This lesson is being piloted (Beta version)

Variables and Assignment

Overview

Teaching: 10 min
Exercises: 10 min
Questions
  • How can I store data in programs?

Objectives
  • Write programs that assign scalar values to variables and perform calculations with those values.

  • Correctly trace value changes in programs that use scalar assignment.

Use variables to store values.

age = 42
first_name = 'Ahmed'

Use print to display values.

print(first_name, 'is', age, 'years old')
Ahmed is 42 years old

Variables must be created before they are used.

print(last_name)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-c1fbb4e96102> in <module>()
----> 1 print(last_name)

NameError: name 'last_name' is not defined

Variables Persist Between Cells

Be aware that it is the order of execution of cells that is important in a Jupyter notebook, not the order in which they appear. Python will remember all the code that was run previously, including any variables you have defined, irrespective of the order in the notebook. Therefore if you define variables lower down the notebook and then (re)run cells further up, those defined further down will still be present. As an example, create two cells with the following content, in this order:

print(myval)
myval = 1

If you execute this in order, the first cell will give an error. However, if you run the first cell after the second cell it will print out 1. To prevent confusion, it can be helpful to use the Kernel -> Restart & Run All option which clears the interpreter and runs everything from a clean slate going top to bottom.

Variables can be used in calculations.

age = age + 3
print('Age in three years:', age)
Age in three years: 45

Use an index to get a single character from a string.

atom_name = 'helium'
print(atom_name[0])
h

Use a slice to get a substring.

atom_name = 'sodium'
print(atom_name[0:3])
sod

Use the built-in function len to find the length of a string.

print(len('helium'))
6

Python is case-sensitive.

Use meaningful variable names.

flabadab = 42
ewr_422_yY = 'Ahmed'
print(ewr_422_yY, 'is', flabadab, 'years old')

Swapping Values

Fill the table showing the values of the variables in this program after each statement is executed.

# Command  # Value of x   # Value of y   # Value of swap #
x = 1.0    #              #              #               #
y = 3.0    #              #              #               #
swap = x   #              #              #               #
x = y      #              #              #               #
y = swap   #              #              #               #

Solution

# Command  # Value of x   # Value of y   # Value of swap #
x = 1.0    # 1.0          # not defined  # not defined   #
y = 3.0    # 1.0          # 3.0          # not defined   #
swap = x   # 1.0          # 3.0          # 1.0           #
x = y      # 3.0          # 3.0          # 1.0           #
y = swap   # 3.0          # 1.0          # 1.0           #

These three lines exchange the values in x and y using the swap variable for temporary storage. This is a fairly common programming idiom.

Predicting Values

What is the final value of position in the program below? (Try to predict the value without running the program, then check your prediction.)

initial = 'left'
position = initial
initial = 'right'

Solution

'left'

The initial variable is assigned the value 'left'. In the second line, the position variable also receives the string value 'left'. In third line, the initial variable is given the value 'right', but the position variable retains its string value of 'left'.

Challenge

If you assign a = 123, what happens if you try to get the second digit of a via a[1]?

Solution

Numbers are not strings or sequences and Python will raise an error if you try to perform an index operation on a number. In the next lesson on types and type conversion we will learn more about types and how to convert between different types. If you want the Nth digit of a number you can convert it into a string using the str built-in function and then perform an index operation on that string.

a = 123
print(a[1])
TypeError: 'int' object is not subscriptable
a = str(123)
print(a[1])
1

Choosing a Name

Which is a better variable name, m, min, or minutes? Why? Hint: think about which code you would rather inherit from someone who is leaving the lab:

  1. ts = m * 60 + s
  2. tot_sec = min * 60 + sec
  3. total_seconds = minutes * 60 + seconds

Solution

minutes is better because min might mean something like “minimum” (and actually is an existing built-in function in Python that we will cover later).

Slicing practice

What does the following program print?

atom_name = 'carbon'
print('atom_name[1:3] is:', atom_name[1:3])

Solution

atom_name[1:3] is: ar

Slicing concepts

  1. What does thing[low:high] do?
  2. What does thing[low:] (without a value after the colon) do?
  3. What does thing[:high] (without a value before the colon) do?
  4. What does thing[:] (just a colon) do?
  5. What does thing[number:some-negative-number] do?
  6. What happens when you choose a high value which is out of range? (i.e., try atom_name[0:15])

Solutions

  1. thing[low:high] returns a slice from low to the value before high
  2. thing[low:] returns a slice from low all the way to the end of thing
  3. thing[:high] returns a slice from the beginning of thing to the value before high
  4. thing[:] returns all of thing
  5. thing[number:some-negative-number] returns a slice from number to some-negative-number values from the end of thing
  6. If a part of the slice is out of range, the operation does not fail. atom_name[0:15] gives the same result as atom_name[0:].

Key Points

  • Use variables to store values.

  • Use print to display values.

  • Variables persist between cells.

  • Variables must be created before they are used.

  • Variables can be used in calculations.

  • Use an index to get a single character from a string.

  • Use a slice to get a substring.

  • Use the built-in function len to find the length of a string.

  • Python is case-sensitive.

  • Use meaningful variable names.