6장 Loop with while and for
Table of contents
- Loop with while and for.
- Repeat with while
- Cancel with break
- Skip Ahead with continue
- Check break Use with else - Iterate with for and in
- Cancel with break
- Skip with continue
- Check break Use with else
- Generate Number Sequences with range() - Other Iterators - Coming Up - Things to Do
- Loop with while and for.
CHAPTER 6 Loop with while and for
For a’ that, an’ a’ that, Our toils obscure, an’ a’ that ...
—Robert Burns, For a’ That and a’ That
Testing with if, elif, and else runs from top to bottom. Sometimes, we need to do
something more than once. We need a loop, and Python gives us two choices: while
and for.
Repeat with while
The simplest looping mechanism in Python is while. Using the interactive inter‐
preter, try this example, which is a simple loop that prints the numbers from 1 to 5:
>>> count = 1
>>> while count <= 5:
... print (count)
... count += 1
...
1
2
3
4
5
>>>
We first assigned the value 1 to count. The while loop compared the value of count
to 5 and continued if count was less than or equal to 5. Inside the loop, we printed the
value of count and then incremented its value by one with the statement count += 1.
Python goes back to the top of the loop, and again compares count with 5. The value
of count is now 2 , so the contents of the while loop are again executed, and count is
incremented to 3.
87
This continues until count is incremented from 5 to 6 at the bottom of the loop. On
the next trip to the top, count <= 5 is now False, and the while loop ends. Python
moves on to the next lines.
Cancel with break
If you want to loop until something occurs, but you’re not sure when that might hap‐
pen, you can use an infinite loop with a break statement. This time, let’s read a line of
input from the keyboard via Python’s input() function and then print it with the first
letter capitalized. We break out of the loop when a line containing only the letter q is
typed:
>>> while True:
... stuff = input("String to capitalize [type q to quit]: ")
... if stuff == "q":
... break
... print (stuff.capitalize())
String to capitalize [type q to quit]: test
Test
String to capitalize [type q to quit]: hey, it works
Hey, it works
String to capitalize [type q to quit]: q
>>>
Skip Ahead with continue
Sometimes, you don’t want to break out of a loop but just want to skip ahead to the
next iteration for some reason. Here’s a contrived example: let’s read an integer, print
its square if it’s odd, and skip it if it’s even. We even added a few comments. Again, we
use q to stop the loop:
>>> while True:
... value = input("Integer, please [q to quit]: ")
... if value == 'q': # quit
... break
... number = int(value)
... if number % 2 == 0: # an even number
... continue
... print (number, "squared is", number*number)
...
Integer, please [q to quit]: 1
1 squared is 1
Integer, please [q to quit]: 2
Integer, please [q to quit]: 3
3 squared is 9
Integer, please [q to quit]: 4
Integer, please [q to quit]: 5
5 squared is 25
88 | Chapter 6: Loop with while and for
Integer, please [q to quit]: q
>>>
Check break Use with else
If the while loop ended normally (no break call), control passes to an optional else.
You use this when you’ve coded a while loop to check for something, and breaking as
soon as it’s found. The else would be run if the while loop completed but the object
was not found:
>>> numbers = [1, 3, 5]
>>> position = 0
>>> while position < len(numbers):
... number = numbers[position]
... if number % 2 == 0:
... print ('Found even number', number)
... break
... position += 1
... else : # break not called
... print ('No even number found')
No even number found
This use of else might seem nonintuitive. Consider it a break
checker.
Iterate with for and in
Python makes frequent use of iterators, for good reason. They make it possible for
you to traverse data structures without knowing how large they are or how they are
implemented. You can even iterate over data that is created on the fly, allowing pro‐
cessing of data streams that would otherwise not fit in the computer’s memory all at
once.
To show iteration, we need something to iterate over. You’ve already seen strings in
Chapter 5, but have not yet read the details on other iterables like lists and tuples
(Chapter 7) or dictionaries (Chapter 8). I’ll show two ways to walk through a string
here, and show iteration for the other types in their own chapters.
Iterate with for and in | 89
It’s legal Python to step through a string like this:
>>> word = 'thud'
>>> offset = 0
>>> while offset < len(word):
... print (word[offset])
... offset += 1
t
h
u
d
But there’s a better, more Pythonic way:
>>> for letter in word:
... print (letter)
t
h
u
d
String iteration produces one character at a time.
Cancel with break
A break in a for loop breaks out of the loop, as it does for a while loop:
>>> word = 'thud'
>>> for letter in word:
... if letter == 'u':
... break
... print (letter)
t
h
Skip with continue
Inserting a continue in a for loop jumps to the next iteration of the loop, as it does
for a while loop.
Check break Use with else
Similar to while, for has an optional else that checks whether the for completed
normally. If break was not called, the else statement is run.
| **90 | Chapter 6: Loop with while and for** |
This is useful when you want to verify that the previous for loop ran to completion
instead of being stopped early with a break:
>>> word = 'thud'
>>> for letter in word:
... if letter == 'x':
... print ("Eek! An 'x'!")
... break
... print (letter)
... else :
... print ("No 'x' in there.")
t
h
u
d
No 'x' in there.
As with while, the use of else with for might seem nonintuitive. It
makes more sense if you think of the for as looking for something,
and else being called if you didn’t find it. To get the same effect
without else, use some variable to indicate whether you found
what you wanted in the for loop.
Generate Number Sequences with range()
The range() function returns a stream of numbers within a specified range. without
first having to create and store a large data structure such as a list or tuple. This lets
you create huge ranges without using all the memory in your computer and crashing
your program.
You use range() similar to how to you use slices: range( start , stop , step ). If you
omit start , the range begins at 0. The only required value is stop ; as with slices, the
last value created will be just before stop. The default value of step is 1 , but you can
go backward with -1.
Like zip(), range() returns an iterable object, so you need to step through the values
with for … in, or convert the object to a sequence like a list. Let’s make the range
0, 1, 2:
>>> for x in range(0,3):
... print (x)
...
0
1
2
>>> list( range(0, 3) )
[0, 1, 2]
Iterate with for and in | 91
Here’s how to make a range from 2 down to 0 :
>>> for x in range(2, -1, -1):
... print (x)
2
1
0
>>> list( range(2, -1, -1) )
[2, 1, 0]
The following snippet uses a step size of 2 to get the even numbers from 0 to 10 :
>>> list( range(0, 11, 2) )
[0, 2, 4, 6, 8, 10]
Other Iterators
Chapter 14 shows iteration over files. In Chapter 10, you can see how to enable itera‐
tion over objects that you’ve defined yourself. Also, Chapter 11 talks about itertools
—a standard Python module with many useful shortcuts.
Coming Up
Chain individual data into lists and tuples.
Things to Do
6.1 Use a for loop to print the values of the list [3, 2, 1, 0].
6.2 Assign the value 7 to the variable guess_me, and the value 1 to the variable num
ber. Write a while loop that compares number with guess_me. Print ‘too low’ if num
ber is less than guess me. If number equals guess_me, print ‘found it!’ and then
exit the loop. If number is greater than guess_me, print ‘oops’ and then exit the loop.
Increment number at the end of the loop.
6.3 Assign the value 5 to the variable guess_me. Use a for loop to iterate a variable
called number over range(10). If number is less than guess_me, print ‘too low’. If it
equals guess_me, print found it! and then break out of the for loop. If number is
greater than guess_me, print ‘oops’ and then exit the loop.