Control Flow

Control flow are the parts of a language that make it do things differently based upon various conditions. In other words: ifs and loops. Python has all of the basic control flow statements that you'd expect.

if Statements

If statement should be familiar to anyone with a passing knowledge of programming. The idea of an if is that you want your script to execute a block of statements only if a certain condition is true. For example, this script won't do anything.

x = 15

if x < 10:

   print "this will never show"

You can use the if...else form of an if statement to do one thing if a condition is true, and something else if the condition is false. This script will print out "this will show!"

x = 15

if x < 10:

   print "this will never show"

else:

   print "this will show!"

Lastly, you can use the if...elif form. This form combines multiple condition checks. "elif" stands for "else if". This form can optionally have a catch-all "else" clause at the end. For example, this script will print out "three":

x = 3

if x == 1:

   print "one"

elif x == 2:

   print "two"

elif x == 3:

   print "three"

else:

   print "not 1-3"

while Loops

A while loop will repeat a block of statements while a condition is true. This code will print out the contents of the items in the list. This code uses a function called len, which is a built-in function that returns the length of a list or string.

listOfFruit = ['Apples', 'Oranges', 'Bananas']

x = 0

while x < len(listOfFruit):

   print listOfFruit[x]

   x = x + 1

for Loops

Python's for loop may be a bit different than what you're used to if you've programmed any C. The for loop is specialized to iterate over the elements of any sequence, like a list. So, we could re-write the example above using a for loop eliminating the counter x:

listOfFruit = ['Apples', 'Oranges', 'Bananas']

for item in listOfFruit:

   print item

Much more graceful! You'll often see the for loop used instead of the while loop, even when you simply want to iterate a given number of times. To do this with the for loop, you can use the built-in function range. The range function returns a variable-size list of integers starting at zero. Calling range(4) will return the list [0, 1, 2, 3]. So, to have a for loop repeat 4 times, you simply can do:

for x in range(4):

   print "this will print 4 times"

break and continue in Loops

You can stop a loop from repeating in its tracks by using the break statement. This code will print out "Loop" exactly two times, and then print "Finished".

for x in range(10):

   if x >= 2:

      break

   print "Loop"

print "Finished"

You can use the continue statement to make a loop stop executing its current iteration and skip to the next one. The following code will print out the numbers 0-9, skipping 4

for x in range(10):

   if x == 4:

      continue

   print x