# 4.6. Iteration Simplifies our Turtle Program¶

To draw a square we’d like to do the same thing four times — move the turtle forward some distance and turn 90 degrees. We previously used 8 lines of Python code to have alex draw the four sides of a square. This next program does exactly the same thing but, with the help of the for statement, uses just three lines (not including the setup code). Remember that the for statement will repeat the forward and left four times, one time for each value in the list.

While “saving some lines of code” might be convenient, it is not the big deal here. What is much more important is that we’ve found a “repeating pattern” of statements, and we reorganized our program to repeat the pattern. Finding the chunks and somehow getting our programs arranged around those chunks is a vital skill when learning How to think like a computer scientist.

The values [0,1,2,3] were provided to make the loop body execute 4 times. We could have used any four values. For example, consider the following program.

In the previous example, there were four integers in the list. This time there are four strings. Since there are four items in the list, the iteration will still occur four times. aColor will take on each color in the list. We can even take this one step further and use the value of aColor as part of the computation.

In this case, the value of aColor is used to change the color attribute of alex. Each iteration causes aColor to change to the next value in the list.

The for-loop is our first example of a compound statement. Syntactically a compound statement is a statement. The level of indentation of a (whole) compound statement is the indentation of its heading. In the example above there are five statements with the same indentation, executed sequentially: the import, 2 assignments, the whole for-loop, and wn.exitonclick(). The for-loop compound statement is executed completely before going on to the next sequential statement, wn.exitonclick().

Mixed up program

The following program uses a turtle to draw a triangle as shown to the left:

But the lines are mixed up. The program should do all necessary set-up and create the turtle. After that, iterate (loop) 3 times, and each time through the loop the turtle should go forward 175 pixels, and then turn left 120 degrees. After the loop, set the window to close when the user clicks in it.

Drag the blocks of statements from the left column to the right column and put them in the right order with the correct indention. Click on <i>Check Me</i> to see if you are right. You will be told if any of the lines are in the wrong order or are incorrectly indented.

        import turtle
---
wn = turtle.Screen()
marie = turtle.Turtle()
---
# repeat 3 times
for i in [0,1,2]:
---
marie.forward(175)
---
marie.left(120)
---
wn.exitonclick()


Mixed up program

The following program uses a turtle to draw a rectangle as shown to the left:

But the lines are mixed up. The program should do all necessary set-up and create the turtle. After that, iterate (loop) 2 times, and each time through the loop the turtle should go forward 175 pixels, turn right 90 degrees, go forward 150 pixels, and turn right 90 degrees. After the loop, set the window to close when the user clicks in it.

Drag the blocks of statements from the left column to the right column and put them in the right order with the correct indention. Click on <i>Check Me</i> to see if you are right. You will be told if any of the lines are in the wrong order or are incorrectly indented.

        import turtle
wn = turtle.Screen()
carlos = turtle.Turtle()
---
# repeat 2 times
for i in [1,2]:
---
carlos.forward(175)
---
carlos.right(90)
---
carlos.forward(150)
carlos.right(90)
---
wn.exitonclick()