Conditionals

Comparison

Lets first begin by being able to check objects, there are a couple of ways to do this

#Equality is ==
print(1 == 1)  # => True
print(2 == 1)  # => False

# Inequality is !=
print(1 != 1)  # => False
print(2 != 1)  # => True

# More comparisons
print(1 < 10)  # => True
print(1 > 10)  # => False
print(2 <= 2)  # => True
print(2 >= 2)  # => True

# Seeing whether a value is in a range
print(1 < 2 and 2 < 3)  # => True
print(2 < 3 and 3 < 2)  # => False

# Chaining makes this look nicer
print(1 < 2 < 3)  # => True
print(2 < 3 < 2)  # => False

We can run this on variables as well

a = 5
print(a == 5) # => True
print(a < 3)  # => False
print(2 < a)  # => True

since everything on both the right and left side are treated as independent expressions,

(is vs. ==) is checks if two variables refer to the same object, but == checks if the objects pointed to have the same values.

a = [1, 2, 3, 4]  # Point a at a new list, [1, 2, 3, 4]
b = a             # Point b at what a is pointing to
print(b is a)     # => True, a and b refer to the same object
print(b == a)     # => True, a's and b's objects are equal
b = [1, 2, 3, 4]  # Point b at a new list, [1, 2, 3, 4]
print(b is a)     # => False, a and b do not refer to the same object
print(b == a)     # => True, a's and b's objects are equal

the "If" statement

We can have code that changes behavior based on conditions being met, to put it simply, whether there is a True or a False value present

Try out the following example

# Let's just make a variable
some_var = 5

# Here is an if statement. Indentation is significant in Python!
# Convention is to use four spaces, not tabs.
# This prints "some_var is smaller than 10"
if some_var > 10:
    print("some_var is totally bigger than 10.")
elif some_var < 10:    # This elif clause is optional.
    print("some_var is smaller than 10.")
else:                  # This is optional too.
    print("some_var is indeed 10.")

Inside this there are three distinct blocks, an if block, an elif and an else

You can read the above code with the following in english:

if the value "some var" is greater than ten, then print "..."
otherwise,  if the value of "some var" is less than 10, then print "..."
otherwise, print "...."

heres another example that python executes code from top down in ifs

some_var = 5
if some_var < 10:
	print("I am less than 10")
elif some_var < 20: # despite this statement being true, the below does not run
	print("I am also less than 20")
else:
	print("I shouldn't run")
print("I am outside of the if statement, I will print")

the elif and else statements are completely optional in a conditional, the following are examples showing that you can have an infinite number of elifs, but a maximum of 1 else

# example using a singular `if` and nothing more
print("Give a number")
user_number = int(input()) 
# we use int() to turn the string result of input into a number

if user_number < 10:
	print("I am less than 10!")
	
print("I will always print!")

# example using multiple `elif`s but no `else`
print("Give a number")
user_number = int(input()) 

if user_number < 10:
	print("I am less than 10!")
elif user_number < 20:
	print("I am less than 20, but more than 10!")
elif user_number < 30:
	print("in between 20 and 30!")
	
print("I will always print!")

# example showing that the if takes only a boolean
if True:
	print("I will always print!")
else:
	print("I will never print")

# example showing a False will never run
if False:
	print("I will never print!")
else:
	print("I will always print")

At this point try out challenge 3!

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