For Loops with Ranges in Python

In this tutorial, we’ll learn about:

• What a for loop is, and how it differs from a while True: loop
• How for loops can be used to repeat actions a specific number of times. Let’s try the difference between a single line program and a for loop.

Single Line Program

1. Connect the M4 to your computer, and load code.py
2. On line 1, write:

print("this is a non-looped program")

1. Open the serial console.
2. Save this program and observe the serial output. You’ll notice you should see a code done running line in your Serial console.

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For Loop

1. Delete all existing code.
2. Write in this code:

for i in range(5):
    print("this is in a loop!")

1. Open the serial console.
2. Save this program and observe the serial output.

Examining a For Loop:

You’ve likely noticed by now that the statement is printed five times. That’s what the for i in range(5): statement does. Let’s examine this:

• The word “for” initiates a loop. This loop will run a specific number of times as defined.
• The variable i is a counter. It will take values from 0 up to 4 in our case (5 values in total because range(5) gives us numbers 0, 1, 2, 3, 4).
• The range(5) function provides a sequence of numbers from 0 to 4.
• The colon : signifies to the computer that the statement is over and the loop body begins. Similar to how we write if x is True statements.
• Much like the indentation in the bullet points here, anything that is indented within the for loop will run for each value in the range.

Try running the following code to see the counter i value throughout loop interations:

for i in range(3):
    print("Loop iteration number:", i)

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Using Delays in For Loops:

We can also use time delays in for loops just like in while loops to see the iteration more clearly. This is generally advised for debugging purposes, but not super helpful for when we’re actually running code, unless the time.sleep() value in minimal.

1. Import the time module and use time.sleep() for delays.
2. Write and run the following code:

import time
for i in range(5):
    print("This is in a loop! We're on interation number:", i)
    time.sleep(1)
print("The loop has finished")

For Loops and While True Loops

A for loop lets us run a block of code a specific number of times. Unlike a while True: loop, which runs forever, a for loop ends after a predetermined number of iterations. This can be useful when you need to repeat actions a set number of times rather than indefinitely. We can even nest in for loops inside of our while True: loop.

1. Delete all existing code.
2. Write and run the following code:

import time

print("starting main loop")
while True:
    for i in range(5):
        print("This is in a loop! We're on interation number:", i)
        time.sleep(1)
    print("The loop has finished. Notice how 'i' will now reset back to '0'")
    time.sleep(1)

1. Observe the serial output.

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For Loops: 2 Arguments (Start & Stop)

a for loop can also take more than a single arguement. Up to this point, we’ve been playing with a single arguement:

for i in range(10):
    print(i)
    time.sleep(0.5)

• in this example, we increase our i iterater variable by 1 every time.
• we also always start at 0 for our loop.

Let’s try something different:

for i in range(0, 10):
    print(i)
    time.sleep(0.5)

• did you notice a difference in the output? What has changed in our arguments?

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We have 2 arguments now, rather than always starting from 0, we can say what number to start from, and what number to end at. Observe:

for i in range(5, 10):
    print(i)
    time.sleep(0.5)

• what number did we start at this time?

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For Loops: 3 Arguments (Start, Stop, Iterater)

Lets try 3 arguments:

for i in range(0, 10, 2):
    print(i)
    time.sleep(0.5)

• What was the print output this time?
• the third argument, 2, is our step iterator. Rather than stepping by 1 each iteration or loop, now we are stepping by 2!

Lets look at a few more examples:

Example 1: Count by 5’s

for i in range(0, 50, 5):
    print(i)
    time.sleep(0.5)

Example 2: Reverse counting down

for i in range(10, 0, -1):
    print(i)
    time.sleep(0.5)