Variables and Assignment

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I store data in programs?

Objectives

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

• Correctly trace value changes in programs that use 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 building.

(Adams was ready for occupancy December 2, 1938. So we’ll say it’s 81. )

building = 'Adams'
age = 81

• 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)

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(building, 'is', age, 'years old')

Adams is 81 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(num_floors)

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

NameError: name 'num_floors' is not defined

• The last line of an error message is usually the most informative.

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 121 to age a few lines ago.

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

Age in three years: 84

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

• The characters (individual letters, numbers, and so on) in a string are ordered. For example, the string ‘AB’ is not the same as ‘BA’. Because of this ordering, we can treat the string as a list of characters.
• Each position in the string (first, second, etc.) is given a number. This number is called an index or sometimes a subscript.
• Indices are numbered from 0 rather than 1.
• Use the position’s index in square brackets to get the character at that position.

(My example is for Field guide to the wildlife of Costa Rica)

call_num = 'QL228.C8 H46 2002'
print(call_num[0])

Q

Use a slice to get a substring.

• A part of a string is called a substring. A substring can be as short as a single character.
• A slice is a part of a string (we’ll learn about slices of other objects soon).
• We take a slice by using [start:stop], where start is replaced with the index of the first element we want and stop is replaced with the index of the element just after the last element we want.
• Taking a slice does not change the contents of the original string. Instead, the slice is a copy of part of the original string.

print(call_num[0:4])

QL228

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

print(len('Laura'))

7

• Nested functions are evaluated from the inside out, just like in mathematics.

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

Slicing

What does the following program print?

lib_name = 'Library of Congress'
print('lib_name[1:3] is:', lib_name[1:3])

1. What does lib_name[low:high] do?
2. What does lib_name[low:] (without a value after the colon) do?
3. What does lib_name[:high] (without a value before the colon) do?
4. What does lib_name[:] (just a colon) do?
5. What does lib_name[number:negative-number] do?

Solution

library_name[1:3] is: ib

1. It will slice the string, starting at the low index and ending an element before the high index
2. It will slice the string, starting at the low index and stopping at the end of the string
3. It will slice the string, starting at the beginning on the string, and ending an element before the high index
4. It will print the entire string
5. It will slice the string, starting the number index, and ending a distance of the absolute value of negative-number elements from the end of the string

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.