For loops

Usage in Python

When do I use for loops?

for loops are traditionally used when you have a block 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, executing the block each time. Contrast the for statement with the ''while'' loop, used when a condition needs to be checked each iteration, or to repeat a block 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 block of code you want to run x number of times, then a block of 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.

for x in xrange(1, 11):
for y in xrange(1, 11):
print '%d * %d = %d' % (x, y, x*y)
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 immediately drop out of the loop and contine execution at the first statement after the block. You can also have an optional else clause, which will run should the for loop exit cleanly - that is, without breaking.

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

range vs xrange

The ''range'' is seen so often in for statements that you might think range is part of the for syntax. It is not: it is a Python built-in function which returns a sequence, which meets the requirement of providing a sequence for the for statement to iterate over. In Python 2.x, range generates the entire sequence when called, while xrange is a generator - it produces values on demand, not all up fromt. You will often see xrange is used much more frequently than range. This is for one reason only - resource usage. For large sequences, the difference in memory usage can be considerable. xrange uses less memory, 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. For Python 3.x, range was changed, you can think of it as being equivalent to the Python 2.x xrange, which no longer is defined in Python 3.x.

Examples

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
Loop over 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 (Python 2)

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

Python Keywords

Keywords are the reserved words in Python.

We cannot use a keyword as a variable name, function name or any other identifier. They are used to define the syntax and structure of the Python language.

In Python, keywords are case sensitive.

There are 33 keywords in Python 3.3. This number can vary slightly in course of time.

All the keywords except,TrueFalse and areNone in lowercase and they must be written as it is. The list of all the keywords is given below.

Keywords in Python programming language
False	class	finally	is	return
None	continue	for	lambda	try
True	def	from	nonlocal	while
and	del	global	not	with
as	elif	if	or	yield
assert	else	import	pass
break	except	in	raise

Looking at all the keywords at once and trying to figure out what they mean might be overwhelming.

If you want to have an overview, here is the complete list of all the keywords with examples.

Python Identifiers

The identifier is the name given to entities like class, functions, variables etc. in Python. It helps to differentiate one entity from another.

Rules for writing identifiers

Identifiers can be a combination of letters in lowercase (a to z) or uppercase (A to Z) or digits (0 to 9) or an underscore (_). Names
like,myClass var_1and,print_this_to_screen all are a valid example.
An identifier cannot start with a digit. 1variable is invalid, but isvariable1 perfectly fine.

Keywords cannot be used as identifiers.


>>> global = 1
  File "<interactive input>", line 1
    global = 1
           ^
SyntaxError: invalid syntax

We cannot use special symbols like !, @, #, $, % etc. in our identifier.


>>> a@ = 0
  File "<interactive input>", line 1
    a@ = 0
     ^
SyntaxError: invalid syntax

The identifier can be of any length.

Things to Python care about

Python is a case-sensitive language. This

means, Variableand arevariable not the same. Always name identifiers that make sense.

While c = 10is valid. Writing wouldcount = 10 make more sense and it would be easier to figure out what it does even when you look at your code after a long gap.

Multiple words can be separated using an

underscore,.this_is_a_long_variable

We can also use the camel-case style of writing, i.e.,

capitalize every first letter of the word except the initial

word without any spaces. For example: camelCaseExample

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for loop in python