Questions:

What kinds of data do programs store?
How can I convert one type to another?

Objectives:

Explain key differences between integers and floating point numbers.
Explain key differences between numbers and character strings.
Use built-in functions to convert between integers, floating point numbers, and strings.

Keypoints:

Every value has a type.
Use the built-in function type to find the type of a value.
Types control what operations can be done on values.
Strings can be added and multiplied.
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.
Convert numbers to strings or vice versa when operating on them.
You can mix integers and floats freely in operations.
Variables only change value when something is assigned to them.

Every value has a type.

Every value in a program has a specific type.
Integer (int): represents positive or negative whole numbers like 3 or -512.
Floating point number (float): represents real numbers like 3.14159 or -2.5.
Character string (usually called "string", str): text.
- Written in either single quotes or double quotes (as long as they match).
- The quote marks aren't printed when the string is displayed.

Use the built-in function `type` to find the type of a value.

Use the built-in function type to find out what type a value has.
Works on variables as well.
- But remember: the value has the type --- the variable is just a label.

print(type(52))

<class 'int'>

fitness = 'average'
print(type(fitness))

<class 'str'>

Types control what operations (or methods) can be performed on a given value.

A value's type determines what the program can do to it. For example, you can subtract two integers but not two strings.

print(5 - 3)

2

print('hello' - 'h')

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/5q/mny3pg2n7h5g21h3v32rfj9wpykqrf/T/ipykernel_11287/13340790.py in <module>
----> 1 print('hello' - 'h')

TypeError: unsupported operand type(s) for -: 'str' and 'str'

You can use the "+" and "*" operators on strings.

"Adding" character strings concatenates them.

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)

Ahmed Walsh

Multiplying a character string by an integer N creates a new string that consists of that character string repeated N times.

separator = '=' * 10
print(separator)

==========

You can 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'
print(atom_name[0])

h

Use a slice to get a substring.

A slice is a part of a string
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.
You can slice any list-like thing and we will come back to slicing several times in this course

atom_name = 'sodium'
print(atom_name[4:6])

um

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

print(len('helium'))

6

Nested functions are evaluated from the inside out, just like in mathematics.
len is a built-in function because it is always available - you do not need to import a particular library to access it
Numbers do not have a length

print(len(52))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/5q/mny3pg2n7h5g21h3v32rfj9wpykqrf/T/ipykernel_11287/2246900780.py in <module>
----> 1 print(len(52))

TypeError: object of type 'int' has no len()

Convert numbers to strings or vice versa when operating on them.

You cannot add numbers and strings.

print(1 + '2')

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/5q/mny3pg2n7h5g21h3v32rfj9wpykqrf/T/ipykernel_11287/3905401405.py in <module>
----> 1 print(1 + '2')

TypeError: unsupported operand type(s) for +: 'int' and 'str'

This is not allowed because it's ambiguous: should 1 + '2' be 3 or '12'?
Some types can be converted to other types by using the type name as a function.

print(1 + int('2'))
print(str(1) + '2')

3
12

You can mix integers and floats freely in operations.

Integers and floating-point numbers can be mixed in arithmetic.
Python 3 automatically converts integers to floats as needed.

print('half is', 1 / 2.0)
print('three squared is', 3.0 ** 2)

half is 0.5
three squared is 9.0

Variables only change value when something is assigned to them.

If we make one cell in a spreadsheet depend on another, and update the latter, the former updates automatically.
This does not happen in programming languages.

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.
The value of second is not updated when we re-assign the value of first.

Tip: By now we have seen plenty of Error messages. Don’t be afraid of Error messages - they won’t break the computer! In the next lesson we will learn how to use these messages to fix our code more efficiently.

Do the quick-test.

Back to Python part one.