Operations and Variables

Operations on Strings and Numeric Types

int operations

As you'd expect: addition, subtraction and multiplication →

• add - +
• subtract - -
• multiply *

Division is Special

• the division operator is /
• what do you think 5/2 gives →
• what type is the result? →

>>> 5/2
2.5
>>> type(5/2)
<class 'float'>
>>> 

>>> 12/4
3.0
>>> type(12/4)
<class 'float'>
>>> 

We Don't Need No Decimal Points

• the integer division operator is // (double forward slash)
• it will always give back an integer: 21//10 → 2
• if the result is a float, it will go to the nearest integer to the left on the number line (or… more simply put, just round down)!
• what will 5//2 give back?
• what will -5//2 give back?

>>> 5//2
2
>>> -5//2
-3

Modulo Operator (Remainder)

• the remainder operator is % (the percent symbol)
• what would 5%2 give back? →

>>> 5%2
1

Exponentiation

• the exponentiation operator is ** (two asterisks)
• what do we get for 2**2? →

>>> 2**2
4

Order of Operations

• as you would expect (remember PEMDAS?)
• what does 12 + 10 / 5 give? →

>>> 12 + 10 / 5
14.0

(Parentheses)

• you can use parentheses to group expressions…
• this will affect odrer of operations
• what will (6 + 4) * 5 return? →

>>> (6 + 4) * 5
50

You Could Always Use It As a Calculator

Translate this formula into Python code →

• take the fraction 9 over 5 and multiply it by 37… then add 32

(9 / 5) * 37 + 32

str Operations

Multiplication!?

• the multiplication operator is *
• it returns a new string with the original string duplicated the number of times specified by the operand on the right side
• what would "hey" * 3 return? →

>>> "hey" * 3
'heyheyhey'

String Concatenation

• use the + to stitch together strings
• it's basically just adding strings together
• example: print("knocked " + "my" + "socks" + "off") →
• why are the words all smashed together?
• how would we fix it?
• let's try different types! - does it work? →
  • with an int? try: 3 + " blind mice" →
  • with a float?
  • what kind of error are we getting?

Let's repeat: string concatenation requires both operands to be strings

Let's Fix It!

You can change from one type to another using functions of the same name as the type you are trying to convert to. Let's look at what autocomplete says about the following functions and demo them. →

• int(), str(), and float()
• unlike print, these functions return a value to your program!
• this is called type conversion or casting

And This Helps… How?

Let's fix our previous string concatenation →

3 + " blind mice"

(of course, this is contrived, since we know that 3 is an int, and we could have just wrapped it in quotes: '3'… but imagine if we didn't know the type beforehand!)

>>> str(3) + " blind mice"
'3 blind mice'

Review

• what operators do we use for exponentiation and integer division?
• what does -10//3 give back?
• what's an easy way of creating a string that has 100 exclamation points in it?
• what's string concatenation?

A Quick Summary of Operators for Numeric Types

Name as many operators that work on numeric types as you can: →

A Quick Summary of String Operators

Name as many operators that work on strings as you can: →

Variables

What's a Variable?

• variable - name that refers to a value
• this terminology is important; very specific… name and value
• we can now use that name instead of the explicit value
• sometimes you'll hear me say the string "literal" - representation of a value within a program… i mean… the thing in quotes

So How Do Variables Actually Work?

some_variable_name = "a value"

• this is an assignment statement - binds a value to a name
• the equals sign assignment token - the "operator" that we use to bind a name to a value
  • name on left
  • value on right
  • eeezy…. just like maths

Another Aside - Interactive Shell

Whenever you type something in the interactive shell, it will always return a value. →

• a value returns a value
• a variable returns a value
• a function can return a value
• if a function doesn't actually return a value, like print, the resulting value will be None (NoneType)

Some More Miscellaneous Comments

• how can I tell if something is printed vs a value is returned?
  • returning a string shows the string in quotes, while printing displays the string without quotes
  • confusing for other types, just know that each line gives back a value to your program, and consequently that value gets shown after each line is entered
• another btw, metasyntactic variables:
  • typical to use nonsense names such as foo, bar, baz as variable names in examples
  • don't use these names, though; use more meaningful names when you write programs!

Variables That Aren't Defined Yet

• what happens when you use a variable that doesn't exist? →

>>> foo
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    foo
NameError: name 'foo' is not defined
>>> 

More About Reassignment

• you can reassign or rebind
• let's see that in action →

>>> a = 23
>>> a
23
>>> a = "foo"

Naming Variables

• you can make them as long as you want… though I suppose it could crash your computer
  • what's an easy way to create a long variable name? →
  • btw, you can autocomplete in the shell by using tab →
• names can only consist of alphanumeric (numbers and letters) characters and the underscore
• the first character has to be a letter or an underscore
• case sensitive - case matters
• the name can't be a keyword or reserved word

Am I a Valid Name?

Which of the following are valid variable names in Python?

1. _foo
2. 1_foo
3. foo
4. 1foo
5. $foo

Let's Actually Use Some Variables

Try the following on your own: →

• create a variable called exclaim, set it eqaul to "!!!" and print out the variable
• create 2 variables, length and width…
  • set them both equal 25 and 8 respectively
  • multiply both variables and and store the result in a variable called area
  • print out the result

User Input