Variables and Decisions¶

Learning Goals:¶

After this week, the student will be able to:

Declare variables
Do simple maths in Python
Write programs using if and else statements
Generate random numbers in python

Introduction¶

This course will teach you how to write code in the Python programming language. For our purposes code is instructions that computers can follow to complete tasks and solve problems for us.

Hello world!¶

To get started with coding, it is customary to write a “Hello world!” program, a program that prints the words “Hello world!” to the screen. In Python, this program looks like this:

print("Hello world!")

Hello world!

To write and run this program, you first need to open a code editor like jupyter lab (or Spyder or repl.it/languages/Python3), then create a file with a suitable name(something like hello.py or hello.ipynb for notebooks) in a folder you can find later. Write print("Hello world!") in your file, and finally run your program.

The “Hello world” program contains two new and useful concepts:

The first is the string "Hello world!" inside the parentheses. In programming, strings are one of the basic data types used to store information. Strings are used to store text. To create one you need to surround the text you want to store with quotes like this: "This is a string".

The second thing is the function print(). Whatever you put inside the parentheses will be written in the console. Nearly every single program in this course will include the print() function, we use it whenever we want our program to output some text. You will become familiar with many other functions during this course, and even make your own. Functions take some input inside the parentheses, and perform some action.

Variables¶

Variables are used to store numbers and text among other things. Variables are useful beacuse we often want to save useful information in our programs in order to access it later. You declare a variable by giving it a name and a value. Let’s start by storing the number 5 in the variable a.

a = 5

Now the variable with the name a holds the value 5. You can try running print(a) to see what happens.

print(a)

The number 5 was printed to the console, since printing the variable a prints its value and not its name. You can create as many variables as you want, and you can name them almost anything.

Extra: Names you can’t use for variables

A variable name can’t be a number, a special character or an important Python keyword like print. If a keyword is used for something else, it changes color in some editors, examples of such keywords are if, else, sum and type. Don’t use these names for your variables, as you’ll overwrite useful functions. Try naming a variable print and see what happens when you then try to print something.

You can think of a variable as being an adress in your computer memory, and at that adress you store some value. When you want to access a value, you use its address to find it.

There are different types of variables that you should familiarize yourself with:

Integers and Floats¶

In Python, whole numbers are stored as integers and decimal numbers are stored as floats. They are declared in the same way though, so you often don’t need to worry about the distinction. When doing calculations in Python, you can think of integers and floats as just being numbers. This is not the case in all other languages though.

To declare an integer, do as before and assign a whole number to a variable. To declare a float, assign a decimal number to a variable.

thisNumber = 42
otherNumber = 2.718

You can add, subtract, multiply and divide in Python like this

total = 5 + 1
difference = 8 - 1
product = 13 * 2
quotient = 21 / 3

To see the actual results we can print the value of the variables

print(total)
print(difference)
print(product)
print(quotient)

You can use variables as if they were numbers when doing calculations:

print(product - total)

There are other operations you can do in Python as well. Exponents are done with two multiplication signs. You can also take a square root by raising a number to the power of 0.5.

exponents = 2**3
print(exponents)
print(25**0.5)

8
5.0

Parentheses work just like in regular maths.

NOTE: Remember to put an operator between numbers and parentheses though, or Python will treat the number like a function and your program won’t run.

parentheses = 5  * (2 + 3)
print(parentheses)

This won’t work: parentheses = 5(2 + 3), neither will this: result = thisNumber(2 + 4), because Python tries to use thisNumber as a function instead of a variable when you use parentheses like this.

In-Class Number Exercises¶

a) Store the result of 1 + 1 in a variable and print it.

b) Store two different numbers in two variables. Add the variables together and print out the result.

c) Do calculations using all of the operators and print out the results.

