Step 9: Repeating Instructions¶
CS20-CP1 Apply various problem-solving strategies to solve programming problems throughout Computer Science 20.
CS20-CP2 Use common coding techniques to enhance code elegance and troubleshoot errors throughout Computer Science 20.
CS20-FP2 Investigate how control structures affect program flow.
Tutorial¶
Often, we will find that we want to repeat a series of instructions a
fixed number of times. For example, in Step 8, you had to tell Reeborg to do the same thing over and over again. There is a way in Python to do this … but it
has too many new concepts to explain right now. I will just show you
the code, and immediately introduce repeat, a simpler replacement
for it, unique to Reeborg’s World. The standard way
is known as a for loop (which we will use later in the course) and is written as follows:
for i in range(n):
# some
# instructions
# here
In Reeborg’s World, we can write a repeat loop as follows:
repeat n: # where n is a natural number, like 3 or 7
# some
# instructions
# here
Note
Using repeat will not work in Python programs meant to be
run outside of Reeborg’s World.
A loop is a block of instructions that is repeated. Each time the loop is repeated is called an iteration, so the code below would have 4 iterations.
The following code will make Reeborg trace a square:
repeat 4:
move()
turn_left()
By using repeat, we can rewrite some function definitions without
having to repeat instructions:
def turn_right():
repeat 3:
turn_left()
You might want to adjust the code that you have saved in the library tab to use the repeat block.
For educators
The rationale for having repeat as an addition to Python’s
standard notation is to avoid having to introduce 4 concepts
at the same time (loops, variables as in _ in for _ in range(n),
builtin functions like range as well as the concept of
function arguments).
By design the n in repeat n must be an integer literal;
it cannot be a variable. When students learn about variables, they
should learn the proper Python syntax to do loops and forget about
the non-standard repeat.
Your Turn¶
Open Step 9 on the Reeborg environment.
Adjust the solution you created for Step 8, this time making use of at least two repeat loops. The world doesn’t change from Step 8, but your solution should be much nicer. Remember to use comments and whitespace to increase the readability of your solution!
repeat, if, def