Introduction
Welcome to the wonderful world of python. Python is a very different language than scheme/racket which we have used so far. Almost all of the syntax (or the structure of the language) will be different than how we have done previous.
Below is a more in depth dive into each aspect of the language
Math
Unlike scheme which uses prefix notation for mathematical operations such as (+ 6 8) which would
add 6 and 8 together. Python on the other hand uses infix notation such as 6 + 8
which is much more similar to the mathematical notation you have learned before.
To add, subtract, multiply, and divide in python you would do the following:
# Addition
1 + 2 # = 3
2 + 4 # = 6
# Subtraction
2 - 3 # = -1
3 - 2 # = 1
# Multiplication
-1 * 2 # = -2
3 * 4 # = 12
# Division
2 / 6 # = .33333
6 / 2 # = 3
As well as the usual operators, python has a few others.
In math class or when using a calculator, it is common to represent an exponent with the ^
character ex: 2^6 would represent 2 to the power of 6, but that is not the case in python. The ^
character already has a meaning as the XOR operator, which does not matter for this class yet, but
is important to note that python will not throw out code that uses the ^ character but will just
produce a weird value. Instead, python uses 2 multiplication symbols to denote exponentiation: **.
Some more examples are below.
2 ** 6 # = 64
3 ** 3 # = 27
Sometimes when you divide, you might not care about the decimal or fractional portion of the result
and for that use case, python has the double division // operator for "integer division" or
division without decimals
2 // 16 # = 0
10 // 8 # = 1
And when dividing you might only care about the remainder and for that, python has the modulo %
operator.
6 % 3 # = 0
7 % 3 # = 1
More Math
Sometimes you need more than just the simple operations and for that, python has the math library. The documentation for the math library can be found on the official python docs but python is a very popular language and is very easily google-able. Do not hesitate to use the internet whenever you need to reference some docs or look something up.
In order to use libraries, you need to import them. To import the math library you would do as
such:
import math
Functions from a library can be used by putting the library name, a dot, and then the name of the
function. ex: math.sin() would be the sin function in the math library
Here are a few useful but non-exhaustive list of the functions available from the math library.
# Trig functions
math.sin(math.pi) # = 0
math.cos(math.pi) # = -1
math.tan(math.pi) # = 0
# Rounding functions
math.floor(10.2) # = 10
math.ceil(10.2) # = 11
# Absolute value
math.fabs(-100) # = 100
# Factorial
math.factorial(4) # = 1 * 2 * 3 * 4 = 24
# Logarithms
math.log(4) # = ln(4) = 1.3862943611
math.log(100, 10) # = log_10(100) = 2
math.log10(100) # = log_10(100) = 2
math.log(100, 5) # = log_5(100) = 2.8613531161
Once again, the docs have an exhaustive list of all methods available from the library.
Booleans
Just like in scheme, we can also deal with true/false conditions or booleans. These are also used in python, although their names are capitalized:
True
False
Just like in scheme, we can also do logical operations with the booleans, such as and and or
which are used much like they are in scheme, just in infix notation instead of prefix notation.
# Logical and
True and True # = True
True and False # = False
False and True # = False
False and False # = False
# Logical or
True or True # = True
True or False # = True
False or True # = True
False or False # = False
# Logical not
not True # = False
not False # = True
There is also a less common logical operation called xor or eXclusive OR which you may never
have to use, but is good to know. It will return True if one and only one of the inputs is True
which can be seen below.
True ^ True # = False
True ^ False # = True
False ^ True # = True
False ^ False # = False
Conditionals
When dealing with numbers, we may want to check if some condition holds true. There are 2 different types of conditionals, equality and inequality. You may remember these from your math class, but if not here is a refresher.
Equality is the comparison of 2 exact numbers, such as _ is equal to _ or _ is not equal to _.