Strings¶

Let’s declare some strings. Remember that you need to surround text with quotes when declaring strings. Single, double and triple quotes will all work.

first = 'Stephen'
middle = "John"
last = '''Fry'''

You can combine strings with +, and make strings repeat by multiplying them with whole numbers,

print(first + middle +  last)
print(first * 5)
print(first + " " + middle + " " + last)

StephenJohnFry
StephenStephenStephenStephenStephen
Stephen John Fry

Strings can contain almost any text, as long as you surround it with quotes.

myText = "Spaces   numbers 2020 'other types of quotes'"
five = "5"
print(myText)
print(five * 2)

Spaces   numbers 2020 'other types of quotes'
55

A problem arises if your text contains quotation marks. print("He cried: "Oh no"") creates the string "He cried: " and the empty string "", and a mess in between that ruins the code. To avoid this problem, python uses the escape character \ - placing this escape character before a special character (like \n, which creates a linebreak) renders it a normal character in the string: print("He cried: \"Oh no\""). You could also use a different type of quotation marks on the outside: print('He cried: "Oh no"').

A simple way of printing strings and numbers together is to use a comma between them.

print("The best number is", 43 * 3, "without a doubt")

The best number is 129 without a doubt

Better printing:¶

A much better way of adding numbers or variables in the middle of text is to use f-strings (the f is for “format”). Put an f in front of the quotation marks, and use curly brackets where you want to add numbers or variables.

print(f"The best number is {43 * 3} without a doubt")

The best number is 129 without a doubt

first = "James"
last = "Bond"
print(f"My name is {last}, {first} {last}.")

My name is Bond, James Bond.

You can also use f-strings to round numbers, which can be useful when trying to make results more readable. To round a number to two decimals, add :.2f at the end of the curly bracket with a number. You can change the 2 to include more or fewer decimals. By adding :.0f after a number you can add it to the string without any decimals.

print(f"The best number is {22/7} without a doubt, or maybe {22/7:.2f}, or maybe even {22/7:.0f}")

The best number is 3.142857142857143 without a doubt, or maybe 3.14, or maybe even 3

If you want to print the name and value of a variable, it can be done like this:

number = 42
print(f"{number = }")

number = 42

In-Class String Exercises¶

a) Store your name in a string. Using this string, make the computer print out that you’re doing a good job.

b) Store you age as an integer. Using this integer, print out that it is your age.

c) Store the value 5.555 in a variable called myFloat. Using string formatting, print the value of myFloat with one decimal.

Changing a variable¶

If you assign two different values to a variable with the same name, the last value is the one that will be stored.

b = "Text"
b = 40 #b now stores an integer, and not the string "Text"
print(b)

You can assign the value of one variable to another.

a = 5
b = a #Now b = 5
a = 17 #This changes a, but not b
print(b)

Comments¶

To write good code you should write good comments when needed. You comment your code by writing # and then your comment anywhere in your code. Comments are ignored by the computer when you run your code. Commenting your code to clarify what it does to yourself and others who might read it is a good practice. You should only add comments when it is needed for clarity, otherwise you only end up making the code less readable.

myName = "Ola Nordmann" #This is a comment
myNumber = 6224

print(myName * 2, myNumber * 2) #The first multiplication makes the string repeat itself, the second doubles the number stored in myNumber

#print(myName + myNumber) this line has been "commented out", so the code here won't be run. If you were you run this code you would get an error anyway, since you can't add a string and integer

Ola NordmannOla Nordmann 12448

In-Class Variable Changing Exercises¶

a) Define a variable myNum with the value 10 and a variable yourNum with the value 42. Use a third variable temp to switch the values of myNum and yourNum.

b) What would the output of this code be?

a = 10
b = 7

a = b
b = a + 5
#b = a + b

print(a)
print(b)

Decisions¶

We often want our program to do different things depending on the state of the program. Much like how we might want to bet different amounts in poker depending on the quality of our hand.

To make our programs make decisions in Python, we use if and else statements.

