An alternative way of implementing loops is by using a for
loop. for
loops tends to be easier and safer to use than while
loops, but cannot
be applied to all iteration problems. while
loops are more general. In
other words, everything that a for
loop can do, a while
loop can do
too, but not the other way around.
The syntax of a for
loop is as follows:
for <variable> in <collection>:
<statements>
A for
loop gets presented with a collection of items, and it will
process these items, in order, one by one. Every cycle through the loop
will put one item in the variable given next to the for
, and can then
be used in the code block under the for
. The variable does not need to
exist before the for
loop is encountered. If it does, it gets
overwritten. It is a real variable, by the way, in the sense that it
still exists after the loop has finished. It will contain the last value
that it got assigned during the processing of the loop.
At this point you might wonder what a “collection” is. There are many different kinds of collections in Python, and in this section I will introduce a few. In later chapters collections will be discussed in more detail.
for
loop with stringsThe only collection introduced until now is the string. A string is a
collection of characters, e.g., the string "banana"
is a collection of
the characters "b", "a", "n", "a", "n"
, and "a"
, in that specific
order. The following code loops through each of these letters:
for letter in "banana":
print( letter )
print( "Done" )
While this code is fairly trivial, let’s go through it step by step (I
did not make a flow chart, as that is not easy for for
loops).
When the for
loop is encountered, Python takes the collection (i.e.,
the string "banana"
) and turns it into an “iterable.” What that is
exactly I will get to in Chapter
241,
but for now assume that it is a list of all the letters in the string,
in the order that they appear in the string. Python then takes the first
of those letters, and puts it in the variable letter
. It then executes
the code block below the for
.
The code block contains only one statement, which is the printing of
letter
. So the program prints “b,” and then loops back to the for
.
Python then takes the next letter, which is an “a,” and it executes the
code block with letter
being "a"
. It then repeats this process for
each of the remaining letters. Once all the letters have been used, the
for
loop ends, and Python executes the last line of the code, which is
the printing of the word “Done.”
To be absolutely clear: In a for
loop you do not have to write code
that explicitly increases some kind of variable that then grabs the next
letter, or something like that. The for
statement handles that
automatically: every time it is looped back to, it takes the next item
from the collection.
for
loop using a variable as collectionIn the code above, the literal string "banana"
was used as the
collection, but it could also be a variable that contains a string. For
instance, the following code runs similar to the previous code:
fruit = "banana"
for letter in fruit:
print( letter )
print( "Done" )
You might wonder if this isn’t dangerous. What happens if the programmer
changes the contents of the variable fruit
in the loop’s code block?
You can try this out using the following code:
fruit = "banana"
for letter in fruit:
print( letter )
if letter == "n":
fruit = "orange"
print( "Done" )
As you can see when you run this code, changing the contents of the
variable fruit
in the loop has no effect on the loop’s processing. The
sequence of characters that the loop processes is only constituted once,
when the for
loop is first entered. Changing the value of fruit
into
"orange"
while the loop is still processing the value "banana"
, does
not stop it from continuing to process "banana"
. This is a great
feature of for
loops, because it means they are guaranteed to end. No
for
loops are endless!12
Note that there is a conditional statement in the loop above. There is nothing that stops you from putting conditions in the code block for a loop. There is also nothing against putting loops in the code block for a condition, or even putting loops inside loops (more on that last option follows later in this chapter). Most readers probably are not surprised to hear that, but for the few who are completely new to programming: as long as you stick to the syntactic requirements, you can use conditional statements and loops wherever you can write Python statements.
for
loop using a range of numbersPython offers a range()
function that generates a collection of
sequential numbers, which is often used for for
loops. The simplest
call to range()
has one parameter, which is a number. It will generate
all integers, starting at zero, up to but not including the parameter.
for x in range( 10 ):
print( x )
range()
can get multiple parameters. If you give two parameters, then
the first will be the starting number (default is zero), while the
second will be the “up to but not including” number. If you give three
parameters, the third will be a step size (default is 1). You can choose
a negative step size if you want to count down. With a negative step
size, make sure that the starting number is higher than the number that
you want to count up to (or down to, in this case).
for x in range( 1, 11, 2 ):
print( x )
Change the three parameters above to observe their effect, until you
fully understand the range()
function.
Use the for
loop and range()
function to print multiples of 3,
starting at 21, counting down to 3, in just two lines of code.
for
loop with manual collectionsIf you want to use a for
loop to cycle through items in a collection
that you create manually, you can do so by listing all your items
between parentheses. This defines a “tuple” for the items of your
collection. Tuples will be discussed in detail in Chapter
123.
for x in ( 10, 100, 1000, 10000 ):
print( x )
Or:
for x in ("apple", "pear", "orange", "banana", "mango", "cherry"):
print( x )
Your collection can even consist of mixed types.
for
loopsTo get strong grips on how to use for
loops, do the following
exercises.
You already created code with a while
loop that asked the user for
five numbers, and displayed their total. Create code for this task, but
now use a for
loop.
Create a countdown function that starts at a certain count, and counts
down to zero. Instead of zero, print “Blast off!” Use a for
loop.
I am not going to ask you to build a for
loop that asks the user to
enter numbers until the user enters zero. Why not?