Week 5 · Session 3

Loops

Doing something more than once without writing it more than once.

Lab 2 Wrote This Three Times

The CRITICAL branch flashes the light three times.

led.on(), time.sleep(0.1), led.off(), time.sleep(0.1), and then the same four lines again, and again
It works. It is also twelve lines that say one thing: flash, three times
Make the warning flash ten times and you are copying forty lines. A loop says it once

The `for` Loop

for flash in range(3): means: run the indented block below, three times
The block is everything indented under the for line, the same rule as if
One pass through the block is an iteration. range(3) is three iterations
The first unindented line after the block runs once, after the last iteration

Try It: Flash Three Times

This is the CRITICAL branch as a loop, with print standing in for the light.

Run it. Then change 3 to 10 and run it again
Add print("Warning over.") at the bottom with no indentation, and see how many times it prints
Indent that same line under the loop and run it once more

for flash in range(3):
    print("on")
    print("off")

What `range` Does

range(3) makes the loop go through 0, then 1, then 2. It stops before 3
range(1, 4) starts at 1 and goes through 1, 2, 3. The stop value is not included
The name after for holds the current value. Use it inside the block, or ignore it and just count trips

Try It: Trace First, Then Run

Write down what this prints before you press Run.

How many lines print, and what is on each one
Then change range(1, 4) to range(4) and predict again before running

for number in range(1, 4):
    print("Flash", number)
print("Done")

An `if` Inside a Loop

The question is asked once per trip, about the value the loop handed over.

flash % 3 == 0 is True for 3, 6 and 9: the remainder is what picks out every third one
Counting from 1 is what makes "every third" line up with the remainder
A counter set to 0 before the loop, plus one inside the if, ends up holding how many trips took that branch

Try It: Every Third One Long

Run it. Which flash numbers are held long, and why those?

Change range(1, 8) to range(7) and run again. Which flashes are long now, and why did that move?
Add a counter: set long = 0 before the loop and long = long + 1 inside the if, then print it after the loop

for flash in range(1, 8):
    if flash % 3 == 0:
        print("Flash", flash, "held long")
    else:
        print("Flash", flash)

Try It: A Loop Inside a Loop

Read it before you run it. Count the lines it will print.

The inner for runs all the way through on every trip of the outer one
Two attempts, and each attempt prints one header, three short and three long. Check your count against the output
Indentation decides which loop a line belongs to. Un-indent the last for and run again

for attempt in range(1, 3):
    print("Attempt", attempt)
    for dot in range(3):
        print("short")
    for dash in range(3):
        print("long")

`while`: Repeat Until Something Changes

For when you do not know the count in advance.

while charge < 40: checks the condition before every trip and stops the first time it is False
Something inside the block has to move the condition toward False, or the loop never ends
Run it. Then change the starting charge to 50 and see how many trips happen

charge = 10
while charge < 40:
    print("Charging:", charge)
    charge = charge + 10
print("Ready to ride.")

Which Loop, When

for ... in range(n) when you know how many times: flash three times
while when you stop on a condition, not a count: keep charging until the battery is ready
An if inside a loop decides each trip on its own. A loop inside a loop repeats a repetition
Lab 3 asks for each of these once, on one lantern