16.1. Iterating with Indexes¶
We have seen that it is easy to loop through every item in a list using the
for value in list:
syntax. But there are limitation to that approach. What if we want to iterate through only
part of the list? What if we want to iterate through the list in reverse order? What
if we want to change items in the list? The
for value in list: can’t do any of these.
Perhaps most surprising is that the
for value in list: can’t change values in a list.
This is because when
value is naming a value in the list, making a change just changes
value is naming - it does not change the list itself. Watch this codelens program
that attempts to set each item in a list to 0 to see how it does not work:
value takes on the value of each item in
list, but it is essentially working with
a copy of the data. Changing
value does nothing to the actual list!
To make this work, or to change the order we iterate through the list. We could try to do that with something like:
index = 0 <- first item index = 0 <- second item index = 0 <- third item
But we don’t want to type out each line separately and we want to make sure we do not
use too many or too few indexes. We want to use a counting loop that will automatically
generate all the needed indexes. This can be created by using the
range function and
asking it to generate
range( len(list) ). That syntax says: “Get the length of the
list. Then generate the range of numbers from 0 to one less than the length.” Which is
exactly what we need. For the example above,
range( len(list) ) would generate
0, 1, 2, which are the indexes of the three values.
Here is the final version of the more complex loop.
Notice that the variable from the
index, is holding the index of
the current item. To work with the “current item”, we need to index into the list using
list[index] will give us the current value.
A common habit among programmer is to use the variable name
i to stand for “index”.
Although this seems to violate the general rule of “use meaningful names”, because all
programmers know that
i probably means “index” when used in a loop,
i does in
fact have meaning.
This program uses
i as the index variable. In it, we have a list of names that
need to be capitalized. To do this, we loop through the indexes 0-3, access the
value at that index (
capitalize() on it, and then store it
back to the same location in the list
names[i] = ...:
Write an indexed based loop and use it to double each item in the list called
You will need to get the existing value, multiply it by two, and store that value back
into the list.
The following program should loop through all the items in
list. Any values
that are negative should be changed to be 0. After the negative items have been
replaced, we want to print the list.
Arrange and indent the blocks correctly. You will not use them all.