Chapter 7. Iteration
This chapter is about iteration, which is the ability to run a block of statements repeatedly. We saw a kind of iteration, using recursion, in “Recursion”. We saw another kind, using a
for loop, in “Simple Repetition”. In this chapter we’ll see yet another kind, using a
while statement. But first I want to say a little more about variable assignment.
As you may have discovered, it is legal to make more than one assignment to the same variable. A new assignment makes an existing variable refer to a new value (and stop referring to the old value).
>>> x = 5 >>> x 5 >>> x = 7 >>> x 7
The first time we display
x, its value is 5; the second time, its value is 7.
Figure 7-1 shows what reassignment looks like in a state diagram.
At this point I want to address a common source of confusion. Because Python uses the equal sign (
=) for assignment, it is tempting to interpret a statement like
a = b as a mathematical proposition of equality; that is, the claim that
b are equal. But this interpretation is wrong.
First, equality is a symmetric relationship and assignment is not. For example, in mathematics, if a=7 then 7=a. But in Python, the statement
a = 7 is legal and
7 = a is not.
Also, in mathematics, a proposition of equality is either true or false for all time. If a=b now, then a will always equal b. In Python, an assignment statement can make two variables equal, but they don’t have to stay that way:
>>> a = 5 >>> b = a # a and b are now equal >>> ...