Conditional statements¶

Sources¶

This lesson is based on the Software Carpentry group’s lessons on Programming with Python.

Basics of conditional statements¶

Conditional statements can change the code behaviour based on meeting certain conditions.

Let’s take a simple example.
```
>>> num = 37
>>> if num > 100:
...     print('greater')
... else:
...     print('not greater')
...
not greater
```
What did we do here? First, we used the if and else statements to determine what parts of the code to execute. Note that both lines containing if or else end with a : and the text beneath is indented. What do these tests do? The if test checks to see whether the variable value for num is greater than 100. If so, ‘greater’ would be written to the screen. Since 37 is smaller than 100, the code beneath the else is executed. The else statement code will run whenever the if test is false.
The combination of if and else is very common, but both are not strictly required.
```
>>> num = 53
>>> if num > 100:
...     print('53 is greater than 100')
...
>>>
```
Note that here we use only the if statement, and because 53 is not greater than 100, nothing is printed to the screen.
We can also have a second test for an if statment by using the elif (else-if) statement.
```
>>> num = -3
>>> if num > 0:
...     print(num, 'is positive')
... elif num == 0:
...     print(num, 'is zero')
... else:
...     print(num, 'is negative')
...
-3 is negative
```
Makes sense, right? Note here that we use the == to test if a value is equal to another. The complete list of these comparison operators is given in the table below.

Operator Meaning

< Less than

<= Less than or equal to

== Equal to

>= Greater than or equal to

> Greater than

!= Not equal to

Example with Urban area classifications:¶

Note that the elif-statement is tested only if the previous if- or elif-statement was not true.

"""
This code will find out if the user lives in a sparsely populated area or an urban area
according to the definition of urban areas in Finland: https://en.wikipedia.org/wiki/Urban_areas_in_Finland
"""

# As population size as input from user
population = input("What is the population of the area (number of people)?: ")

#Convert user input as integer:
population = int(population)

# If population is zero or negative
if population <= 0:
        print("Ghost town!")

# If population equals one
elif population == 1:
        print("Living alone!")

# If population is less than 200
elif population < 200:
    print("Sparsely populated area")

# If population is more than 200
elif population > 200:

    # Ask the user for maximum distance between buildings
    building_dist = input("What is the maximum distance between buildings in the area (m)?: ")
    #Convert user input as integer:
    building_dist = int(building_dist)

    #If building distance is greater than 200
    if building_dist >= 200:
        print("Sparsely populated area")

    #In case population was more than 200, and building distance less than 200
    else:
        print("Urban area")

We can also use and and or to have multiple conditions.

>>> if (1 > 0) and (-1 > 0):
...     print('Both parts are true')
... else:
...     print('One part is not true')
...
One part is not true
>>> if (1 < 0) or (-1 < 0):
...     print('At least one test is true')
...
At least one test is true

This can be quite handy.

String comparisons¶

# Define two text strings:
text1 ="What's the story"
text2 = "Morning glory?"

#Check if strings are equal or not
text1 == text2
text1 != text2


#Check if a letter/work is found in a string
'a' in text1
'a' in text2
'glory' in text2
'Glory' in text2

# Check if string starts with spesific word /letter:
filename.startswith("Toto")
filename.startswith('.shp')

Operator	Meaning
`<`	Less than
`<=`	Less than or equal to
`==`	Equal to
`>=`	Greater than or equal to
`>`	Greater than
`!=`	Not equal to