Embedded Files
When we drew the square, it was quite tedious. We had to move then turn, move then turn, etc. etc. four times. If we were drawing a hexagon, or an octogon, or a polygon with 42 sides, it would have been a nightmare to duplicate all that code.
A basic building block of all programs is to be able to repeat some code over and over again. In computer science, we refer to this repetitive idea as iteration. In this section, we will explore some mechanisms for basic iteration.
In Python, the for statement allows us to write programs that implement iteration. As a simple example, let’s say we have some friends, and we’d like to send them each an email inviting them to our party. We don’t quite know how to send email yet, so for the moment we’ll just print a message for each friend.
Take a look at the output produced when you press the run button. There is one line printed for each friend. Here’s how it works:
- name in this
forstatement is called the loop variable. - The list of names in the square brackets is called a Python list. Lists are very useful. We will have much more to say about them later.
- Line 2 is the loop body. The loop body is always indented. The indentation determines exactly what statements are “in the loop”. The loop body is performed one time for each name in the list.
- On each iteration or pass of the loop, first a check is done to see if there are still more items to be processed. If there are none left (this is called the terminating condition of the loop), the loop has finished. Program execution continues at the next statement after the loop body.
- If there are items still to be processed, the loop variable is updated to refer to the next item in the list. This means, in this case, that the loop body is executed here 7 times, and each time name will refer to a different friend.
- At the end of each execution of the body of the loop, Python returns to the
forstatement, to see if there are more items to be handled.
Flow of Execution of the for Loop
Flow of Execution of the for Loop
As a program executes, the interpreter always keeps track of which statement is about to be executed. We call this the control flow, or the flow of execution of the program. When humans execute programs, they often use their finger to point to each statement in turn. So you could think of control flow as “Python’s moving finger”.
As a program executes, the interpreter always keeps track of which statement is about to be executed. We call this the control flow, or the flow of execution of the program. When humans execute programs, they often use their finger to point to each statement in turn. So you could think of control flow as “Python’s moving finger”.
Control flow until now has been strictly top to bottom, one statement at a time. We call this type of control sequential. Sequential flow of control is always assumed to be the default behavior for a computer program. The for statement changes this.
Control flow until now has been strictly top to bottom, one statement at a time. We call this type of control sequential. Sequential flow of control is always assumed to be the default behavior for a computer program. The for statement changes this.
Flow of control is often easy to visualize and understand if we draw a flowchart. This flowchart shows the exact steps and logic of how the for statement executes.
Flow of control is often easy to visualize and understand if we draw a flowchart. This flowchart shows the exact steps and logic of how the for statement executes.
A codelens demonstration is a good way to help you visualize exactly how the flow of control works with the for loop. Try stepping forward and backward through the program by pressing the buttons. You can see the value of name change as the loop iterates through the list of friends.
Iteration Simplifies our Turtle Program
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.
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.
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.
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 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 causesaColor to change to the next value in the list.
In this case, the value of aColor is used to change the color attribute of alex. Each iteration causesaColor to change to the next value in the list.
Mixed up program
turtle-6-1: 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.
Mixed up program
turtle-6-2: 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.
Page updated
Google Sites
Report abuse