12. For loops

12.1. For loops with strings

for char in str:

the string is looped through with each character in the string, str, being placed in the char variable for use in the for loop block code.

The code below loops through each character in a string and does something with it.

for welcome_character in welcome_string takes each character in the string welcome_string and puts it in the variable welcome_character.

display.scroll(welcome_character) then scrolls the character.

from microbit import *

welcome_string = 'Hello'
while True:
    for welcome_character in welcome_string:
        display.scroll(welcome_character)
    sleep(300)

---

Tasks

1. Write a for-loop to scroll each letter in ‘winner’ individually.

2. Write a for-loop to scroll each digit in ‘2023’ individually.

Q1

Write a for-loop to scroll each letter in ‘winner’ individually.

from microbit import *

string = 'winner'
while True:
    for character in string:
        display.scroll(character)
    sleep(300)

Q2

Write a for-loop to scroll each digit in ‘1966’ individually.

from microbit import *

string = '1966'
while True:
    for character in string:
        display.scroll(character)
    sleep(300)

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12.2. Add actions to a for-loop

Apart from simply scrolling the character in a string, as in the examples above, other actions can be added to the for-loop.

The code below adds an underscore, ‘_’, between each character.’

from microbit import *

welcome_string = 'Hello'
spacing_character = "_"
while True:
    for welcome_character in welcome_string:
        display.scroll(welcome_character)
        display.scroll(spacing_character)
    sleep(300)

Tasks

1. Write a for-loop to scroll each letter in ‘ace’ individually with an ‘*’ between them.

2. Write a for-loop to scroll each digit in ‘8850’ individually with a ‘-’ between them.

Q1

Write a for-loop to scroll each letter in ‘ace’ individually with an ‘*’ between them.

from microbit import *

string = 'ace'
spacing_character = "*"
while True:
    for character in string:
        display.scroll(character)
        display.scroll(spacing_character)
    sleep(300)

Q2

Write a for-loop to scroll each digit in ‘8850’ individually with a ‘-’ between them.

from microbit import *

string = '2023'
spacing_character = "-"
while True:
    for character in string:
        display.scroll(character)
        display.scroll(spacing_character)
    sleep(300)

The disadvantage of this approach is that the spacing character is also added to the end, after the last character.

e.g “a*c*e*”

See the `Spacing characters under EXT: for-loops <https://pc-microbit-micropython.readthedocs.io/en/latest/lessons/For_loops_2.html>`_#spacing-characters>`_ for ideas to get “a*c*e” instead of “a*c*e*”.

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12.3. For loops with lists

for item in lst:

The list is looped through with each element in the list, lst, being placed in the item variable for use in the for loop block code.

Python can loop through each element in a list and do something with it.

In the code below, each element in the list is displayed.

The sleep is placed after the for-loop to create a short delay before looping through the list again.

from microbit import *

wise_men = ['Melchior', 'Caspar', 'Balthazar']
while True:
    for wise_man in wise_men:
        display.scroll(wise_man, delay=80)
    sleep(300)

In the code below, each number in the list is displayed via a for-loop.

The sleep is placed within the for-loop to create a short delay before the next number is shown.

from microbit import *

primes = [2, 3, 5, 7]
while True:
    for num in primes:
        display.show(num)
        sleep(300)

---

Tasks

1. Write a for-loop to scroll each name in the list ['Bugs', 'Daffy', 'Marvin'].

2. Write a for-loop to scroll each number in the list [1, 2, 3, 5, 8].

Q1

Write a for-loop to scroll each name in the list ['Bugs', 'Daffy', 'Marvin'].

from microbit import *

names_list = ['Bugs', 'Daffy', 'Marvin']
while True:
    for name in names_list:
        display.scroll(name)
    sleep(300)

Q2

Write a for-loop to scroll each number in the list [1, 2, 3, 5, 8].

from microbit import *

num_list = [1, 2, 3, 5, 8]
while True:
    for num in num_list:
        display.scroll(num)
    sleep(300)

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12.4. Nested For loops

A loop within another loop within is called a nested loop.

