Control Flow & Loops

What is control flow?

By default, Python runs your code one statement at a time, top to bottom. "Control flow" is the set of language features that let you change that order — branch into different paths, repeat blocks, or skip them entirely. The two main tools are conditional statements (if/elif/else) and loops (for/while).

Comparison and boolean operators

Before we branch, we need a way to ask questions. Python's comparison operators return either `True` or `False` — values of type `bool`.

print(5 > 3)        # True
print(5 == 5)       # True (equality is two equals signs)
print(5 != 6)       # True
print("a" in "cat") # True (membership)

# Combining conditions
age = 25
print(age >= 18 and age < 65)  # True (both must be true)
print(age < 13 or age > 65)    # False (at least one must be true)
print(not (age == 25))         # False (flips it)

if / elif / else

The `if` statement runs a block only if a condition is true. `elif` ("else if") chains additional conditions, and `else` runs when nothing else matched. Note the colons at the end of each line and the indentation — Python uses indentation to mark which lines belong to which block. The standard is 4 spaces.

score = 78

if score >= 90:
    grade = "A"
elif score >= 75:
    grade = "B"
elif score >= 60:
    grade = "C"
else:
    grade = "F"

print(grade)

Python also has a one-line conditional expression (sometimes called the ternary operator). It's handy when you just want to pick between two values:

age = 20
status = "adult" if age >= 18 else "minor"
print(status)

for loops

A `for` loop repeats a block once for each item in some collection (list, string, range, etc.). The variable after `for` takes on each value in turn.

# Loop over a range of numbers (0 through 4)
for i in range(5):
    print(i)

# Loop over the characters in a string
for letter in "hello":
    print(letter)

# Loop over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

`range(n)` produces 0, 1, 2, ..., n-1. `range(start, stop)` produces start, start+1, ..., stop-1. `range(start, stop, step)` skips by `step` each time. This is one of the most-used functions in all of Python.

while loops

A `while` loop keeps running as long as a condition is true. Use it when you don't know in advance how many iterations you need.

n = 5
while n > 0:
    print(n)
    n -= 1   # shorthand for n = n - 1
print("Liftoff!")

break and continue

`break` exits the loop immediately. `continue` skips the rest of the current iteration and goes to the next one. Both work in `for` and `while`.

for n in range(20):
    if n == 10:
        break          # stop entirely at 10
    if n % 2 == 1:
        continue       # skip odd numbers
    print(n)