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Variables

Overview

Teaching: 0 min
Exercises: 60 min
Questions

  • How do we store data in variabes?

Objectives

  • Be able to create variables

  • Understand that different types of variables store different kinds of information

  • Be able to perform simple arithmatic operations on variables

Creating Variables

If we want to, we can essentially use Python as a fancy calculator. Type the command below into the Spyder console and press `Enter’:

5 + 7

You should see the result returned as output in the console:

12

Here’s another example. If I type:

636/8

I should get back the output

79

However, this isn’t very efficient. The real value of using Spyder/Python for something like this is that we can store that value in a variable and use it again later.

Imagine that we have computed colony area in pixels from one of our plate images and we want to convert it to actual area in square mm. Based on the size properties of our image or some kind of scale marker, we have figured out the number of pixels. That corresponds to a square mm of area. We can create variables for each of these values and use them to do calculations.

Type the commands below in the console, hitting Enter after each line.

AreaInPixels = 6929
mm2PerPixel = 0.0277
AreaInPixels*mm2PerPixel

This will return the value of the last calculation:

191.9333

If we think that we will need to use this value later, we can also store it in a variable:

AreaInMm = AreaInPixels * mm2PerPixel

Notice that this doesn’t give us any output! If we want to print the value of AreaInMm to the console, we can get it by simply calling the variable name:

AreaInMm

Returns the output

191.9333

The assignment operator, =

A variable is just a name for a value, like x or AreaInMm. We assign a variable name to a value by using the assignment operator, =. This is not the same as saying that the variable name equals the value. Later, when we use test for equality, we will use a different operator (==).

The variable name is just a convenient handle that we assign to, and can use to refer to, a value. You can change the value assigned to a variable at any time, just by reassigning it.

This code

mm2PerPixel = 0.03
mm2PerPixel

Returns the output

0.03

The value of mm2PerPixel has been updated.

Now, check the value of areaInPixels

AreaInPixels

It still has the same value:

6929

The value for mm2PerPixel changed, but the value for AreaInPixels did not! If we imagine the variable as a sticky note with a name written on it, assignment is like putting the sticky note on a particular value:

This means that assigning a value to one variable does not change the values of other variables. Since AreaInPixels doesn’t remember where it came from, it isn’t automatically updated when mm2PerPixel changes. This is different from the way spreadsheets work.

Mathematical operations

Python uses all of the usual mathematical operators to perform operations on numbers and variables. You have already seen addition several

Returns the output

36838.39164889

remainder = 10.0%8
remainder

Returns the output

2.0

Different types of variables store different types of data

Python lets us make different “types” of variables. So far you have seen ints (short for integers, numbers without decimal points) and floats (floating point numbers, numbers with decimal points). Python also allows you to make variables that are strings (strings of text, very useful in Biology when dealing with sequence data) and other types of variables that we will introduce you to later.

name = 'Erin'
word = 'image'

You can use many of the same operators on text strings that you can on floats or ints:

name + word

Returns the output

'Erinimage'

name * 2

Returns the output

'ErinErin'

However, not all operators are supported for all combinations of variables:

name + remainder

Will result in an error message

Traceback (most recent call last):

  File "<ipython-input-17-5359e87c9757>", line 1, in <module>
    name + remainder

TypeError: Can't convert 'int' object to str implicitly

Since Python doesn’t know exactly how to add a word to a number, it returns an error message. Error messages are usually very descriptive, and can tell you a lot about problems in your code!

Good practices- variable names

Variables should be given descriptive names. Python requires that variable names must begin with a letter. They are also case sensitive (Text, text, and tEXT are all different variables). Beyond this, Python doesn’t care what you call your variables.
You could name them all after different kinds of fruit if you want! However, this would be very confusing when you went back later to look at your code.

You should do your best to give your variables descriptive, useful names that describe the data they hold.

DIVAS variable name conventions

In the DIVAS project, variables should start with a lowercase letter, and capitalize the first letter of each successive word (“camel case”).

For example, plateArea, or colonyCount.

Examples/Exercises

Variable Switching

Draw diagrams showing what variables refer to what values after each statement in the following program:

avgCount = 47.5
time = 122
avgCount = avgCount * 2.0
time = time - 24

Solution

On line 1 of the code, the value 47.5 is assigned to the variable avgCount. Similarly, line 2 assigns 122 to the variable name time.

On line 3, avgCount is reassigned. This line of code takes the value stored in avgCount (currently 47.5), multiplies it by 2, and then stores the new value 95.0 to the variable name avgCount. The effect is that the value stored in avgCount is updated to be 95.0.

The 4th line similarly takes the old value of time, subtracts 24, and reassigns the variable name time to point to this new value (98).

Sorting Out References

What does the following program print out? Why?

first, second = 'Plate302', 'Ecoli'
third, fourth = second, first
print(third, fourth)

Solution

If you run the code above, you should get printed output that looks like this:

Ecoli Plate302

To understand what is happening, you can think of the first line of code as storing the value “Plate302” somewhere in your computer’s memory and then assigning it the handle first. At the same time, it stores the value “Ecoli” and assigns it the handle second. Then, the second line of code creates new handles pointing to those same values. So third points at the same value/memory location as second (“Ecoli”) and fourth points at the same value/location as first (“Plate302”).

Variable Switching

Given the following two variables

density1 = 4.59
density2 = 3.28

write code to “switch” the values so that the final result is that

density1 = 3.28
density2 = 4.59

Solution

The trick here is to create a temporary variable so that the values stored are not “lost”

temp1 = density1
density1 = density2
density2 = temp1
print(density1, density2)

Key Points

  • Creating variables allows you to “reuse” a number or value in multiple locations in your code

  • Python stores different kinds of information (text, numbers, decimal numbers, and others) as different “types” of variables

  • You should give your variables descriptive names! This will make your code easier to read!

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