Iteration

Don't forget to get setup before you start:

• 💻 Run mwc update.
• 💻 Move into the lab directory: cd Desktop/making_with_code/mwc1/unit1/lab_iteration.
• 💻 Run poetry shell to enter the Python environment for this lab.

In the Names Lab, you learned how to write functions with arguments, such as square:

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from turtle import *
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def square(side_length):
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    forward(side_length)
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    right(90)
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    forward(side_length)
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    right(90)
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    forward(side_length)
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    right(90)
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    forward(side_length)
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    right(90)

This is a powerful function--instead of repeating the code block every time you want to draw a square, you can just call square(50), square(100), or whatever size square you want.

But what if you want to draw a lot of squares? In this lab you will learn about iteration, the process of doing something multiple times.

Lists

A list is a data structure which holds any number of values, separated by commas and surrounded by square brackets. For example, [], [50], and [100, 200, 300, 400] are all lists of numbers.

Once you create a list, you can iterate through it using a "for-loop". (Iteration is the process of going through a collection of things one at a time.) Line 13 goes through sizes, one number at a time. Each number is temporarily assigned to the variable size, and then the code block runs.

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sizes = [20, 40, 60, 80, 100]
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for size in sizes:
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    square(size)

Sometimes we just want to repeat the code block, and we don't even use the temporarily-assigned variable. The example below doesn't use the temporarily-assigned variable size, so the values of the numbers in the list don't matter. All that matters is how many numbers there are in the list.

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for size in sizes:
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    square(20)
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    penup()
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    forward(40)
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    pendown()

Ranges

Lists are great, but what if you want to draw a lot of squares? You don't necessarily want to type out a 100-item list just to iterate over it. Python has a built-in function called range which automatically creates a list-like object. range can be called in a few different ways.

💻 Run python to open Python's interactive mode. Enter the following commands to try out using ranges.

range(max) generates the numbers from 0 up to max (not including max).

>>> for number in range(5):
...     print(number)
...
0
1
2
3
4

range(min, max) generates the numbers from min up to max (not including max)

>>> for number in range(5, 10):
...     print(number)
...
5
6
7
8
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range(min, max, stride) generates the numbers from min up to max, counting by stride

>>> for number in range(0, 100, 20):
...     print(number)
...
0
20
40
60
80

Draw with ranges

drawtiles.py provides an example of what you can do with ranges. This program takes a function to draw a single tile, draw_tile from tile.py, and draws it over and over in a grid.

💻 Run python drawtiles.py --help to find out how to use this program.

(lab-iteration-py3.11) lab_iteration % python drawtiles.py --help
usage: python drawtiles.py [-h] [--fast] width height size

Draws a grid of tiles.

positional arguments:
  width       How many tiles across the grid should be
  height      How many tiles high the grid should be
  size        Side length of each tile

options:
  -h, --help  show this help message and exit
  --fast      Skip turtle animation and show the result

The first line of the help message shows how to run the program. You need to provide three arguments: width, height, and size, and can optionally also add the --fast flag. The rest of the help message explains what each argument does.

💻 Run python drawtiles.py 5 3 40. If you're impatient, add --fast.

Try running drawtiles.py with different arguments. Then open tile.py and try changing the draw_tile() function. Create your own tile pattern.