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Conditionals and Loops (2/3)

Comparison operators

In addition to using Booleans, you can also create conditionals using various comparison operators. For example, a conditional can test the size of a number:

num = 7
if num > 5:
    print('num is greater than 5')

Or the contents of a string:

t = 'this is text'
if t == 'this is text':
    print('the text matches')

In this example I use the double equals (==) operator to check if one thing equals another. This is the standard way to check equality, since the single equals (=) is reserved for assigning values to variables. The most common comparison operators are:

  • == or is - check if two values are equal
  • != or is not - check if two values are not equal
  • > - check if one number is larger than another
  • >= - check if one number is larger or equal to another
  • < - check if one number is smaller than another
  • <= - check if one number is smaller or equal to another

In all cases, a succeeding condition will compute to a value of True, while a failed condition will compute to a value of False. You can check this by passing a Conditional statement into the type() function and observing that the type is bool:

print(type(5 > 4))

Elif statements

Between the if ...: and else: statments, you can also use any number of elif ...: (a concatenation of else and if) statement to chain together conditions to create more complex logics:

num_1 = 3
num_2 = 7

if num_1 > 5:
    print('num_1 is greater than 5')
elif num_2 > 5:
    print('num_2 is greater than 5')
else:
    print("they're both too small!")

This creates a chain of tests that happen in order. If the first test passes, that block of code is executed, and the rest of the Conditional is skipped. If it fails, the second test (written after the elif keyword) is analyzed, and so on. If none of the tests pass, the code following the else: statement is executed.

Combining conditions

With Python, you can also combine multiple tests within a single line of code by joining them using a set of special logic operators:

  • and - returns True if both conditions are True, False otherwise
  • or - returns True if either conditions are True and False only if they are both False
  • not - added in front of any conditional or boolean, this operator reverses the boolean, making a True value False and vice versa.

Using these logic operators we can write the following compound conditionals:

num_1 = 3
num_2 = 7

if num_1 < 5 and num_2 < 5:
    print("they're both too small!")

if num_1 < 5 or num_2 < 5:
    print("at least one of them is too small!")

if not num_2 < 5:
    print("num_1 is at least 5")

The 'in' operator

Finally, we can create a conditional to check if an item or value is stored within a list using the spacial in operator. We can also combine it with not to check if something is currently not stored inside a list. For example:

myList = [1, 2, 3, 4, 5]
if 5 in myList:
    print("5 is in the list")

myDictionary = {'a': 1, 'b': 2, 'c': 3}
if 'd' not in myDictionary.keys():
    print("there is no item with a key of 'd'")
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