Equality can be expressed in python as such:
1 == 2 # = False
6 == 6 # = True
6 != 9 # = True
12 != 12 # = False
Inequality is checking the relation of 1 number with respect to another or _ is greater than _ or
_ is less than _. These operations can be expressed in python as such:
1 > 2 # = False
6 > 6 # = False
6 >= 9 # = False
12 >= 12 # = True
6 < 9 # = True
12 < 12 # = False
1 <= 2 # = True
6 <= 6 # = True
Strings
Similarly to scheme, strings, which are just text, can be created using quotation marks (") aka
double quotes. But unlike scheme python also lets you use apostrophes (') aka single quotes to
surround your text. Both act exactly the same.
"Strings can be defined using double quotes"
'or using single quotes'
When using double quotes, you are allowed to use apostrophes in the text as usual
"they're late"
And when using single quotes, you can use double quotes inside of the text with no problem
'And then he said "Oh no you did not!"'
When you want to use both in one string, things get a bit more complex. The quotes that you write around your string, signal to the language that everything inside of it is text, but when the text contains the same quote you used to tell the language about the text, the language will mistakenly see it as you ending the string.
You can see this in the syntax highlighting (coloring) of the code below changing after that quote, meaning that python would not read this how we wanted.
"They're having a lot of fun and yelling "yippee""
'They're having a lot of fun and yelling "yippee"'
To get around this issue, we can use what is called "escaping quotes". If you want to use the
character that you used to make a string (ie. ' inside a single quoted string or " inside a
double quoted string) you can put a backslash \ character in front of it to tell the language to
treat it as part of the string.
'They\'re having a lot of fun and yelling "yippee"'
"They're having a lot of fun and yelling \"yippee\""
Note the colors staying the same throughout the string. Both strings above are exactly equal, the only difference is what quote we are using.
Sometimes it is useful to combine 2 strings, and python allows us to do so using the + operator.
"Hello" + "world" # = "Helloworld"
We may also want to create a string that has multiple lines of text. We can do so using a multiline strings which are defined using 3 double quotes
"""Hello there
this string
spans across
multiple lines :D"""
You can also use the newline character \n instead to denote multiple lines in a string
"Hello there\nthis string\nspans across\nmultiple lines :D"
The above 2 strings are exactly equal, they are just different ways of writing them
Comments
A lot of the time, code is complicated. Reading code is 2x harder than writing that same code. To help ourselves with our programming, programming languages have comments, or lines/sections of code that are not read by the language, but instead by the programmer. I have been using comments in these notes with notes and information on what is in the code that I am showing you. These comments are denoted by a different, dimmer color than the rest of the code.
Comments in python are denoted using the pound sign # aka hash or hashtag.
# Cool single line comment
Comments can appear after code, but any text after the # is interpreted as a comment.
2 + 5 # Cool comment after code
If you want a comment that spans multiple lines, you can use a multiline comment which will just get ignored when you run the program since you do nothing with it
"""
Weird whacky multiline string acting as a
multiline comment
"""
But it is probably better to just use multiple comments, one per line like a sane person
# Or just use
# Multiple lines of these
# Like a sane human being
Turtle
You cant have a programming lesson without a bit of fun, so here is our fun for this lesson. We get to draw! yaay woot woot.... Just me? ok...
In python, we can use the turtle library. The documentation for the turtle library can be found
on the official python docs Just like with the math library, we have to
import the library to use it.
import turtle
With all good art, we need color, so we can go ahead and set the color of the turtle with the color method
turtle.color('red', 'yellow')
To move the turtle, we have 2 options. We can either move in a direction, ie forward, backward,
or we could move to an exact position.
# Turtle movement
turtle.forward(10)
turtle.left(120)
# Turtle teleportation
turtle.setx(-100)
turtle.sety(100)
The coordinates of the canvas that we can draw on are on the x axis, -180 to 180 and on the y axis, -200 to 200. The center of the screen being 0,0.
We can also stop the turtle from drawing a path, or change its color mid stroke using the functions below.
# Turtle pen control
turtle.penup()
turtle.pencolor('red')
turtle.pendown()
Once again, the docs have an exhaustive list of all methods available from the library.
Remember, python is very google-able, please feel free to google to your heart's content.
Too Long; Didn't Read
If you want an abridged version, you can look at just the code that I have gone over here