Operators

Operators are symbols built into python that allow for special functionality such as addition, subtraction, and validation.

Arithmetic operators

Arithmetic operators are operators used to do basic arithmetic (hence the name). What this means is that they are the basis for any calculations you need to make in a program. Most of these will be pretty obvious so I will try and make it more interesting by telling you some of the usefulness (weirdness) you wouldn't expect out of them, as well as some use cases.

basic-math

Addition

This should be pretty basic and boring, in fact you have already seen it before, but for the sake of completeness, here is how to do addition in python:

print(5 + 3) # Prints: 8

# You can also use variables

sum_value = 5 + 8

print(sum_value) # Prints 8

# You can also use variables in place of numbers

sum_2 = 6 + sum_value 

print(sum_2) # Prints: 14 

Well that was boring, but... there are more interesting things you can do with addition. For example, if you have multiple lists you can actually add them together to combine the two (order matters):

my_list = [1,2,3,4] # Initialize my_list

my_list_2 = [5,6,7,8] # Initialize my_list_2

my_list = my_list + my_list_2 # Take the current value of my_list and add my_list_2 to it

print(my_list) # Prints: [1, 2, 3, 4, 5, 6, 7, 8]

As you can see in the above example, when the my_list_2 variable is added to the first my_list variable it is tacked on to the end of it (called concatenation in computer science).

Here is where things get weird, if you recall in the first challenge (yes you should do those), I mentioned that strings are like lists. This means operands also work on strings:

name = "Hello " # Set name variable to an string 'hello '

name = name + "World!" # Take the current name value and add 'world' to it

print(name) # Prints: Hello World!

Just a heads up combining these properties together gives some interesting effects that I won't go into detail with, but I will show you. For example if you add a string and a list:

my_list = [] # Initialize an empty list

name = "John" # Initialize name variable to John

my_list = my_list + name # Adding the current my_list variable to the string name

print(my_list) # Prints: ['j','o','h','n']

Now this ^^ is weird, the reason is that strings are actually lists of individual characters, and so as we saw before they are concatenated together.

Subtraction

Just like addition, subtraction is somewhat boring the syntax for it is pretty simple:

print(5 - 3) # Prints: 2

# You can also use variables

value = 5 - 8

print(value) # Prints -3

# You can also use variables in place of numbers

value_2 = value - 6 

print(value_2) # Prints: -9

Unlike addition there are no fun tricks with it though, subtracting from a string or list will just give you an error.

Multiplication

Now multiplication is back to getting weird. First lets get the simple syntax down:

print(5 * 3) # Prints: 15

# You can also use variables

value = 5 * 8 

print(value) # Prints 40

# You can also use variables in place of numbers

value_2 = value * 6

print(sum_2) # Prints: 120

Now for the weird part, with lists and strings you can multiply them, and the results are very fun...

my_list = [1,2,3,4] # Initialize my_list

my_list = my_list * 3 # Take my_list and multiply it by 3

print(my_list) # Prints: [1,2,3,4,1,2,3,4,1,2,3,4]

As you can see, when you multiply a list (or string) by a number, it concatenates the value of the list (or string) the number of times you multiply it by. So in this case since the list is multiplied by 3, the value of the list is present 3 times in a row.

Division

And going back to the boring we have division, division has no special uses, but it does have 2 forms integer and floating point division.

Floating Point Division

This is your typical division, it will always return a float. The syntax is as follows:

print(5 / 2) # Prints: 2.5

# You can also use variables

value = 5 / 3

print(value) # Prints 1.6666666666666667

# You can also use variables in place of numbers

value_2 = value / 2 

print(value_2) # Prints: 0.8333333333333334
Integer Division

This sort of division will always return an int. If your value comes out to a float (anything with a decimal) then it takes the floor of the division (always rounds down even if above 0.5).

