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A very quick and simple introduction to Python (part 2)

02 Nov 2014

Version 1.0 (last updated on Nov 8th, 2014)

Welcome back to the second part of the quick Python overview. In this section, we will cover the basics of if-else statements, while and for loops and creating methods. Once again, all comments and questions are encouraged and very welcome. Please feel free to email me at camillamon[at]gmail.com.

##2.0 Control-flow in Python programs If-else statements and for and while loops form the core of control-flow in Python programs. Let’s take a brief tour of these structures, starting with the if-else statement.

###2.1 If and else Typically in a more complicated program, we have to take different actions depending on some previous result. For example, suppose we have a number and we want to check whether it is divisible by 5. One very simple way to do this is to use the if-statement.

The syntax of the if-statement in Python is as follows:

if test:
  do something
else:
  do something else

Here is a small example illustrating the concept:

random_number=10
if random_number%5==0:
  print "Divisible by 5"
else:
  print "Not divisible by 5"

Let’s step through the example above line by line. In the first line, we create the number object (‘10’) and give it the name random_number. Next, we want to find out if, the object that the name random_number points to is divisible by 5 or not. In order to do this, we employ the modulus operation, which gives us the remainder of random_number divided by 5. If the remainder is 0 (ie. random_number is divisible by 5), then we will print out “Divisible by 5”.

If you want, run this program using the interactive Python shell or the online Python shell. Which one of the statements is printed to the terminal?

####2.1.1 A more complicated example In the little program above, we only tested for one condition (whether or not the number we stored in random_number is divisible by 5). Usually in real programming life, testing for one condition is not enough for what we aim to achieve. So let’s take our little example one step further.

Suppose that we still want to check whether the number stored in random_number is divisible by 5. If no, we want to check whether it is divisible by 3. This means that our program has to branch into three different ‘logical ‘ branches.

  • random_number is divisible by 5
  • random_number is not divisible by 5 but is divisible by 3
  • random_number is neither divisible by 5 nor divisible by 3

In Python, this could be achieved in the following manner:

random_number=10 #or assign a number of your choice
if random_number%5==0:
  print "The number is divisible by 5!"
elif random_number%3==0:
  print "The number is divisible by 3!"
else:
  print "The number is divisible neither by 5 nor 3 "

##Excercises:

  1. Find out if the year 1044 is a leap year. A year is a leap year, if it is divisible by 4 and 400, but not divisible by a 100.

###2.2 The while loop Now that we are familiar with if, else and elif statements, can take a look at the while loop. The while loop executes while some condition is true and is especially useful if we want to execute a block of code repeatedly. Let’s illustrate this with a simple example. Suppose we want to print out all of the numbers from 1 to 10.

number=1   #the initial number
while number<11:  
  print number
  number+=1

As we can see from the example above, a while statement is declared with the following syntax

while test:
  do something

The loop will keep executing until the test becomes false. In the little number printing example, the test in the while statement evaluates whether the number object referenced by number is less than 11. If yes, the statements inside the while block are executed.

####A Word of Warning: Do not write infinite loops! When writing your first while loops, it’s easy to forget to make sure that the loop terminates. What would happen if we leave out the statement number+=1 from the while loop we wrote above?

###Exercises:

  1. Write a small program that checks the numbers from 1 to 25 and prints only those that are divisible by 5

###2.3 For loops The for ‘loop is a close cousin of the while loop. It is design to iterate (or step through) items for example in a list or string. Let’s look at the general syntax of the for loop.

#general syntax for a for-loop
for element in object:
  execute code here

The for loop begins with a header similar to do that of the while loop. There is one key difference, though. The header for the for-loop also includes something called an assignment target which we called element in the script above.

You can think of the assignment target as a box. When we loop through an iterable object such as a list, every element takes a turn jumping into the box. While the element ‘lives in the box’, we can carry out operations on it.

If all of this seems nebulous right now, do not worry! We will make all of this concrete by working through a for loop example.

We are given a list of elements (these may be strings or numbers of a mixture of both) and we have to print out each element.

random_elements=["apple", "Jack", 12, 1+8, "athlete"]

#let's print out each element using the for-loop
for element in random_elements:
  print element

"""
#The output should be :
apple
Jack
12
9
athlete
"""

It does not matter what we call the assignment target. Instead of element, we could have called it word or chocolatebar or even simply x.

Now it’s time to write your own for-loop.

###Exercises:

  1. Searching for a string in a list: You are given the following list (copy and paste this into your Python shell or text editor)
strings=['absb', 'hello', 'hghgtjk', 'apples', 'icecream']

Use a for-loop to iterate through the list. When you encounter the string “apples”, print out the words “I found apples” on the console.

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