Variables and Assignment

Overview

Teaching: 15 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.

• Variables are names for values.
• In Python the = symbol assigns the value on the right to the name on the left.
• The variable is created when a value is assigned to it.
• Here, Python assigns an age to a variable age and a name in quotation marks to a variable first_name.

age = 42
first_name = 'Ahmed'

• Variable names:
  • cannot start with a digit
  • cannot contain spaces, quotation marks, or other punctuation
  • may contain an underscore (typically used to separate words in long variable names)
• Underscores at the start like __alistairs_real_age have a special meaning so we won’t do that until we understand the convention.

Use print to display values.

• Python has a built-in function called print that prints things as text.
• Call the function (i.e., tell Python to run it) by using its name.
• Provide values to the function (i.e., the things to print) in parentheses.
• To add a string to the printout, wrap the string in single quotations.
• The values passed to the function are called ‘arguments’

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

Ahmed is 42 years old

• print automatically puts a single space between items to separate them.
• And wraps around to a new line at the end.

Variables must be created before they are used.

• If a variable doesn’t exist yet, or if the name has been mis-spelled, Python reports an error.
  • Unlike some languages, which “guess” a default value.

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

• The last line of an error message is usually the most informative.
• We will look at error messages in detail later.

Variables Persist Between Cells

Variables defined in one cell exist in all other cells once executed, so the relative location of cells in the notebook do not matter (i.e., cells lower down can still affect those above). Remember: Notebook cells are just a way to organize a program: as far as Python is concerned, all of the source code is one long set of instructions.

Variables can be used in calculations.

• We can use variables in calculations just as if they were values.
  • Remember, we assigned 42 to age a few lines ago.

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

Age in three years: 45

Variables can be re-assigned

• A variable’s value can be replaced with another value by using the assignment operator again.

first = 1
second = 5 * first
first = 2
print('first is', first, 'and second is', second)

first is 2 and second is 5

• The computer reads the value of first when doing the multiplication, creates a new value, and assigns it to second.
• After that, second does not remember where it came from.

Python is case-sensitive.

• Python thinks that upper- and lower-case letters are different, so Name and name are different variables.
• There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.

Use meaningful variable names.

• Python doesn’t care what you call variables as long as they obey the rules (alphanumeric characters and the underscore).

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

• Use meaningful variable names to help other people understand what the program does.
• The most important “other person” is your future self.

Use comments to add documentation to programs.

# This sentence isn't executed by Python.
adjustment = 0.5   # Neither is this - anything after '#' is ignored.
print(adjustments)

0.5

Swapping Values

Draw a table showing the values of the variables in this program after each statement is executed. In simple terms, what do the last three lines of this program do?

x = 1.0
y = 3.0
swap = x
x = y
y = swap

Solution

swap = x  #  x->1.0 y->3.0 swap->1.0
x = y     #  x->3.0 y->3.0 swap->1.0
y = swap  #  x->3.0 y->1.0 swap->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"

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 does in Python, but we haven’t seen that yet).

What Did You Have For Dinner Last Night?

In a new notebook cell create variables that contain relevant data regarding the “what, where, and when” of last night’s dinner, then include those variables in a print() command as part of a full sentence.

Paste the output into the EitherPad, so we can all vicariously enjoy your meal.

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.

• Use comments to add documentation to programs.