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.

Python is a programming Language

• Bash is another programming language, used for the shell
• Like Bash, python operates on the read-evaulate-print loop (REPL). When you enter an expression, python reads your code, evaulates and executes it, prints the output, and then loops back to the next line or block of code.

Python can be used as a simple calculator

• The usual mathematical operations are supported (we will go into more detail on this later)
• Two main kinds of number: int for integers, and float for numbers with decimals.
• Exponents use **

In [1]: 1 + 3
Out[1]: 4

In [2]: 4 * 5
Out[2]: 20

In [3]: 1/2
Out[3]: 0.5

In [4]: 2**3
Out[4]: 8

• The usual order of operations are obeyed

In [5]: 6/2 + 3 * 5**2
Out[5]: 78.0

• You can also type arbitrary sequences of characters in quotes, called strings.
• You can perform operations on those strings

In [6]: 'hello'
Out[6]: 'hello'

In [7]: 'hello' + 'world'
Out[7]: 'helloworld'

In [8]: 3 * 'hello'
OUt[8]: 'hellohellohello'

• Operations are generally intuitive.
• Some do not work because they do not make sense (we will discuss this more here)

In [9]: 'hello' + 3

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-c9c9952ff2b4> in <module>
----> 1 'hello' + 3

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

• This is the first of many errors we will encounter in python.
• The last line of an error message is usually the most informative.
• We will look at error messages in detail later.

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 quotes to a variable first_name.

In [1]: age = 42

In [2]: first_name = 'Ahmed'

• Variable names
  • can only contain letters, digits, and underscore _ (typically used to separate words in long variable names)
  • cannot start with a digit
• Variable names that start with underscores like __alistairs_real_age have a special meaning so we won’t do that until we understand the convention.

Use print to display values in the midst of your script.

• 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 or double quotes.
• The values passed to the function are called arguments

In [3]: 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.
• Also note the lack of quotes around the strings.

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.)

In [1]: last_name

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-e1aeda7b4fde> in <module>
----> 1 last_name

NameError: name 'last_name' is not defined

Variables can be used in calculations.

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

age = age + 3
age

45

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.
• Use the position’s index in square brackets to get the character at that position.

atom_name = 'helium'
atom_name[0]

'h'

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.
• An item in a list is called an element. Whenever we treat a string as if it were a list, the string’s elements are its individual characters.
• A slice is a part of a string (or, more generally, any list-like thing).
• 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.
• Mathematically, you might say that a slice selects [start:stop).
• The difference between stop and start is the slice’s length.
• Taking a slice does not change the contents of the original string. Instead, the slice is a copy of part of the original string.

atom_name = 'sodium'
atom_name[0:3]

'sod'

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

len('helium')

6

• Nested functions are evaluated from the inside out, 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.

• 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.

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
a[1]

TypeError: 'int' object is not subscriptable

a = str(123)
a[1]

'2'

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.

• Values are displayed as output.

• Use print to render strings.

• 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.