for and while loops

Why it matters

Computers are good at doing the same thing many times without getting bored or making mistakes. Adding up a thousand readings, checking every row of a file, retrying a calculation until it converges: all of these are loops. Python has two kinds. A for loop goes through a collection item by item. A while loop keeps going as long as a condition is true.

for loops over a list

readings = [21.5, 22.0, 23.25, 22.75]

for value in readings:
    print("Reading:", value)

Read it as "for each value in readings, run the indented block". On each pass, called an iteration, the loop variable value holds the next item. The loop ends by itself after the last item.

You can loop over any collection, including the characters of a string:

for letter in "DNA":
    print(letter)     # D, then N, then A

The accumulator pattern

The most common loop builds up a result. Start with an empty or zero value before the loop, update it inside, and use it after:

readings = [21.5, 22.0, 23.25, 22.75]

total = 0
count = 0
for value in readings:
    total += value
    count += 1

print(f"Average: {total / count:.2f}")   # Average: 22.38

The same pattern finds a maximum, counts matches or collects values into a new list:

high = []
for value in readings:
    if value > 22:
        high.append(value)     # append adds an item to the end of a list
print(high)                    # [23.25, 22.75]

(Python also has built-ins such as sum, max and len for the simple cases. Knowing the pattern lets you handle the cases they do not cover.)

Counting with range

range produces a sequence of whole numbers, which is how you repeat something a fixed number of times:

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

for year in range(2020, 2025):
    print(year)         # 2020 to 2024

for n in range(10, 0, -2):
    print(n)            # 10 8 6 4 2   (a step of -2)

range(start, stop, step) includes start and excludes stop. That "up to but not including" rule is everywhere in Python, and it is the source of many off-by-one bugs. To add the numbers from 1 to 100 you need range(1, 101).

If you need both the position and the item, use enumerate:

for position, name in enumerate(["Ada", "Grace", "Katherine"], start=1):
    print(position, name)    # 1 Ada / 2 Grace / 3 Katherine

while loops

Use while when you do not know in advance how many iterations you need, only when to stop:

balance = 1000.0
years = 0
while balance < 2000:
    balance *= 1.05      # 5% growth each year
    years += 1
print(f"Doubles after {years} years")   # Doubles after 15 years

Python checks the condition before every iteration. When it becomes false, the loop ends. Something inside the loop must eventually make the condition false, or the loop runs forever. If your program seems frozen, an infinite while loop is the usual suspect (the browser runner stops it after a few seconds; in a terminal, press Ctrl+C).

break and continue

• break leaves the loop immediately.
• continue skips the rest of this iteration and moves to the next one.

values = [4, 8, -1, 15, 16]

for v in values:
    if v < 0:
        print("Found a negative value, stopping")
        break
    print("Processing", v)

for v in values:
    if v < 0:
        continue          # skip bad data, keep going
    print("Valid:", v)

Nested loops

A loop inside a loop runs the inner loop completely for each pass of the outer one. A multiplication table is the classic example:

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

Output:

   1   2   3
   2   4   6
   3   6   9

Common mistakes

• Off by one with range. range(1, 10) stops at 9. Ask yourself whether the last value should be included.
• Resetting the accumulator inside the loop. If total = 0 sits inside the loop, it is wiped on every pass. Put it before the loop.
• Forgetting to update the while condition. No years += 1 or balance *= ... means an infinite loop.
• Changing a list while looping over it. Removing items from the list you are looping over skips elements. Build a new list instead.
• Wrong indentation. A line indented under the loop runs every time; a line at the outer level runs once, after the loop. Moving a print one level in or out changes the output completely.

Quick recap

• for item in collection: visits each item once.
• Accumulator pattern: set up before, update inside, use after.
• range(start, stop, step) counts and excludes stop.
• while condition: repeats until the condition is false; make sure it can become false.
• break exits a loop; continue skips to the next iteration.

Try it

In the Do tab you will write a for loop that sums a range of numbers, another that counts values above a threshold, and a while loop that finds how many years an investment takes to double.