The code below loops through each list and displays the result of joining each string.

from microbit import *

col_letters = ['A', 'B', 'C']
row_nums  = ['1', '2', '3', '4']
while True:
    for col in col_letters:
        for row in row_nums:
            display.scroll(col + row, delay=200)

for col in col_letters takes each element in the list col_letters and puts it in the variable col for use in the loop.

for row in row_nums takes each element in the list row_nums and puts it in the variable row for use in the loop.

Each time the outer loop, for col in col_letters, runs 3 times since there are 3 elements in ['A', 'B', 'C'].

Each time the outer loop is run, the inner loop, for row in row_nums, runs 4 times since there are 4 elements in ['1', '2', '3', '4'].

The nested loops run a total of 3 * 4 or 12 times.

The + in col + row does a text join. When col = ‘A’ and row = ‘1’, col + row will result in 'A1'.

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The nested loops below perform multiplication tables for 5 and 6.

The nested loops run a total of 2 * 9 or 18 times.

from microbit import *

nums_1_list = [5, 6]
nums_2_list = [1, 2, 3, 4, 5, 6, 7, 8, 9]
while True:
    for num_1 in nums_1_list:
        for num_2 in nums_2_list:
            display.scroll(num_1 * num_2, delay=80)

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Tasks

1. Write a for-loop that scrolls all the letter pairings from the 2 lists: [“A”, “B”, “C”] and [“X”, “Y”, “Z”] taking one letter from each with the first letter always beng form the first list given.

2. Write a nested for-loop that tuns on and off pixels at points (x,y) using the lists: x_positions = [0, 1, 2, 3, 4] and y_positions = [4, 3, 2, 1, 0]. Use display.set_pixel(x, y, 9) to turn on the pixel and display.set_pixel(x, y, 0) to turn it off after a short sleep.

Q1

Write a for-loop that scrolls all the letter pairings from the 2 lists: [“A”, “B”, “C”] and [“X”, “Y”, “Z”] taking one letter from each with the first letter always beng form the first list given.

from microbit import *

letters_1 = ["A", "B", "C"]
letters_2 = ["X", "Y", "Z"]

while True:
    for letter1 in letters_1:
        for letter2 in letters_2:
            display.scroll(letter1 + letter2, delay=100)

Q2

Write a nested for-loop that tuns on and off pixels at points (x,y) using the lists: x_positions = [0, 1, 2, 3, 4] and y_positions = [4, 3, 2, 1, 0]. Use display.set_pixel(x, y, 9) to turn on the pixel and display.set_pixel(x, y, 0) to turn it off after a short sleep.

from microbit import *

x_positions = [0, 1, 2, 3, 4]
y_positions = [4, 3, 2, 1, 0]

while True:
    for x in x_positions:
        for y in y_positions:
            display.set_pixel(x, y, 9)
            sleep(200)
            display.set_pixel(x, y, 0)

For examples of using nested for-loops specific to the microbit display, see the Setting Pixels page.

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12.5. For loops with mixed lists

Mixed lists can be used with display.show.

Strings, integers and floats, and Images can all be displayed with display.show.

A short delay is used in display.show for when there are multiple characters in a string or number.

A short sleep is used in the for-loop so that there is a noticeable gap between each list element no matter whether they are strings, numbers or images.

A longer sleep is used after the for-loop before it repeats.

