Loops & Control Flow

Loops & Control Flow#

So far our programs have run straight from top to bottom. Real programs need to make decisions (“if this customer is overdue, flag the invoice”) and repeat work (“for every row in the dataset, clean it up”). That’s what control flow is for, and Mojo’s control flow will feel comfortingly familiar if you’ve ever touched Python.

Making decisions with if#

The if statement runs a block of code only when a condition is true:

def main():
    var balance = 1200

    if balance > 1000:
        print("Premium account")

Add elif to test more conditions, and else for the fallback:

def classify(balance: Int):
    if balance > 10000:
        print("Status: VIP")
    elif balance > 1000:
        print("Status: Premium")
    else:
        print("Status: Standard")

def main():
    classify(25000)   # Status: VIP
    classify(1200)    # Status: Premium
    classify(50)      # Status: Standard

Two beginner notes that trip people up:

  • The condition does not need parentheses, but the line must end with a colon :.

  • The body is defined by indentation, not braces. Pick four spaces and stay consistent, exactly like Python.

Conditional expressions#

Sometimes you just want to pick one of two values. Mojo has the inline if … else expression for that:

var a = 7
var b = 12
var smaller = a if a < b else b   # smaller == 7

Read it left to right: “use a if a < b, otherwise use b.”

Repeating with for#

A for loop walks through the items of a collection, one at a time. This is the loop you’ll use most.

def main():
    var temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]

    for temp in temps:
        print(temp, "°C")

Each time around, temp holds the next value from the list. Clean and readable.

Counting with range#

When you want to repeat something a set number of times, or you need the index, use range:

def main():
    for i in range(4):
        print(i)        # prints 0, 1, 2, 3

range is more flexible than it first looks. It accepts start, stop, and step:

def main():
    # start at 9, stop before 0, step down by 3
    for x in range(9, 0, -3):
        print(x)        # prints 9, 6, 3

A very common pattern is to loop over a list by index so you also know the position:

def main():
    var temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]

    for day in range(len(temps)):
        print("Day", day + 1, "→", temps[day], "°C")

We use day + 1 because range starts at zero, but humans count days starting from one.

Repeating with while#

A while loop keeps going as long as its condition stays true. Use it when you don’t know in advance how many times you’ll loop.

def main():
    var countdown = 5
    while countdown > 0:
        print(countdown)
        countdown -= 1
    print("Liftoff! 🔥")

The danger with while is the infinite loop: if the condition never becomes false, your program runs forever. Here, countdown -= 1 is what guarantees we eventually stop. Always make sure something inside the loop moves you toward the exit.

Steering loops: break and continue#

Two keywords give you finer control inside any loop.

break exits the loop immediately:

def main():
    var invoices: List[Int] = [200, 450, -1, 700]

    for amount in invoices:
        if amount < 0:
            print("Bad record found, stopping.")
            break
        print("Processing invoice:", amount)

continue skips the rest of the current pass and jumps to the next item:

def main():
    var invoices: List[Int] = [200, 450, -1, 700]
    var total = 0

    for amount in invoices:
        if amount < 0:
            continue          # ignore bad records, keep going
        total += amount

    print("Clean total:", total)   # 1350

break says “I’m done with this loop entirely.” continue says “skip this one, but keep looping.”

A small, real example#

Let’s tie it together. Imagine we’re scanning a week of account balances and want to count how many days were below a threshold, the kind of quick check you’d do before feeding data to a model.

def main():
    var balances: List[Int] = [1200, 800, 1500, 300, 950, 2000, 600]
    var threshold = 1000
    var low_days = 0

    for day in range(len(balances)):
        var balance = balances[day]
        if balance < threshold:
            low_days += 1
            print("Day", day + 1, "is low:", balance)

    print("Total low-balance days:", low_days)

Run it and you’ll see exactly which days fell short, plus a final count. That single pattern, loop over data, test a condition, accumulate a result, is the backbone of an enormous amount of data work.

Try it yourself#

A few small exercises to make this stick. Solutions follow.

  1. Print the numbers 1 through 10, but skip every multiple of 3.

  2. Given var prices: List[Float64] = [9.99, 19.50, 4.25, 30.00], compute and print the total.

  3. Starting from 100, repeatedly halve a number with a while loop, printing each step, until it drops below 1.

Solutions#

# 1.
def main():
    for n in range(1, 11):
        if n % 3 == 0:
            continue
        print(n)

# 2.
def main():
    var prices: List[Float64] = [9.99, 19.50, 4.25, 30.00]
    var total: Float64 = 0.0
    for price in prices:
        total += price
    print("Total:", total)

# 3.
def main():
    var value: Float64 = 100.0
    while value >= 1.0:
        print(value)
        value /= 2.0

You can now branch and repeat, which means you can express real logic. Next we’ll package that logic into reusable functions.