for loops and range

The for loop in Python is used to iterate over a sequence of items: a list, a string, a range, a dict… anything that is "iterable".

parole = ["ciao", "mondo", "!"]
for p in parole:
    print(p)
# ciao
# mondo
# !

The pattern is always the same: for <variable> in <iterable>: followed by the indented block that will be executed once for each item.

range(...): generating integer intervals

range produces a lazy sequence of integers. Three forms:

range(5)         # 0, 1, 2, 3, 4
range(2, 6)      # 2, 3, 4, 5
range(0, 10, 2)  # 0, 2, 4, 6, 8     (step)
range(5, 0, -1)  # 5, 4, 3, 2, 1     (negative step)

range is exclusive on the right end, like slice. It is a lazy object: it does not allocate memory for all the numbers, it produces them one at a time.

totale = 0
for i in range(1, 11):
    totale = totale + i
totale  # 55  (1+2+...+10)

Iterating with an index: enumerate

When you need the item and its index together, use enumerate:

parole = ["ciao", "mondo", "!"]
for i, p in enumerate(parole):
    print(i, p)
# 0 ciao
# 1 mondo
# 2 !

enumerate produces (index, value) pairs that we assign directly to two variables via tuple unpacking (we'll see it in the module dedicated to data structures).

Iterating two sequences in parallel: zip

nomi = ["Ada", "Linus", "Grace"]
anni = [36, 54, 79]
for n, a in zip(nomi, anni):
    print(f"{n} ha {a} anni")

zip stops at the shortest sequence.

range() and Python's laziness

In Python, the range() function does not allocate an actual list in memory: it returns a lazy generator that yields numbers one at a time as requested (for example, in a for loop). This allows iterating over millions of integers without bloating RAM. To inspect range values as a list, you can cast it explicitly with list(range(5)).

Try it yourself

Review exercise

Additional challenge