When the computer reaches an if statement, it checks whether the logical statement is true or not. If it is, the code below the if statement that is indented is run. If not, the indented code is skipped. Indented code is code with blank space to its left. We normally use the “tab” key to indent code.

myNumber = 5

if myNumber > 3:
    print("Hello!")

if myNumber > 8:
    print("Goodbye!")

Hello!

We can expand this program with an else statement. The indented code under the else statement is run whenever the if statement above “fails”.

myNumber = 5

if myNumber > 8:
    print("The number was bigger than 8")
else:
    print("The number was not bigger than 8")

The number was not bigger than 8

We can also use strings in the logical statements in the if statements:

name = "John"

if name == "Kant":
    print("I'm sorry, but I Kant remember your name, what was it again?")
else:
    print(f"Hello {name}.")

Hello John.

We can also put if statement inside other if or else statements. These nested if statements work just like the ones we just looked at, but the program reaches them only if the outside if statement “succeeded”.

name = "John"
age = 31

if name == "Kant":
    if age == 297:
        print("Guten tag mein mann!")
    else:
        print("I'm sorry, but I Kant remember your name, what was it again?")
else:
    print(f"Hello {name}")

Hello John

In-Class Decisions Exercises¶

a) Write a program that checks whether an integer variable is greater than 10.

b) Write a program which greets children with the phrase “Hey kid”, and adults by their name.

c) Write a program that uses a nested if statement.

Elif statements and larger decision blocks¶

We can expand this program again by combining an if statement and an else statement into an elif statement. The elif statement is run when an if statement right above it fails.

When you have multiple decisions chained together like this, the final else only “succeeds” when all prevous ifs and elifs failed or were skipped. Only an if can start such a chain, and when you reach an else the chain is stopped.

The trolley problem is a thought experiment where a trolley is about to run over five people on the main track, but you can divert it to a side track where it will run over only one person.

Let’s write a program that chooses what action to take depending on which action saves the most lives.

mainTrack = 5
sideTrack = 1

if mainTrack > sideTrack:
    print("Change the tracks!")
elif mainTrack == sideTrack:
    print("I honestly don't care what you do.")
else:
    print("Don't change the tracks!")

Change the tracks!

We can also put if statement inside other if or else statements. These nested if statements work just like the ones we just looked at, but the program reaches them only if the outside if statement “succeeded”.

Let’s write a program which does the same as last time, only now it can also choose to blow up the trolley if that saves the most lives.

mainTrack = 5
sideTrack = 8
passengers = 4

if mainTrack > sideTrack:
    if sideTrack > passengers:
        print("Blow up the trolley!")
    else:
        print("Switch the tracks!")
        
elif mainTrack == sideTrack == passengers:
    print("I honestly don't care what you do.")
    
else:
    if mainTrack > passengers:
        print("Blow up the trolley!")
    else:
        print("Don't switch the tracks!")

Blow up the trolley!

1. First the variables mainTrack, sideTrack and passengers are declared

2. Then the program reaches the first if statement. mainTrack is not greater than sideTrack, so the program skips the indented code under the first if statement.

3. Then it reaches an elif statement. mainTrack, sideTrack and passengers are not all equal, so this elif statement “fails”. The indented code is skipped and the program reaches an else sentence.

4. Everything on the level of this else statement failed, so the indented code under it is run.

5. There it reaches an if statement. mainTrack is greater than passengers, so the if statement succeeds and the indented code is run. “Blow up the trolley!” is printed.

6. Finally, the last else is reached and since the previous if succeeded, it is skipped. The program has now reached the end.

This example brings up an important question when writing programs that make decisions in the real world. Should a program choose whether a self driving car should sacrifice a passenger or a pedestrian if it comes down to it during a potential collision? And if so, what should it choose? Should this decision be made by the car manufacturers?