The syntax for integer division is pretty simple:

print(5 // 3) # Prints: 1

# You can also use variables

value = 5 // 8

print(value) # Prints 0 (Remember it ALWAYS rounds down)

# You can also use variables in place of numbers

value_2 = value // 6 

print(value_2) # Prints: 0
Shortcuts

Many of the operations you are doing are going to involve taking the original value of a variable doing an operation and storing the result back in the variable. For example:

variable_1 = 5 # Initialize the variable to 5

variable_1 = variable_1 + 5 # Take the current value of the variable and add 5

variable_1 = variable_1 - 5 # Take the current value of the variable and subtract 5

variable_1 = variable_1 / 2 # Take the current value of the variable and floating point divide by 2

variable_1 = variable_1 // 2 # Take the current value of the variable and integer divide by 2

For addition, subtraction, multiplication and division there is a shortcut to do the above operations. The general form is variable_name <operator>= value

variable_1 = 5 # Initialize the variable to 5

variable_1 += 5 # Take the current value of the variable and add 5

variable_1 -= 5 # Take the current value of the variable and subtract 5

variable_1 /= 2 # Take the current value of the variable and floating point divide by 2

variable_1 //= 2 # Take the current value of the variable and integer divide by 2
Modulus

What this actually does is returns the remainder to the division of the two terms. This is not commonly used other than to check if something is evenly divisible by another number. This is because if a number is divisible by another then the modulus will be 0:

print(5 % 3) # Prints: 2

# You can also use variables

value = 10 % 5

print(value) # Prints 0 (Therefore 10 is evenly divisible by 5)

# You can also use variables in place of numbers

value_2 = value % 6 

print(value_2) # Prints: 0 (Because 0/anything is always 0)

Note that this is an incredibly slow way to do this check, but it is used often enough that it's worth learning.

calculation

Logical operators

Logical operators are symbols that are used to make comparisons between values. They all return Boolean values when used (True or False), and are useful especially when combined with if statements (explained later). All of these comparisons can be made with int's or float's

Greater than

Used to check if a value is larger than another value:

print(5 > 3) # Prints: True; since 5 is greater than 3

result = 5 > 3 # You can store the result in a variable

print(result) # Prints: True

print(3 > 5) # Prints: False; since 3 is NOT greater than 5

print(5 > 5) # Prints: False; since 5 is NOT greater than 5 (they are equal)
Greater than or equal to

Used to check if a value is larger than or equal to another value:

print(5 >= 3) # Prints: True; since 5 is greater than 3

result = 5 >= 3 # You can store the result in a variable

print(result) # Prints: True

print(3 >= 5) # Prints: False; since 3 is NOT greater than 5

print(5 >= 5) # Prints: True; since 5 is equal to 5
Less than

Used to check if a value is smaller than another value:

print(5 < 3) # Prints: False; since 5 is NOT less than 3

result = 5 < 3 # You can store the result in a variable

print(result) # Prints: False

print(3 < 5) # Prints: True; since 3 is less than 5

print(5 < 5) # Prints: False; since 5 is NOT less than 5 (they are equal)
Less than or equal to

Used to check if a value is smaller than or equal to another value:

print(5 <= 3) # Prints: False; since 5 is NOT less than OR equal to 3

result = 5 <= 3 # You can store the result in a variable

print(result) # Prints: False

print(3 <= 5) # Prints: True; since 3 is less than 5

print(5 <= 5) # Prints: True; since 5 is equal to 5
Not

Not is an operand that flips the Boolean value of what follows; So something that would be True will be False and something that is False would be True. The syntax is not <boolean> here is an example:

print(not 5 < 3) # Prints: True; since the statement 5 < 3 evaluates to False

result = not 5 < 3 # You can store the result in a variable

print(result) # Prints: True

print(not 3 < 5) # Prints: False; Since the statement 3 < 5 evaluates to True
And

The and operand takes two Boolean values, if they are both True it will return True, otherwise it is always False. The syntax is <boolean> and <boolean> for example:

print(5 < 3 and 8 > 6) # Prints: True; since both statements evaluate to True

print(5 > 3 and 8 > 6) # Prints: False; since the first statement evaluates to False

I have provided a table below which will tell you how this works in all situations, these tables are called truth tables and they are quite useful. Assume that a and b are placeholders for statements that evaluate to True or false.

| a     | b     | a and b | Example               |
| ----- | ----- | ------- | --------------------- |
| True  | True  | True    | ```5 < 3 and 8 > 6``` |
| False | True  | False   | ```8 > 6 and 5 > 3``` |
| True  | False | False   | ```5 > 3 and 8 > 6``` |
| False | False | False   | ```5 < 3 and 8 > 6``` |
Or

The or operand takes two Boolean values, if either are True it will return True, if both are False it will return False. The syntax is <boolean> or<boolean> for example:

print(5 < 3 or 8 > 6) # Prints: True; since both statements evaluate to True

print(5 > 3 or 8 > 6) # Prints: True; since the second statement evaluates to True

