Making Choices

Overview

Teaching: 30 min
Exercises: 0 min

Questions

• How can my programs do different things based on data values?

Objectives

• Write conditional statements including if, elif, and else branches.

• Correctly evaluate expressions containing and and or.

In our previous lessons, we examined the seasonal behaviour of sea waves. How can we use this data to make practical decisions?

How can we use Python to identify thresholds, and take a different action for each? In this lesson, we’ll learn how to write code that runs only when certain conditions are true.

Conditionals

We can ask Python to take different actions, depending on a condition, with an if statement:

num = 37
if num > 100:
    print('greater')
else:
    print('not greater')
print('done')

not greater
done

The second line of this code uses the keyword if to tell Python that we want to make a choice. If the test that follows the if statement is true, the body of the if (i.e., the set of lines indented underneath it) is executed, and “greater” is printed. If the test is false, the body of the else is executed instead, and “not greater” is printed. Only one or the other is ever executed before continuing on with program execution to print “done”:

Conditional statements don’t have to include an else. If there isn’t one, Python simply does nothing if the test is false:

num = 53
print('before conditional...')
if num > 100:
    print(num, 'is greater than 100')
print('...after conditional')

before conditional...
...after conditional

We can also chain several tests together using elif, which is short for “else if”. The following Python code uses elif to print the sign of a number.

num = -3

if num > 0:
    print(num, 'is positive')
elif num == 0:
    print(num, 'is zero')
else:
    print(num, 'is negative')

-3 is negative

Note that to test for equality we use a double equals sign == rather than a single equals sign = which is used to assign values.

Comparing in Python

Along with the > and == operators we have already used for comparing values in our conditionals, there are a few more options to know about:

• >: greater than
• <: less than
• ==: equal to
• !=: does not equal
• >=: greater than or equal to
• <=: less than or equal to

We can also combine tests using and and or. and is only true if both parts are true:

if (1 > 0) and (-1 >= 0):
    print('both parts are true')
else:
    print('at least one part is false')

at least one part is false

while or is true if at least one part is true:

if (1 < 0) or (1 >= 0):
    print('at least one test is true')

at least one test is true

True and False

True and False are special words in Python called booleans, which represent truth values. A statement such as 1 < 0 returns the value False, while -1 < 0 returns the value True.

Checking our Data

Now that we’ve seen how conditionals work, we can use them to look for thresholds in our wave data. We are about to use functions provided by the numpy module again. Therefore, if you’re working in a new Python session, make sure to load the module with:

import numpy

If you are operating a boat, for carrying passengers of working offshore, you need to know that it will be save to go to sea. Ideally you wouldn’t want to have passengers transported if the wave height is above 3 metres

Let’s look at our wave data, and find which months we can operate the boats, based on the monthly mean wave-height.

We could look at each month individually:

data = numpy.loadtxt(fname='data/wavesmonthly.csv', delimiter=',', skiprows=1)
reshaped_data = numpy.reshape(data[:,2], [37,12])

month0 = numpy.mean(reshaped_data, axis=0)[0]

if month0 < 3:
    print("Can take passengers this month")

Survey vehicles can operate in stormier waters, with wave heights up to 4m

elif month0 < 4:
    print("Can take survey vehicles (but not passengers)")

And if neither of these conditions are true, then it’s too stormy, and nothing can go out.

else:
    print("Can't take any boats out to sea")

Let’s test that out for January:

month0 = numpy.mean(reshaped_data, axis=0)[0]

if month0 < 3:
    print("Can take passengers this month")
elif month0 < 4:
    print("Can take survey vehicles (but not passengers)")
else:
    print("Can't take any boats out to sea")

Can't take any boats out to sea

Now let’s try for June

month5 = numpy.mean(reshaped_data, axis=0)[5]

if month5 < 3:
    print("Can take passengers this month")
elif month5 < 4:
    print("Can take survey vehicles (but not passengers)")
else:
    print("Can't take any boats out to sea")

Can take passengers this month

Notice how the statement stops as soon as it reaches a condition which is True.

We could test for all months less manually, using a for loop:

for month_index, monthly_waveheight in enumerate(numpy.mean(reshaped_data, axis=0)):
    if monthly_waveheight < 3:
        print(f"Month {month_index}: we can take passengers this month")
    elif monthly_waveheight < 4:
        print(f"Month {month_index}: we can take survey vehicles (but not passengers) this month")
    else:
        print(f"Month {month_index}: we can't take any boats out to sea this month")

Month 0: can't take any boats out to sea this month
Month 1: can't take any boats out to sea this month
Month 2: can't take any boats out to sea this month
Month 3: can take survey vehicles (but not passengers) this month
Month 4: can take passengers this month
Month 5: can take passengers this month
Month 6: can take passengers this month
Month 7: can take passengers this month
Month 8: can take passengers this month
Month 9: can take survey vehicles (but not passengers) this month
Month 10: can take survey vehicles (but not passengers) this month
Month 11: can't take any boats out to sea this month

The enumerate function is the Pythonic way of getting the index of values in a loop - in this case it allows us to list the month number in the output. We need a variable to store this value in, which we’re calling month_index. Because the loop returns two variables form each iteration (month index and monthly waveheight), we need two variables.

We can see that we could suggest a timetable for passenger ferries in May - September (inclusive), we could plan to take survey boats out in April, October, and November; and we should not plan any sea-going activities in January, February, March, or December.

