Loops¶
Introduction¶
You might have noticed that the code we’ve written so far has been very repetitive. Python has a feature called a “loop”, which we can use to make this code less repetitive! For now, try this example:
for name in ["John", "Sam", "Jill"]:
print("Hello " + name)
This is incredibly helpful if we want to do the same thing multiple times‒like drawing the edges of a square‒but only want to write that action once. Here’s another version of a loop, try this:
for i in range(10):
print(i)
Notice how we write only one line of code using i, but it takes on 10
different values? The code runs once for each value we give it.
The range(n) function can be considered a shorthand
for 0, 1, 2, ..., n-1. If you want to know more about it, you can use
the help in the Python shell by typing help(range).
Use the q key to exit the help again.
You can also loop over elements of your choice:
total = 0
for i in [5, 7, 11, 13]:
print(i)
total = total + i
print(total)
Write this example in a new code file and run it, to check it works how you might expect.
Note
Notice how above, the lines of code that run multiple times, are the ones that
are indented. This is an important concept in Python‒that’s how it
knows which lines are inside the for loop, to be run once for each value
you give it, and which are part of the rest of your program.
Thonny and Noteable will both help you by automatically indenting the line
after for .... :
Sometimes you want to repeat some code a number of times, but don’t care about
the value of the index variable (i in the code above). If that’s the case
use _ instead. This shows that we don’t care about its value, or
won’t use it. Here’s a simple example:
for _ in range(10):
print("Hello!")
So in this code for _ in range(10): just means “do the following 10 times”.
You may or may not be wondering about the variable i - why is it used all
the time above? Well, it simply stands for “index” and is one of the most
common variable names ever found in code. But if you are looping over something
other than just numbers, be sure to name it something better! For instance (just
for illustration, don’t type this in):
for drink in list_of_beverages:
print("Would you like a " + drink + "?")
This is immediately clearer to understand than if we had used i
instead of drink.
Drawing a dashed line¶
Exercise¶
Draw a dashed line. You can move the turtle without the turtle drawing its
movement by using the turtle.penup() function; to tell it to draw again,
use turtle.pendown().
Solution¶
for i in range(10):
turtle.forward(15)
turtle.penup()
turtle.forward(5)
turtle.pendown()
Bonus¶
Can you make the dashes become larger as the line progresses?
Hint
Feeling lost? Print your loop variable (in this case i) at every run of the loop:
for i in range(10):
print(i)
# write more code here
Can you utilize i‒commonly called the index variable or loop
variable‒to get increasing step sizes?
More Efficient Squares¶
Exercise¶
The squares we were drawing at the start of this tutorial had a lot of repeated lines of code. Can you write out a square drawing program in fewer lines by utilizing loops?
Solution¶
for _ in range(4):
turtle.forward(100)
turtle.left(90)
Bonus¶
Try nesting loops, by putting one right under (inside) the other, with some drawing code that’s inside both. Here’s what it can look like:
for ...:
for ...:
# drawing code inside the inner loop goes here
...
# you can put some code here to move
# around after!
...
Replace the ...’s with your own code, and see if you can come up with
something funny or interesting! Mistakes are encouraged!
Comments¶
In the example above, the line that starts with a
#is called a comment. In Python, anything that goes on a line after#is ignored by the computer. Use comments to explain what your program does, without changing the behaviour for the computer. They can also be used to easily and temporarily disable, or “comment out” some lines of code.Comments can also go at the end of a line, like this: