``for`` Loops ============= If you were to instruct your pet turtle to draw a square in the sand, your commands would quickly become quite tedious. You must tell your pet to move then turn, move then turn, etc. four times. If you want your turtle to draw a hexagon, an octagon, or a polygon with 42 sides, it would be a nightmare to repeat all the commands! .. admonition:: Example Turtle statements for drawing a square: .. sourcecode:: python :linenos: import turtle pet = turtle.Turtle() pet.forward(50) pet.left(90) pet.forward(50) pet.left(90) pet.forward(50) pet.left(90) pet.forward(50) pet.left(90) Note: If you have not practiced with turtles in class yet, check out the :ref:`Python Turtles ` appendix. Time to use iteration! The first Python iteration tool we will learn is the ``for`` loop. ``for`` Loop Syntax ------------------- .. index:: single: loop; for .. index:: ! for loop A **for loop** repeats a specific set of code statements a specific number of times. When a ``for`` loop begins, we must declare exactly how many times it will run. Play around with the basic syntax of a ``for`` loop. .. admonition:: Try It This program prints the integers 0 through 20, one number per line. .. raw:: html #. What do you notice about the numbers printed in the console vs. the value inside ``range()``? #. Try changing the value inside ``range()``. What values throw an error? #. What happens when you indent line 4 to match line 2? Let's break down the syntax piece by piece, so we can begin to understand how ``for`` loops are structured. .. index:: single: for loop; variable #. In line 1, ``num`` is called the **loop variable**. Each time the loop executes, ``num`` gets assigned a new value. #. Line 2 begins the loop body. The loop body is ALWAYS indented. The indentation determines exactly what statements are “in the loop”. You saw this when you indented line 4 to match line 2. Originally, line 4 was outside of the loop and ran only once. By indenting line 4, it becomes part of the loop body and gets repeated multiple times. #. The *first* unindented line after the ``for`` statement marks the end of the loop body. #. The loop body can contain any number of statements. #. The number of times the loop body runs depends on the value in ``range()``. Line By Line ^^^^^^^^^^^^ Let's modify the code just a little to follow the operation of a ``for`` loop. .. admonition:: Example .. sourcecode:: Python :linenos: for num in range(4): print(num) print("Hello" * num) print("Done!") **Console Output** :: 0 1 Hello 2 HelloHello 3 HelloHelloHello Done! #. The first time Python executes the ``for`` statement in line 1, ``num`` is assigned a value of ``0``. #. Next, Python checks if the value of ``num`` is less than the value inside ``range``. Since ``num < 4`` evaluates to ``True``, the loop body executes. #. Line 2 prints the current value of ``num``. #. Line 3 prints the string ``Hello`` zero times. #. Python reaches the end of the loop body (the indented lines). At this point, it increases the value of ``num`` by 1 and then MOVES BACK TO THE ``for`` STATEMENT (line 1). #. The new value of ``num`` (``1``) gets compared to the ``range`` value. Since ``num < 4`` still returns ``True``, the loop body executes again. #. Lines 2 and 3 run with the new value of ``num``, so we see ``1`` and ``Hello`` printed to the console. #. Python again reaches the end of the loop body, increases the value of ``num`` and moves back up to the ``for`` statement. #. This process continues until the value of ``num`` reaches the end of the specified ``range``. Once the comparison ``num < 4`` returns ``False``, the loop ends. Since Python adds 1 after each iteration, this occurs when ``num`` is 4 (so ``4 < 4`` is ``False``). At that point, the loop body will have run exactly 4 times, with ``num`` taking the values 0, 1, 2, and 3. #. Once the loop finishes, Python proceeds to line 5 and prints ``Done!`` one time. We can use a picture to show the *flow of execution* of this ``for`` loop: .. _for-loop-control-flow: .. figure:: figures/for-loop-diagram.png :alt: Diagram showing the flow of a program with a for loop. Flow of execution of a ``for`` loop Notice that even though line 1 uses ``range(4)``, the value ``4`` is NOT included in the output. Why? Begin Counting at 0 ^^^^^^^^^^^^^^^^^^^ .. index:: ! zero-based indexing Iterating a certain number of times is a very common thing to do, and Python gives us the built-in ``range`` keyword to provide a set of values for the loop variable to use. The sequence provided by ``range`` always starts with ``0``. If you ask for ``range(4)``, then you will get 4 values starting with 0. In other words, 0, 1, 2, and finally 3. Notice that 4 is not included since we started with 0. Likewise, ``range(10)`` provides 10 values, 0 through 9. Starting a count at 0 instead of at 1 is called **zero-based indexing** and is very common in computer programming. .. admonition:: Note Programmers like to count from 0! For ``range(n)``, the loop variable will take each integer value from 0 up to BUT NOT INCLUDING ``n``. Check Your Understanding ------------------------ .. admonition:: Question How does python know what lines are contained in the loop body? .. raw:: html

The lines are indented by the same amount from the for statement.
There is always exactly one line in the loop body.
The loop body ends with an empty line.
The loop body ends at the next for statement.

.. Answer = a. .. admonition:: Question How many lines does the following code print? .. sourcecode:: python :linenos: for number in range(10): print("I have", 12 - number, "cookies. I'm going to eat one!") .. raw:: html

.. Answer = c. .. admonition:: Question For the code above, what is the value of ``number`` the *third* time Python executes the loop? .. raw:: html

.. Answer = b. .. admonition:: Question For the same code, what is the LAST line printed by the program? .. raw:: html

I have 2 cookies. I'm going to eat one!
I have 3 cookies. I'm going to eat one!
I have 10 cookies. I'm going to eat one!
I have 12 cookies. I'm going to eat one!

.. Answer = b.