For loops

Usage in Python

• When do I use for loops?

for loops are traditionally used when you have a piece of code which you want to repeat a fixed number of times. The Python for statement iterates over the members of a sequence in order. Contrast the for statement with the WhileLoop, used when a condition needs to be checked each iteration, or to repeat a piece of code forever. For example:

For loop from 0 to 2, therefore running 3 times.

for x in range(0, 3):
    print "We're on time %d" % (x)

While loop from 1 to infinity, therefore running forever.

x = 1
while True:
    print "To infinity and beyond! We're getting close, on %d now!" % (x)
    x += 1

As you can see, these loop constructs serve different purposes. The for loop runs for a fixed amount - in this case, 3, while the while loop runs until the loop condition changes; in this example, the condition is the boolean True which will never change, so it could theoretically run forever. You could use a for loop with a huge number in order to gain the same effect as a while loop, but what's the point of doing that when you have a construct that already exists? As the old saying goes, "why try to reinvent the wheel?".

• How do they work?

If you've done any programming before, you have undoubtedly come across a for loop or an equivalent to it. Many languages have conditions in the syntax of their for loop - such as a relational expression to determine if the loop is done, and an increment expression to determine the next loop value. In Python this is controlled instead by generating the appropriate sequence. Basically, any object with an iterable method can be used in a for loop. Even strings, despite not having an iterable method - but we'll not get on to that here. Having an iterable method basically means that the data can be presented in list form, where there are multiple values in an orderly fashion. You can define your own iterables by creating an object with next() and iter() methods. This means that you'll rarely be dealing with raw numbers when it comes to for loops in Python - great for just about anyone!

• Nested loops

When you have a piece of code you want to run x number of times, then code within that code which you want to run y number of times, you use what is known as a "nested loop". In Python, these are heavily used whenever someone has a list of lists - an iterable object within an iterable object.

• Early exits

Like the while loop, the for loop can be made to exit before the given object is finished. This is done using the break statement, which will stop the code from executing any further. You can also have an optional else clause, which will run should the for loop exit cleanly - that is, without breaking.

Things to remember

• range vs xrange

The range function creates a list containing numbers defined by the input. The xrange function creates a number generator. You will often see that xrange is used much more frequently than range. This is for one reason only - resource usage. The range function generates a list of numbers all at once, where as xrange generates them as needed. This means that less memory is used, and should the for loop exit early, there's no need to waste time creating the unused numbers. This effect is tiny in smaller lists, but increases rapidly in larger lists as you can see in the examples below.

Examples

Nested loops

for x in xrange(1, 11):
    for y in xrange(1, 11):
        print '%d * %d = %d' % (x, y, x*y)

Early exit

for x in xrange(3):
    if x == 1:
        break

For..Else

for x in xrange(3):
    print x
else:
    print 'Final x = %d' % (x)

Strings as an iterable

string = "Hello World"
for x in string:
    print x

Lists as an iterable

collection = ['hey', 5, 'd']
for x in collection:
    print x

Lists of lists

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]
for list in list_of_lists:
    for x in list:
        print x

Creating your own iterable

class Iterable(object):

    def __init__(self,values):
        self.values = values
        self.location = 0

    def __iter__(self):
        return self

    def next(self):
        if self.location == len(self.values):
            raise StopIteration
        value = self.values[self.location]
        self.location += 1
        return value

range vs xrange

import time

#use time.time() on Linux

start = time.clock()
for x in range(10000000):
    pass
stop = time.clock()

print stop - start

start = time.clock()
for x in xrange(10000000):
    pass
stop = time.clock()

print stop - start

Time on small ranges

import time

#use time.time() on Linux

start = time.clock()

for x in range(1000):
    pass
stop = time.clock()

print stop-start

start = time.clock()
for x in xrange(1000):
    pass
stop = time.clock()

print stop-start

Your own range generator using yield

def my_range(start, end, step):
    while start <= end:
        yield start
        start += step

for x in my_range(1, 10, 0.5):
    print x