This is a relatively crude example, but shows how we can use these programming constructs to help make some decisions using the data.

In this way, we have asked Python to do something different depending on the condition of our data. Here we printed messages in all cases, but we could also imagine not using the else catch-all so that messages are only printed when something is wrong, freeing us from having to manually examine every plot for features we’ve seen before.

How Many Paths?

Consider this code:

if 4 > 5:
    print('A')
elif 4 == 5:
    print('B')
elif 4 < 5:
    print('C')

Which of the following would be printed if you were to run this code? Why did you pick this answer?

1. A
2. B
3. C
4. B and C

Solution

C gets printed because the first two conditions, 4 > 5 and 4 == 5, are not true, but 4 < 5 is true.

What Is Truth?

True and False booleans are not the only values in Python that are true and false. In fact, any value can be used in an if or elif. After reading and running the code below, explain what the rule is for which values are considered true and which are considered false.

if '':
    print('empty string is true')
if 'word':
    print('word is true')
if []:
    print('empty list is true')
if [1, 2, 3]:
    print('non-empty list is true')
if 0:
    print('zero is true')
if 1:
    print('one is true')

That’s Not Not What I Meant

Sometimes it is useful to check whether some condition is not true. The Boolean operator not can do this explicitly. After reading and running the code below, write some if statements that use not to test the rule that you formulated in the previous challenge.

if not '':
    print('empty string is not true')
if not 'word':
    print('word is not true')
if not not True:
    print('not not True is true')

Close Enough

Write some conditions that print True if the variable a is within 10% of the variable b and False otherwise. Compare your implementation with your partner’s: do you get the same answer for all possible pairs of numbers?

Hint

There is a built-in function abs that returns the absolute value of a number:

print(abs(-12))

12

Solution 1

a = 5
b = 5.1

if abs(a - b) <= 0.1 * abs(b):
    print('True')
else:
    print('False')

Solution 2

print(abs(a - b) <= 0.1 * abs(b))

This works because the Booleans True and False have string representations which can be printed.

In-Place Operators

Python (and most other languages in the C family) provides in-place operators that work like this:

x = 1  # original value
x += 1 # add one to x, assigning result back to x
x *= 3 # multiply x by 3
print(x)

6

Write some code that sums the positive and negative numbers in a list separately, using in-place operators. Do you think the result is more or less readable than writing the same without in-place operators?

Solution

positive_sum = 0
negative_sum = 0
test_list = [3, 4, 6, 1, -1, -5, 0, 7, -8]
for num in test_list:
    if num > 0:
        positive_sum += num
    elif num == 0:
        pass
    else:
        negative_sum += num
print(positive_sum, negative_sum)

Here pass means “don’t do anything”. In this particular case, it’s not actually needed, since if num == 0 neither sum needs to change, but it illustrates the use of elif and pass.

Sorting a List Into Buckets

In our data folder, we have some CSV files (those with .csv file extensions), and one NetCDF file (with a .nc file extension). We’d like to break these files into two lists called csv_files and nc_files, respectively.

Add code to the template below to do this. Note that the string method startswith returns True if and only if the string it is called on ends with the string passed as an argument, that is:

'String'.endswith('ing')

True

But

'String'.endswith('ING')

False

Use the following Python code as your starting point:

filenames = ['wavesmonthly.csv',
         'waves_00s.csv',
         'waves_10s.csv',
         'waves_80s.csv',
         'waves_90s.csv',
         'multyear_hs_avg.nc']
csv_files = []
nc_files = []
other_files = []

Your solution should:

1. loop over the names of the files
2. figure out which group each filename belongs in
3. append the filename to that list

In the end the two lists should be:

csv_files = ['waves_00s.csv', 'waves_10s.csv', 'waves_80s.csv', 'waves_90s.csv', 'wavesmonthly.csv']
nc_files = ['multyear_hs_avg.nc']

Solution

filenames = ['wavesmonthly.csv',
        'waves_00s.csv',
        'waves_10s.csv',
        'waves_80s.csv',
        'waves_90s.csv',
        'multyear_hs_avg.nc']
csv_files = []
nc_files = []
other_files = []

for filename in filenames:
    if filename.endswith('csv'):
        csv_files.append(filename)
    elif filename.endswith('nc'):
        nc_files.append(filename)
    else:
        other_files.append(filename)

print('csv_files:', csv_files)
print('nc_files:', nc_files)
print('other_files:', other_files)

Counting Vowels

1. Write a loop that counts the number of vowels in a character string.
2. Test it on a few individual words and full sentences.
3. Once you are done, compare your solution to your neighbor’s. Did you make the same decisions about how to handle the letter ‘y’ (which some people think is a vowel, and some do not)?

Solution

vowels = 'aeiouAEIOU'
sentence = 'Mary had a little lamb.'
count = 0
for char in sentence:
    if char in vowels:
        count += 1

print('The number of vowels in this string is ' + str(count))

Key Points

• Use if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.

• The bodies of the branches of conditional statements must be indented.

• Use == to test for equality.

• X and Y is only true if both X and Y are true.

• X or Y is true if either X or Y, or both, are true.

• Zero, the empty string, and the empty list are considered false; all other numbers, strings, and lists are considered true.

• True and False represent truth values.