For and while loops in Python

A loop repeats a block of code, so a Python program can do one job many times without you writing it out again. It is how a program adds up a column of prices, checks every line of a log or counts the days to a deadline.

Typing print a hundred times would be silly. A loop does it in two lines.

A for loop takes a run of values and repeats its indented lines once for each. The name after for holds the current value.

range makes the run of whole numbers. range(5) gives 0 to 4. range(1, 6) gives 1 to 5, because it stops before the second number.

python3 -c "for n in range(3): print(n)"

for n in range(1, 6):
    print("Round", n)
# Change 6 to 9 and run again.

To add up many values, make a variable before the loop and start it at 0. Inside the loop, add each value to it. This variable is an accumulator.

After the loop it holds the whole total. The trace table shows each pass.

total = 0
for n in range(1, 5):
    total = total + n
    print("n is", n, "and total is", total)
print("Final total:", total)

printf 'total = 0\nfor n in range(1, 5):\n    total = total + n\nprint(total)\n' > total.py

python3 total.py

A while loop does not need a run of values. It tests a condition before every pass, and it repeats while the condition is True.

Something inside the loop must change the answer, or it never stops. A loop that never ends is an infinite loop. In a terminal, press Ctrl and C together to stop one.

balance = 100
months = 0
while balance < 150:
    balance = balance + 20
    months = months + 1
print(months, "months to reach", balance)
# Change 150 to 200 and run again.

Sometimes you can stop as soon as you find what you want. The command break leaves the loop at once, and the program carries on after it.

The next program searches for the first number whose square is over 50.

for n in range(1, 100):
    if n * n > 50:
        print("First square over 50 is", n * n)
        break

A loop can sit inside another loop. The inner loop runs all its passes for each pass of the outer one.

With 2 passes outside and 3 inside, the inner lines run 2 times 3, which is 6 times.

for row in range(1, 3):
    for col in range(1, 4):
        print(row, "x", col, "=", row * col)
# The inner loop runs fully for each pass of the outer one.

python3 -c "for a in range(1, 4): print(a, a * 2, a * 3)"

Change total.py so that it adds the even numbers from 2 to 20. You can do it with a third number in range, or with an if inside the loop.

Write down the answer you expect first. A trace table for the first three passes is enough to check it.