PyInfo
Python Fundamentals contents

Loops

while and for loops, range, break / continue / else, enumerate and zip, and nested loops.

Beginner4 min readloopsrangeiteration

Read first: Conditional Statements, Lists and Tuples

A loop repeats a block of code. Python has two: while (repeat while a condition holds) and for (repeat for every item of a sequence).

while

python
i, total = 1, 0
while i <= 5:
    total += i
    i += 1
assert total == 15

Use while when you do not know in advance how many iterations you need, for example the sum of digits of a number:

python
def digit_sum(n):
    total = 0
    while n > 0:
        total += n % 10       # last digit
        n //= 10              # drop the last digit
    return total

assert digit_sum(12345) == 15

for and range

for walks over anything iterable: lists, strings, dicts, sets, files.

python
total = 0
for x in [4, 8, 15]:
    total += x
assert total == 27

letters = [ch for ch in "abc"]
assert letters == ["a", "b", "c"]

range generates numbers lazily (no list is built):

Call Values
range(5) 0, 1, 2, 3, 4
range(2, 6) 2, 3, 4, 5
range(10, 0, -3) 10, 7, 4, 1
python
assert list(range(5)) == [0, 1, 2, 3, 4]
assert list(range(2, 6)) == [2, 3, 4, 5]
assert list(range(10, 0, -3)) == [10, 7, 4, 1]
assert sum(range(1, 101)) == 5050

The stop value is always excluded. range(n) is the usual way to repeat something nn times; name the variable _ when you do not use it.

Loop with a position: enumerate

Do not write for i in range(len(a)) when you need both the index and the value:

python
fruits = ["apple", "pear", "plum"]
pairs = [(i, f) for i, f in enumerate(fruits)]
assert pairs == [(0, "apple"), (1, "pear"), (2, "plum")]
assert list(enumerate(fruits, start=1))[0] == (1, "apple")

Two sequences at once: zip

python
names = ["Ana", "Bob"]
scores = [9, 7]
assert list(zip(names, scores)) == [("Ana", 9), ("Bob", 7)]
assert dict(zip(names, scores)) == {"Ana": 9, "Bob": 7}

break, continue and else

  • break leaves the loop immediately.
  • continue skips to the next iteration.
  • A loop's else block runs only if the loop finished without break.
python
def is_prime(n):
    if n < 2:
        return False
    for d in range(2, int(n ** 0.5) + 1):
        if n % d == 0:
            return False          # found a divisor
    return True

assert [p for p in range(20) if is_prime(p)] == [2, 3, 5, 7, 11, 13, 17, 19]

Same test with for ... else:

python
def first_divisor(n):
    for d in range(2, n):
        if n % d == 0:
            result = d
            break
    else:                          # no break: nothing divided n
        result = None
    return result

assert first_divisor(15) == 3 and first_divisor(13) is None

pass does nothing and is a placeholder where a statement is required.

Nested loops

Loops can be nested; the inner loop runs completely for each step of the outer one.

python
table = [[i * j for j in range(1, 4)] for i in range(1, 4)]
assert table == [[1, 2, 3], [2, 4, 6], [3, 6, 9]]

pattern = "\n".join("*" * i for i in range(1, 4))
assert pattern == "*\n**\n***"

Two nested loops over nn elements do n2n^2 steps. With n=105n = 10^5 that is far too slow in Python, and it is the point where algorithmic thinking starts. See complexity in the contest track.

Exercises

  1. Print the multiplication table of a number read from input.
  2. Sum all odd numbers between 1 and nn.
  3. Read nn and print the first nn Fibonacci numbers.
  4. Reverse the digits of an integer with a while loop.
  5. Print a right triangle of stars of height nn.
  6. Find all perfect numbers below 10000 (a number equal to the sum of its proper divisors).

Practice

Apply this on the platform and get an instant verdict.

suma cifrelorsuma numere imparenumar prim