Here is the truth table for the or operator:

| a     | b     | a or b | Example              |
| ----- | ----- | ------ | -------------------- |
| True  | True  | True   | ```5 < 3 or 8 > 6``` |
| False | True  | True   | ```8 > 6 or 5 > 3``` |
| True  | False | True   | ```5 > 3 or 8 > 6``` |
| False | False | False  | ```5 < 3 or 8 > 6``` |
In

The in operator is as far as I know one that is specific to python (and really useful). It takes two operands one being a value (int, string etc.) and the other being a collection of some sort (list, string). The operator will return True if the value is in the collection, for example lets say you wanted to check if a name taken from the command line is john, the code would look like this:

name = input("Enter name: ") # Take someones name from the command line

print("john" in name) # Prints: True if john is name given, or False otherwise

Another example would be checking if a list contains a specific int:

print(4 in [1,2,3]) # Prints: False since no 4 is present in the list

print(4 in [1,2,3,4]) # Prints: True since 4 is present in the list

operators

Bitwise operators

DISCLAIMER: If you are new to programming feel free to skip on to conditionals as this section won't be relevant for you.

For anyone who has used lower level languages bitwise operators are available in python (binary AND, OR, XOR, Ones Compliment, Left and Right shifts). I am not going to go into detail, but if you are interested here is the syntax:

| Operator | Description                                                  | Example                                                                                          |
| -------- | ------------------------------------------------------------ | ------------------------------------------------------------                                     |
| &        | Binary AND: Operator copies a bit to the result if it exists in both operands | (a & b) (means 0000 1100)                                                       |
| \|       | Binary OR: It copies a bit if it exists in either operand.   | (a \| b) = 61 (means 0011 1101)                                                                  |
| ^        | Binary XOR: It copies the bit if it is set in one operand but not both. | (a ^ b) = 49 (means 0011 0001)                                                        |
| !        | Binary Ones Complement: It is unary and has the effect of 'flipping' bits. | (~a ) = -61 (means 1100 0011 in 2's complement form due to a signed binary number. |
| <<       | Binary Left Shift: The left operands value is moved left by the number of bits specified by the right operand. | a << 2 = 240 (means 1111 0000)                 |
| \>>      | Binary Right Shift: The left operands value is moved right by the number of bits specified by the right operand. | a \>> 2 = 15 (means 0000 1111)               |

Chart taken from here

Conditionals

Conditional statements are a statement that takes a logical operator and executes code if the operator is True. Technically this is also a part of control flows so I will just show you one of what's to come in the next module.

if, else, and elif statements

if

This conditional is exactly how it sounds, if some operator or boolean is True then do something. The syntax for this statement looks something like this:

if condition:
    # Do stuff at this indentation level

You'll notice a few things about this:

  1. first you need to put a colon after your if statement; This is because you can combine statements together (I will show this later), so the colon is there to say "This if statement is complete".
  2. After the colon you will need to indent all the code you want to run by at least one space or tab (Typical convention is 4 spaces or 1 tab); This tells python which code you want to run if the condition is True, and which to run regardless of if the statement is True.

So for example lets say you have a number (x) and if x < 3 you want to add 2 to it, and no matter what the value of x is you want to print x the code looks like this:

x = 2 # Setting up the x variable

if x < 3:
    x += 2 # This will run if x < 3, otherwise it will be skipped over

print(X) # Since this is on a lower indentation level, this code will run regardless

if x < 3:
    x += 2 # This will run if x < 3, otherwise it will be skipped over

print(X) # Since this is on a lower indentation level, this code will run regardless

Try to figure out what happens without reading my explanation below, once you have a guess, keep reading.

What happens in the example:

  1. Setting up a variable called x, which is set to 2
  2. Because at this point x < 3 the code will add 2 to the current value of x, making it 4
  3. Since the value of x is 4 the print statement will print 4
  4. Since x is now not less than 3 nothing will be added to the value
  5. Since the value of x is still 4 the print statement will print 4

else

else statements are used in conjunction with if statements. They allow you to set what should happen if the if statements' condition is False. Here is the syntax:

if condition:
    # Do stuff
else:
    # Do different stuff (Only happens if the above if statement condition is False)

Modifying the earlier example, let's setup two sets of if-else statements in which if x < 3 we take the current x value and add 2 to it, otherwise (else) subtract 1. Here is the code:

x = 2 # Setting up the x variable

if x < 3:
    x += 2 # This will run if x < 3, otherwise it will be skipped over
else:
    x -= 1 # This will run if x is not less than 3

print(X) # Since this is on a lower indentation level, this code will run regardless

if x < 3:
    x += 2 # This will run if x < 3, otherwise it will be skipped over
else:
    x -= 1 # This will run if x is not less than 3

print(X) # Since this is on a lower indentation level, this code will run regardless

Try to figure out what happens without reading my explanation below, once you have a guess, keep reading.

