Chapter 3.2 - Complex conditions - Computer Programming for the Geosciences

Chapter 3.2.1 - `if` statement review¶

The most basic if statements syntax is as follows:

if (test):
    code

In this example, code will only run if test is True.

Much like the evaluation order for setting variables, test is the first thing that is processed in the if statement.

The only requirement for test is that it is a single bool after it is evaluated. This is simple if only one test is used:

temperature = 50

if (temperature > 95):
    code

In this case, temperature > 95 is recognized by Python as a test that needs to be changed into a bool. Since 50 is not greater tha 95, the bool in this case is False.

Chapter 3.2.2 - Complex conditions¶

What if you needed multiple tests? For example, say that you need to make sure the temperature was between 0 C and 100 C:

temperature = 20

condition1 = temperature >= 0
condition2 = temperature <= 100

How can we use both condition1 and condition2? By using the following keywords:

and or &: this combines two tests by requiring both tests to be True to result in True
or or |: this combines two tests by requiring at least one test to be True to result in True

Here is a basic example with two conditions a and b combined with and:

a = True
b = False

print("A =", a)
print("B =", b)

if a and b:
    print("both a and b are True")
else:
    print("a, b, or both a and b are False")

Here is the same example, except combined with or

a = True
b = False

print("A =", a)
print("B =", b)

if a or b:
    print("Either a is True, b is True, or both a and b are True")
else:
    print("both a and b are False")

We can combine as many tests as we like, as long as you can simplify the tests down to a single bool

Try to change a, b, c, and d to get a different result.

a = True
b = False
c = True
d = False

print("A =", a)
print("B =", b)
print("C =", c)
print("D =", d)

if (a and b) or (c and d):
    print("either a and b are both True OR c and d are both True")
else:
    print("either a and b are False AND either c or d are False")

Slowing down: Different ways to think about this:¶

1. Use bool math:

A very low-level way to look at this is to convert True to 1 and False to 0 in your head.

If you have two conditions that are either True (1) or False (0), and will require the sum to be 2 to result in True, whereas or will require the sum to be at least 1 to result in True.

This can be helpful for complex conditions:

a = True
b = False
c = True
d = False
e = True
d = False

Do order of operation from left to right and based on the parentheses:

Complex condition to solve -> (a or b) and (c or (d and e)) or (f)

(a or b) -> (True or False) -> (1 + 0) -> 1 (True since 1 is >= 1 for OR) -> 1
(d and e) -> (False and True) -> (0 + 1) -> 1 (False 1 < 2 for AND) -> 0
(c or (d and e)) -> (True or (False)) (1 + (0)) -> 1 (True since 1 is >= 1 for OR) -> 1
(a or b) and (c or (d and e)) -> (True) and (True) -> (1) + (1) -> 2 (True since 2 >= 2 for AND)
(a or b) and (c or (d and e)) or (f) -> (True) or (False) -> (1) + (0) -> 1 (True since 1 >= 1 for OR)

This will result in True. How would you change the code below to result in False?

a = True
b = False
c = True
d = False
e = True
f = False

(a or b) and (c or (d and e)) or (f)

2. Use “truth tables”

You can look at every possible combination of two bool by using truth tables. These tables simulate what would happen if one of the four possible combinations of and or or occurred:

and truth table:

Condition 1	Condition2	`Condition 1 and Condition 2`
`True`	`True`	`True`
`True`	`False`	`False`
`False`	`True`	`False`
`False`	`False`	`False`

or truth table:

Condition 1	Condition2	`Condition 1 or Condition 2`
`True`	`True`	`True`
`True`	`False`	`True`
`False`	`True`	`True`
`False`	`False`	`False`

Example: if Condition 1 is False and Condition 2 is True, we can interpret the truth tables as follows:

and truth table:

Condition 1	Condition2	`Condition 1 and Condition 2`	Result
`True`	`True`	`True`
`False`	`False`	`False`
`True`	`True`	`False`	✔
`False`	`False`	`False`

or truth table:

Condition 1	Condition2	`Condition 1 or Condition 2`	Result
`True`	`True`	`True`
`True`	`False`	`True`
`False`	`True`	`True`	✔
`False`	`False`	`False`

Combining or and and to see all possible combinations for both cases, and specifically if a is False and b is True:

Variable 1	Variable 2	Conditional Test 1	Conditional Test 2	Result
`a`	`b`	`a and b`	`a or b`
`True`	`True`	`True`	`True`
`True`	`False`	`False`	`True`
`False`	`True`	`False`	`True`	✔
`False`	`False`	`False`	`False`

Try modifying the numbers below to see how it changes the results.¶

What does the following code do “in English”?

temperature = 0
humidity = 100

if (temperature <= 32) and (humidity == 100):
    print("freezing fog")
elif (temperature >= 32) and (humidity == 100):
    print("fog")
else:
    print("clear")

Chapter 3.2.3 - Practice¶

You have an observation that includes (unit or format, data type):

temperature (degrees F, float)
year (YYYY, int)
month (MM, int)
state (abbreviation, str).

You want to print temperature only if the following conditions are met:

The year is before 2000
The month is either June, July, or August
The state is Illinois

Write a complex condition that combines multiple tests using and or or in an if / else statement to determine if you should print the temperature:

temperature = input("What is the temperature?")
year = input("What is the year?")
month = input("What is the month (1 - 12)?")
state = input("What is the state (abbreviation)?")

#### Your code below

print(temperature, year, month, state)

Create a complex condition using if / elif / else that you may need for your own project:

Chapter 3 Flow Control

Chapter 3.1 - if / elif / else statements

Chapter 3 Flow Control

Chapter 3.3 - Loops

Chapter 3.2 - Complex conditions

Chapter 3.2.1 - if statement review¶

Chapter 3.2.2 - Complex conditions¶

Slowing down: Different ways to think about this:¶

Try modifying the numbers below to see how it changes the results.¶

Chapter 3.2.3 - Practice¶

Chapter 3.2.1 - `if` statement review¶