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Conditions


Python uses boolean variables to evaluate conditions. The boolean values True and False are returned when an expression is compared or evaluated. For example:

x = 2
print(x == 2) # prints out True
print(x == 3) # prints out False
print(x < 3) # prints out True

Notice that variable assignment is done using a single equals operator "=", whereas comparison between two variables is done using the double equals operator "==". The "not equals" operator is marked as "!=".

Boolean operators

The "and" and "or" boolean operators allow building complex boolean expressions, for example:

name = "John"
age = 23
if name == "John" and age == 23:
    print("Your name is John, and you are also 23 years old.")

if name == "John" or name == "Rick":
    print("Your name is either John or Rick.")

The "in" operator

The "in" operator could be used to check if a specified object exists within an iterable object container, such as a list:

name = "John"
if name in ["John", "Rick"]:
    print("Your name is either John or Rick.")

Python uses indentation to define code blocks, instead of brackets. The standard Python indentation is 4 spaces, although tabs and any other space size will work, as long as it is consistent. Notice that code blocks do not need any termination.

Here is an example for using Python's "if" statement using code blocks:

if <statement is true>:
    <do something>
    ....
    ....
elif <another statement is true>: # else if
    <do something else>
    ....
    ....
else:
    <do another thing>
    ....
    ....

For example:

x = 2
if x == 2:
    print("x equals two!")
else:
    print("x does not equal to two.")

A statement is evaulated as true if one of the following is correct: 1. The "True" boolean variable is given, or calculated using an expression, such as an arithmetic comparison. 2. An object which is not considered "empty" is passed.

Here are some examples for objects which are considered as empty: 1. An empty string: "" 2. An empty list: [] 3. The number zero: 0 4. The false boolean variable: False

The 'is' operator

Unlike the double equals operator "==", the "is" operator does not match the values of the variables, but the instances themselves. For example:

x = [1,2,3]
y = [1,2,3]
print(x == y) # Prints out True
print(x is y) # Prints out False

The "not" operator

Using "not" before a boolean expression inverts it:

print(not False) # Prints out True
print((not False) == (False)) # Prints out False

Exercise

Change the variables in the first section, so that each if statement resolves as True.

# change this code number = 10 second_number = 10 first_array = [] second_array = [1,2,3] if number > 15: print("1") if first_array: print("2") if len(second_array) == 2: print("3") if len(first_array) + len(second_array) == 5: print("4") if first_array and first_array[0] == 1: print("5") if not second_number: print("6") # change this code number = 16 second_number = 0 first_array = [1,2,3] second_array = [1,2] if number > 15: print("1") if first_array: print("2") if len(second_array) == 2: print("3") if len(first_array) + len(second_array) == 5: print("4") if first_array and first_array[0] == 1: print("5") if not second_number: print("6") test_output_contains("1", no_output_msg= "Did you print out 1 if `number` is greater than 15?") test_output_contains("2", no_output_msg= "Did you print out 2 if there exists a list `first_array`?") test_output_contains("3", no_output_msg= "Did you print out 3 if the length of `second_array` is 2?") test_output_contains("4", no_output_msg= "Did you print out 4 if len(first_array) + len(second_array) == 5?") test_output_contains("5", no_output_msg= "Did you print out 5 if first_array and first_array[0] == 1?") test_output_contains("6", no_output_msg= "Did you print out 6 if not second_number?") success_msg("Great Work!")

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