What does this code mean?

from microbit import *

mixed_list = ['I', Image.HEART, 3.14]
while True:
    for element in mixed_list:
        display.show(element, delay=200)
        sleep(700)
    sleep(1000)

---

Tasks

1. Create a mixed list to display the message to be asleep at 10 o’clock.

2. Create a mixed list to display you’re 3 favourite animals with their number order.

Q1

Create a mixed list to display the message to be asleep at 10 o’clock.

from microbit import *

mixed_list = [Image.ASLEEP, '@', Image.CLOCK10]
while True:
    for element in mixed_list:
        display.show(element, delay=200)
        sleep(700)
    sleep(1000)

Q2

Create a mixed list to display you’re 3 favourite animals in number order.

from microbit import *

mixed_list = ['#1', Image.RABBIT, '#2', Image.DUCK, '#3', Image.TORTOISE]
while True:
    for element in mixed_list:
        display.show(element, delay=200)
        sleep(700)
    sleep(1000)

12.6. EXT: storing values in a list

Exercises

1. Here is some fun code that displays a heart image as a series of 3 pixels. Change it form 3 to 5 pixels.

2. Here is some fun code that displays a heart image as a series of shown pixels. Change the pop parameter to pop the last item via -1 to clear the image in reverse.

Q1

Here is some fun code that displays a heart image as a series of 3 pixels. Change it form 3 to 5 pixels.

from microbit import *

heart = [
    [0, 1, 1, 1, 0],
    [1, 1, 1, 1, 1],
    [1, 1, 1, 1, 1],
    [0, 1, 1, 1, 0],
    [0, 0, 1, 0, 0]
]

lit_pixels = []  # Keep track of three active pixels

while True:
    for y in range(5):
        for x in range(5):
            if heart[y][x]:  # Only process active parts of the heart shape
                display.set_pixel(x, y, 9)
                lit_pixels.append((x, y))  # Store pixel coordinates

                # If more than 3 pixels are lit, remove the oldest one
                if len(lit_pixels) > 3:
                    old_x, old_y = lit_pixels.pop(0)
                    display.set_pixel(old_x, old_y, 0)

                sleep(100)

    sleep(500)
    display.clear()
    sleep(500)

Q2

Here is some fun code that displays a heart image as a series of shown pixels. Change the pop parameter to pop the last item via -1 to clear the image in reverse.

from microbit import *

heart = [
    [0, 1, 1, 1, 0],
    [1, 1, 1, 1, 1],
    [1, 1, 1, 1, 1],
    [0, 1, 1, 1, 0],
    [0, 0, 1, 0, 0]
]

lit_pixels = []  # Keep track of active pixels

while True:
    # Light up pixels one by one
    for y in range(5):
        for x in range(5):
            if heart[y][x]:  # Only process active parts of the heart shape
                display.set_pixel(x, y, 9)
                lit_pixels.append((x, y))  # Store pixel coordinates
                sleep(100)

    # Fade out pixels one by one instead of clearing all at once
    while lit_pixels:
        old_x, old_y = lit_pixels.pop(0)
        display.set_pixel(old_x, old_y, 0)
        sleep(100)

    sleep(500)

Q3

Here is some fun code that displays a heart image as a series of shown pixels, then clears it by choosing random pixels from the stored list of pixels. Modify it to dim the pixels instead of turning them off.

from microbit import *
import random  # Import random module

heart = [
    [0, 1, 1, 1, 0],
    [1, 1, 1, 1, 1],
    [1, 1, 1, 1, 1],
    [0, 1, 1, 1, 0],
    [0, 0, 1, 0, 0]
]

lit_pixels = []  # Keep track of active pixels

while True:
    # Light up pixels one by one
    for y in range(5):
        for x in range(5):
            if heart[y][x]:  # Only process active parts of the heart shape
                display.set_pixel(x, y, 9)
                lit_pixels.append((x, y))  # Store pixel coordinates
                sleep(100)

    # Remove pixels randomly instead of sequentially
    while lit_pixels:
        random_index = random.randint(0, len(lit_pixels) - 1)  # Get a random pixel index
        old_x, old_y = lit_pixels.pop(random_index)  # Remove a random pixel
        display.set_pixel(old_x, old_y, 0)
        sleep(50)

    sleep(500)