What happens in the example:

  1. Setting up a variable called x, which is set to 2
  2. Because at this point x < 3 the code will add 2 to the current value of x, making it 4
  3. Since the value of x is 4 the print statement will print 4
  4. Since x is now not less than 3 the else statement is invoked and x has a 1 subtracted from it
  5. Since the value of x is now 3 the print statement will print 3

else-ifn't

source

elif

elif statements are a way to chain if statements. For example lets say you had 4 conditions you wanted to check for you could combine them together using logical operators, or to make it more legible you could have 4 different elif statements.

The syntax is as follows:

if conidion: # You MUST have an initial if statement to use an elif statement
    # Do stuff

elif condition_2:
    # Do other stuff

else: # This is optional, but useful if you want a catch-all condition
    # Do other other stuff

Let's take a more realistic example, let's say you wanted to make a program that takes in input from the command line and then if it is a value between 1 and 5, print the text version of the number (i.e. 5 would print "Five"). Here is the code:

user_value = int(input("Enter a number between 0 and 5: "))

if user_value == 0:
    print("Zero")

elif user_value == 1:
    print("One")

elif user_value == 2:
    print("Two")

elif user_value == 3:
    print("Three")

elif user_value == 4:
    print("Four")

elif user_value == 5:
    print("Five")

else: # If value is not between 0 and 5
    print("Value provided is not between 0 and 5!")

Exercises

""" =========== Exercise 1 ============= I have provided some starter code below that creates a result variable, and a number_1 variable. Your goal is to make number_1 equal 11 after the operations that have been done to it. """ result = 0 number_1 = 5 number_1 += 52 # Do more operations on number 1 until it equals eleven result = number_1 print(result == 11) """ =========== Exercise 2 ============= Take input from the command line, and convert it to an int. Now pick a range (i.e. 0-10), and create a set of conditional statements that prints the string representation of the number input by the user. For example if someone put in 8, then it would print 'eight'. Hint: Use if, elif and else statements. """ """ =========== Exercise 3 ============= Before running the code below try to figure out which print statement will execute and why. Then uncomment the code and check if you were right. """ # number = 0 # number += 15 # number //= 2 # number *= 6 # number -= 4 # if number < 10: # print("Less than 10") # elif 10 <= number <= 20: # print("Between 10 and 20") # elif 20 <= number <= 30: # print("Between 20 and 30") # elif 30 <= number <= 40: # print("Between 30 and 40") # else: # print("¯\_(ツ)_/¯")

Challenges

""" =========== Challenge 1 ============= Take an input from the command line, then convert it to an int and if it is even print 'the number is even' otherwise print 'the number is odd'. """ """ =========== Challenge 2 ============= Take an input from the command line, and convert it to an int. Validate the number is within the range 1-5, and then for each possible value (1-5), write the code to make the input 11. I.e. Someone inputs 1 the result is 11, if someone inputs 2 the result is 11 etc. If someone puts in a number not in range (1-5) print: 'value not between 1-5 please try again' Hint: You should have between 6-7 if/elif/else statements """ """ =========== Challenge 3 ============= There are functions in python that can be used to determine if strings contain certain characters. For example the function isdigit() returns True if ALL the characters in the string are digits. Here is an example of it's usage: numbers = "1234567" letters = "Hello 4" print(numbers.isdigit()) # prints True print(letters.isdigit()) # prints False There are two other similar functions called endswith() and islower(). endswith() takes a string as an argument and returns true if the string it's being used on ends with the string provided. islower() returns true if the string provided is all lowercase Now take input at the command line, and using if statements print the following statements if conditions are met: 1. if the string is all numbers print "All numbers" 2. If the string is all lowercase print "All lowercase" 3. If the string ends with "yes" print "Ends in yes" Otherwise print "None of the conditions have been met" """