Logical statements and booleans¶

We often compare numbers in if and elif statements. These are the five different logical operators you can use to compare numbers:

Let a = 5

a < 5 returns False
a > 5 returns False
a == 5 returns True (equal, you NEED two equal signs)
a <= 5 returns True (less than or equal)
a >= 5 returns True
a != 5 returns False (not equal)

You can store the value of these logical statemens in variables called booleans. Booleans can hold the values True or False and can be used for logical operations. Python uses the and, or and not keywords for logical operations.

sunny = True
ownSunscreen = False

if sunny and not ownSunscreen:
    print("Go buy some sunscreen!")

Go buy some sunscreen!

True and True -> True
True or True -> True
True or False -> True
not True -> False
not False -> True
True and False -> False
False and False -> False
False or False -> False

In-Class Decisions With `elif` Exercises¶

a) Write a program that is nice to people called “Alice” and rude to people called “Bob”, while other people are given a standard greeting.

b) Write a program that makes a decision in the trolley problem, such that the most people are killed.

c) Try comparing two strings using the < operator. Write a program that compares two words and prints out the one which comes first alphabetically (only use lower case letters).

Random number generation¶

By using built-in functionality in your computer and clever maths, Python can generate psuedorandom numbers. They’re not actually random numbers, since they’re generated from a starting number using a sequence of equations. But psuedorandom numbers are still distributed like random numbers, which is what we care about(therefore, we will just be calling them random numbers from now on). To generate them we need to import the randint() function like this:

from random import randint

Python includes a lot of functionality right out of the box, but we often need even more functionality. To access this functionality, we import modules. In this case, we imported the randint function from the random module so that we can generate random numbers. Now we can use this function freely in our program, much like how we would use the print() function.

The randint() function takes two arguments, a starting number and an ending number. The function then returns a random between the starting number and the ending number. What that means is that randint(1, 6) can return any of the numbers 1, 2, 3, 4, 5 or 6, with equal probability.

Using this we can simulate the throw of a die! Write these lines of code and run it multiple times to see what happens.

dice = randint(1,6) #generates a random number between 1 and 6, like a die
print(dice)

We can also simulate a cointoss like this:

coin = randint(0, 1)

if coin == 0:
    print("Heads!")
else:
    print("Tails!")

Tails!

In-Class Random number exercises¶

The variable dice holds a random value from 1 to 6, like a diceroll.

a) What is the probability that A will be printed?

b) What is the probability that B will be printed?

c) What is the probability that B will be printed given that A has been printed?

dice = randint(1,6)

if dice > 3:
    print("A")

if dice == 6:
    print("B")

d) Does this change the behaviour of the code?

dice = randint(1,6)

if dice > 3:
    print("A")
    if dice == 6:
        print("B")

Programming languages, useful software and many other items of interest

Variables and Decisions

Contents

Variables and Decisions¶

Learning Goals:¶

Introduction¶

Hello world!¶

Variables¶

Integers and Floats¶

In-Class Number Exercises¶

Strings¶

Better printing:¶

In-Class String Exercises¶

Changing a variable¶

Comments¶

In-Class Variable Changing Exercises¶

Decisions¶

In-Class Decisions Exercises¶

Elif statements and larger decision blocks¶

Logical statements and booleans¶

In-Class Decisions With `elif` Exercises¶

Random number generation¶

In-Class Random number exercises¶

Programming languages, useful software and many other items of interest

Variables and Decisions

Contents

Variables and Decisions¶

Learning Goals:¶

Introduction¶

Hello world!¶

Variables¶

Integers and Floats¶

In-Class Number Exercises¶

Strings¶

Better printing:¶

In-Class String Exercises¶

Changing a variable¶

Comments¶

In-Class Variable Changing Exercises¶

Decisions¶

In-Class Decisions Exercises¶

Elif statements and larger decision blocks¶

Logical statements and booleans¶

In-Class Decisions With elif Exercises¶

Random number generation¶

In-Class Random number exercises¶

In-Class Decisions With `elif` Exercises¶