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Variables & Data Types

Variables

In Python, we store values using names called variables. We can assign a variable with an = sign:

max_coverage = 6000
minCoverage = 35
antibiotic = 'Streptomycin'
antibiotic2 = 'Penicillin'
You will note a few things about variables:

  • can incorporate letters, digits and underscores
  • cannot start with a digit
  • these are case sensitive

Variables, once we assign them to some value, can be passed into functions to accomplish certain tasks. Functions, generally speaking, take in some input and spit out some output. Let's use the simplist use case, the print() function:

print('The maximum coverage is ', max_coverage)

The maximum coverage is  6000

Here the function print() took in two character values and printed a combined string of words.

Note

variables are available to use between blocks. However, the order in which you run blocks matters so make sure to run your code blocks in order!

Data Types

  • integer: a positive/negative whole number (34, -675)
  • float: a floating point number (4.67, -2034.67)
  • string: a character string written with either single or double quotes ('Streptomycin', "antibiotic")
  • bool: a TRUE/FALSE value

So you have a variable, how do you determine the type? Well we can use the type() function:

type(max_coverage)

int

If you want to convert between data types you can specify with the following functions:

  • int(): to convert to an integer
  • float(): to convert to a floating point number
  • str(): to convert to a string

Calculations

You can use Python like a calculator using the following symbols:

Operator Name Example
+ Addition x + y
- Subtraction x - y
* Multiplication x * y
/ Division x / y
% Modulus x % y
** Exponentiation x ** y
// Floor division x // y

Let's try an few example:

35 / 7 - 5 + 4 * 4 + 2**2

20.0

We note that Python calculations follow the order of operations when performing a calculation. We should also bring up two non-standard operations that you may or may not be familiar with: Modulus and Floor division. Modulus is the remainder after division so:

7 % 2

1

Floor division is a division operation for which you round the result down to a whole number:

7 // 2

3

Strings & Operators

You can use + and * with string data as well to add and multiply, take for instance:

antibiotic + antibiotic

'StreptomycinStreptomycin'

antibiotic * 4

'StreptomycinStreptomycinStreptomycinStreptomycin'

Indexing

Unlike the other data types, strings have lengths. We can use the len() function to check how long a string is:

print(antibiotic)
len(antibiotic)

'Streptomycin'
12

We can slice strings if needed to! However, the letters you are grabbing are zero-indexed meaning that the first letter is letter 0, the second letter is letter 1, and so on:

antibiotic[0]

'S'

antibiotic[1]

't'

We can grab more letters using the format [start:stop]:

antibiotic[1:5]

'trep'

Comments

When assigning variables we can add descriptions to our code to give our code context. We do this by writing our description after a # symbol:

# creating a variable for time of day
time_of_day = 'Morning'

Everything after the # is not processed as Python code even within a code block in a Jupyter notebook.