Decisions & loops

So far your programs have run every line once, top to bottom. Real programs make decisions ("if the password is wrong, ask again") and repeat work ("for every item in the cart, add up the price"). These two ideas, conditions and loops, are called control flow, and with them you can write any computation there is. This lesson covers both, plus the patterns you'll use with them every day.

#if: running code only sometimes

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

temperature = int(input("Temperature in °C: "))

if temperature > 30:
    print("It's hot. Drink water.")
    print("Maybe stay inside.")

print("Have a nice day.")

Three details matter:

• The condition is any expression that's True or False, usually a comparison.
• The line ends with a colon :.
• The lines that belong to the if are indented, by four spaces. Indentation isn't decoration in Python: it's how Python knows which lines are inside the block. The unindented print runs either way.

Getting indentation wrong is the most common beginner error:

IndentationError: expected an indented block after 'if' statement on line 3

Your editor indents automatically after a colon; let it.

#else and elif: choosing between options

else runs when the condition is false:

age = int(input("Age: "))

if age >= 18:
    print("You can vote.")
else:
    print(f"You can vote in {18 - age} years.")

For more than two options, chain conditions with elif ("else if"). Python checks them in order and runs only the first block whose condition is true:

score = int(input("Score (0-100): "))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Grade: {grade}")

A score of 95 is also >= 80, but it gets an A because the first true branch wins and the rest are skipped. Order your conditions from most specific to least.

Conditions can be combined:

if age >= 18 and has_id:
    print("Welcome in.")

if day == "Saturday" or day == "Sunday":
    print("Weekend!")

#while: repeat as long as something is true

A while loop repeats its block for as long as its condition stays true. It checks the condition before every round:

count = 3
while count > 0:
    print(count)
    count -= 1
print("Lift off!")

3
2
1
Lift off!

Something inside the loop must eventually make the condition false. If you forget count -= 1, the condition stays true forever and the program never stops: an infinite loop. Press Ctrl+C in the terminal to stop it.

while is the right tool when you don't know in advance how many times to repeat, such as asking until the user gives a valid answer:

answer = input("Continue? (yes/no) ").lower()
while answer != "yes" and answer != "no":
    answer = input("Please type yes or no: ").lower()
print(f"You chose {answer}.")

#for: repeat for each item

A for loop runs its block once for each item in a sequence. The loop variable takes each value in turn:

for letter in "cat":
    print(letter)

c
a
t

To repeat a fixed number of times, loop over range:

for i in range(5):        # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):     # 1 to 5 (the end is never included)
    print(i)

for i in range(0, 20, 5): # 0, 5, 10, 15 (a step of 5)
    print(i)

range(5) produces five numbers starting at 0, and the end value is always excluded. That's the same "up to but not including" rule as string slicing. It feels odd for a day, then becomes natural.

In the next lessons you'll loop over lists of things (orders, names, lines in a file), which is where for really shines.

#Common loop patterns

A handful of patterns cover most loops you'll write.

Accumulate a total:

total = 0
for n in range(1, 101):
    total += n
print(total)    # 5050

Count things that match a condition:

sentence = "the quick brown fox jumps over the lazy dog"
vowels = 0
for ch in sentence:
    if ch in "aeiou":
        vowels += 1
print(vowels)   # 11

Find something and stop early with break, which exits the loop immediately:

for n in range(2, 100):
    if n % 7 == 0 and n % 5 == 0:
        print(f"First number divisible by 5 and 7: {n}")
        break

Skip some items with continue, which jumps straight to the next round:

for n in range(1, 11):
    if n % 2 == 0:
        continue          # skip even numbers
    print(n)              # prints 1, 3, 5, 7, 9

A while True loop with break is a clean way to write "keep going until something happens", especially for menus and input:

while True:
    command = input("> ").strip().lower()
    if command == "quit":
        break
    print(f"You typed: {command}")
print("Bye!")

#Loops inside loops

A loop can contain another loop. The inner loop runs completely for every round of the outer one:

for row in range(1, 4):
    line = ""
    for col in range(1, 4):
        line += f"{row * col:4}"
    print(line)

   1   2   3
   2   4   6
   3   6   9

Nested loops are natural for grids and tables. Watch out for how quickly the work grows: two nested loops over 1,000 items each run the inner block a million times.

#Tracing code by hand

When a loop doesn't do what you expect, trace it: go line by line on paper, writing down each variable's value after each step.

total = 0
for n in range(1, 4):
    total += n * n

It feels slow, and it's the single most effective way to understand code, yours or anyone else's. You'll learn faster ways to watch a running program in the debugging lesson, but they're the same idea.

#Practice

Write guess.py, a number-guessing game:

1. The program picks a secret whole number from 1 to 100. Use these two lines to get a random one:

   import random
   secret = random.randint(1, 100)

2. The player has at most 7 guesses. After each guess, say "Too low", "Too high" or "Correct!".

3. If the player guesses correctly, say how many guesses it took and stop. If they run out, reveal the number.

4. Bonus: if the player types something that isn't a number, or a number outside 1 to 100, tell them and don't count it as a guess. (Hint: "42".isdigit() is True, "abc".isdigit() is False.)

# guess.py
import random

secret = random.randint(1, 100)
max_guesses = 7
guesses = 0

print("I'm thinking of a number from 1 to 100.")

while guesses < max_guesses:
    text = input(f"Guess {guesses + 1} of {max_guesses}: ").strip()
    if not text.isdigit():
        print("Please type a whole number.")
        continue
    guess = int(text)
    if guess < 1 or guess > 100:
        print("The number is between 1 and 100.")
        continue

    guesses += 1
    if guess < secret:
        print("Too low.")
    elif guess > secret:
        print("Too high.")
    else:
        print(f"Correct! You got it in {guesses} guesses.")
        break
else:
    print(f"Out of guesses. The number was {secret}.")

#Key takeaways

• if, elif and else choose which block runs; only the first true branch runs, and indentation defines the blocks.
• while repeats as long as a condition is true; make sure something eventually makes it false.
• for runs once per item; range(start, stop, step) excludes the stop value.
• break leaves a loop, continue skips to the next round, and while True plus break handles "until" loops cleanly.
• Accumulate, count, find and skip are the core loop patterns; trace by hand when a loop surprises you.