe card—a list containing all the steps. Now, how do we follow the instructions? You wouldn't just read the first step; you'd read and perform every step, one after another, until the recipe is complete.

A for loop is how you tell the computer to do this. It automates repetition. Instead of writing code to print recipe[0], then recipe[1], then recipe[2], you can create a simple loop to do it for you.

The logic is just like cooking: "For each step on my recipe card, I will perform that step."

The loop will automatically:

1. Start with the first step.
2. Perform the action specified for that step.
3. Move to the next step.
4. Repeat until all steps are complete.

Let's use a for loop to print every step in our recipe list from the last lesson.

recipe = [
  "Preheat oven to 400°F",
  "Mix all ingredients in a bowl",
  "Pour into baking dish",
  "Bake for 25 minutes"
]

# For each 'step' in the 'recipe' list...
# ...put it in a temporary variable called 'step' and run the code below.
for step in recipe:
  print(step)

# The output will be all four steps, printed one by one.

let recipe = [
  "Preheat oven to 400°F",
  "Mix all ingredients in a bowl",
  "Pour into baking dish",
  "Bake for 25 minutes"
];

// For each 'step' in the 'recipe' array...
// ...put it in a temporary variable called 'step' and run the code below.
for (let step of recipe) {
  console.log(step);
}

// The output will be all four steps, printed one by one.

The combination of lists (to store your ingredients and steps) and loops (to process them) is one of the most powerful pairings in all of programming. You'll use it constantly to work with collections of ingredients, menu items, orders, or any other data you can imagine.

Beyond Printing: Using Loops to Calculate

The real power of loops comes from doing calculations. Imagine your recipe has multiple stages, each with a different duration. A loop can act as a calculator to find the total time.

Let's say we have a list of cooking times in minutes (e.g., for prepping, baking, and resting). We can loop through them to calculate the sum. To do this, we'll create a "running total" variable that starts at 0 and adds the time from each step.

# A list of durations for each stage of cooking
cooking_times = [15, 30, 10] # prep, bake, rest

# Start our running total at 0
total_time = 0

# For each number in our list...
for time in cooking_times:
  # ...add it to our running total
  total_time = total_time + time
  # After the first loop, total_time is 15
  # After the second loop, total_time is 15 + 30 = 45
  # After the third loop, total_time is 45 + 10 = 55

# After the loop is finished, print the final result
print("Total cooking time:", total_time, "minutes")

// An array of durations for each stage of cooking
let cookingTimes = [15, 30, 10]; // prep, bake, rest

// Start our running total at 0
let totalTime = 0;

// For each number in our array...
for (let time of cookingTimes) {
  // ...add it to our running total
  totalTime = totalTime + time;
  // After the first loop, totalTime is 15
  // After the second loop, totalTime is 15 + 30 = 45
  // After the third loop, totalTime is 45 + 10 = 55
}

// After the loop is finished, log the final result
console.log("Total cooking time: " + totalTime + " minutes");

That's it. We didn't just read the list; we used a loop to process it and produce a single, meaningful result. This technique of aggregating data is fundamental. You'll use it to sum up sales, count users, or calculate averages—all by combining a list and a for loop.

Mini-Exercise 💡

The for loop is powerful because you can change the list, and the loop code still works perfectly.

1. Start with the code from the "Beyond Printing" section that calculates total_time.
2. Add a new number to the cooking_times list. For example, add 5 minutes for "cooling".
3. Run the code again. Notice how you didn't have to change the loop at all. It automatically included the new item in its calculation.
4. Try adding another one!

What If You Don't Have a List?

For loops are perfect when you have a list of items and you want to do something for each item. But what if you don't have a list? What if you need to repeat an action until a certain condition is met?

• "Keep stirring until the sauce is thick."
• "Wait while the oven is preheating."
• "Knead the dough until it's smooth."

You don't know exactly how many stirs or how many minutes that will take. You just know the goal. For these situations, we need a different kind of loop.

In the next section, we'll explore while loops, which are designed for exactly this purpose: repeating an action as long as a certain condition remains true.