Solutions




""" =========== Exercise 1 ============= I have provided some starter code below that creates a result variable, and a number_1 variable. Your goal is to make number_1 equal 11 after the operations that have been done to it. """ result = 0 number_1 = 5 number_1 += 52 # Do more operations on number 1 until it equals eleven # There are a ton of ways to do this here is one: number_1 //= 3 # number_1 == 19 number_1 -= 8 # number_1 == 11 result = number_1 print(result == 11) """ =========== Exercise 2 ============= Take input from the command line, and convert it to an int. Now pick a range (i.e. 0-10), and create a set of conditional statements that prints the string representation of the number input by the user. For example if someone put in 8, then it would print 'eight'. Hint: Use if, elif and else statements. """ user_choice = int(input("Enter a number between 0-10:")) if user_choice == 0: print("Zero") elif user_choice == 1: print("One") elif user_choice == 2: print("Two") elif user_choice == 3: print("Three") elif user_choice == 4: print("Four") elif user_choice == 5: print("Five") elif user_choice == 6: print("Six") elif user_choice == 7: print("Seven") elif user_choice == 8: print("Eight") elif user_choice == 9: print("Nine") elif user_choice == 10: print("Ten") else: print("Number is out of range") """ =========== Exercise 3 ============= Before running the code below try to figure out which print statement will execute and why. Then uncomment the code and check if you were right. """ # number = 0 # number += 15 # number //= 2 # number *= 6 # number -= 4 # if number < 10: # print("Less than 10") # elif 10 <= number <= 20: # print("Between 10 and 20") # elif 20 <= number <= 30: # print("Between 20 and 30") # elif 30 <= number <= 40: # print("Between 30 and 40") # else: # print("¯\_(ツ)_/¯") """Here is the explanation number starts as 0 then it is 'plus-equalled' to 15 it's then 'integer-divide-equalled' to 2 making it 7 then 'multiply-equalled' to 6 making it 42, then finally 'minus-equalled' to 4 making it 38. Because the number is now 38 the program will print: 'Between 30 and 40' """ """============== Challenges ================""" """ =========== Challenge 1 ============= Take an input from the command line, then convert it to an int and if it is even print 'the number is even' otherwise print 'the number is odd'. """ number = int(input("Please enter a number: ")) if number % 2 == 0: print("Number is Even") else: print("Number is Odd") """ =========== Challenge 2 ============= Take an input from the command line, and convert it to an int. Validate the number is within the range 1-5, and then for each possible value (1-5), write the code to make the input 11. I.e. Someone inputs 1 the result is 11, if someone inputs 2 the result is 11 etc. If someone puts in a number not in range (1-5) print: 'value not between 1-5 please try again' Hint: You should have between 6-7 if/elif/else statements """ number = int(input("Please enter a number between 1-5: ")) # These answers are not the only possible answers if number == 1: number += 10 # 1 + 10 is eleven elif number == 2: number *= 5 # 2 * 5 is 10 number += 1 # 10 + 1 is 11 elif number == 3: number *= 4 # 3 * 4 is 12 number -= 1 # 12 - 1 is 11 elif number == 4: number *= 3 # 4 * 3 is 12 number -= 1 # 12 - 1 is 11 elif number == 5: number += 6 # 5 + 6 is 11 else: print("Number is not in range 1-5") """ =========== Challenge 3 ============= There are functions in python that can be used to determine if strings contain certain characters. For example the function isdigit() returns True if ALL the characters in the string are digits. Here is an example of it's usage: numbers = "1234567" letters = "Hello 4" print(numbers.isdigit()) # prints True print(letters.isdigit()) # prints False There are two other similar functions called endswith() and islower(). endswith() takes a string as an argument and returns true if the string it's being used on ends with the string provided. islower() returns true if the string provided is all lowercase Now take input at the command line, and using if statements print the following statements if conditions are met: 1. if the string is all numbers print "All numbers" 2. If the string is all lowercase print "All lowercase" 3. If the string ends with "yes" print "Ends in yes" Otherwise print "None of the conditions have been met" """ user_input = input("Enter some text:") if user_input.isdigit(): print("All numbers") elif user_input.islower(): print("All lowercase") # Because the chain of statements would end with the above elif, this nested if is required if user_input.endswith("yes") or user_input.endswith("Yes"): print("Ends in Yes") elif user_input.endswith("yes") or user_input.endswith("Yes"): print("Ends in yes") else: print("None of the conditions have been met")