(ProgWerk) Making Choices
Last updated on 2024-03-10 | Edit this page
Estimated time: 30 minutes
Overview
Questions
- How can my programs do different things based on data values?
Objectives
- Write conditional statements including
if
,elif
, andelse
branches. - Correctly evaluate expressions containing
and
andor
.
In our last lesson, we discovered something suspicious was going on in our inflammation data by drawing some plots. How can we use Python to automatically recognize the different features we saw, 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:
The following example will lead to a syntax error in the Python
prompt, as it seems to expect exactly one top-level statement per
invocation. Removing print('done')
from the example will
fix the problem.
IPython executes the example from a single prompt without throwing an error.
OUTPUT
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:
PYTHON
num = 53
print('before conditional...')
if num > 100:
print(num, 'is greater than 100')
print('...after conditional')
OUTPUT
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.
PYTHON
num = -3
if num > 0:
print(num, 'is positive')
elif num == 0:
print(num, 'is zero')
else:
print(num, 'is negative')
OUTPUT
-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.
We can also combine tests using and
and or
.
and
is only true if both parts are true:
PYTHON
if (1 > 0) and (-1 >= 0):
print('both parts are true')
else:
print('at least one part is false')
OUTPUT
at least one part is false
while or
is true if at least one part is true:
OUTPUT
at least one test is true
Checking our Data
Now that we’ve seen how conditionals work, we can use them to check
for the suspicious features we saw in our inflammation 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 and data with:
From the first couple of plots, we saw that maximum daily inflammation exhibits a strange behavior and raises one unit a day. Wouldn’t it be a good idea to detect such behavior and report it as suspicious? Let’s do that! However, instead of checking every single day of the study, let’s merely check if maximum inflammation in the beginning (day 0) and in the middle (day 20) of the study are equal to the corresponding day numbers.
PYTHON
max_inflammation_0 = numpy.amax(data, axis=0)[0]
max_inflammation_20 = numpy.amax(data, axis=0)[20]
if max_inflammation_0 == 0 and max_inflammation_20 == 20:
print('Suspicious looking maxima!')
We also saw a different problem in the third dataset; the minima per
day were all zero (looks like a healthy person snuck into our study). We
can also check for this with an elif
condition:
And if neither of these conditions are true, we can use
else
to give the all-clear:
Let’s test that out:
PYTHON
data = numpy.loadtxt(fname='inflammation-01.csv', delimiter=',')
max_inflammation_0 = numpy.amax(data, axis=0)[0]
max_inflammation_20 = numpy.amax(data, axis=0)[20]
if max_inflammation_0 == 0 and max_inflammation_20 == 20:
print('Suspicious looking maxima!')
elif numpy.sum(numpy.amin(data, axis=0)) == 0:
print('Minima add up to zero!')
else:
print('Seems OK!')
OUTPUT
Suspicious looking maxima!
PYTHON
data = numpy.loadtxt(fname='inflammation-03.csv', delimiter=',')
max_inflammation_0 = numpy.amax(data, axis=0)[0]
max_inflammation_20 = numpy.amax(data, axis=0)[20]
if max_inflammation_0 == 0 and max_inflammation_20 == 20:
print('Suspicious looking maxima!')
elif numpy.sum(numpy.amin(data, axis=0)) == 0:
print('Minima add up to zero!')
else:
print('Seems OK!')
OUTPUT
Minima add up to zero!
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.
C gets printed because the first two conditions,
4 > 5
and 4 == 5
, are not true, but
4 < 5
is true. In this case only one of these conditions
can be true for at a time, but in other scenarios multiple
elif
conditions could be met. In these scenarios only the
action associated with the first true elif
condition will
occur, starting from the top of the conditional section.
This contrasts with the case of multiple if
statements,
where every action can occur as long as their condition is met.
There is a built-in
function abs
that returns the absolute value of a
number:
OUTPUT
12
In-Place Operators
Python (and most other languages in the C family) provides in-place operators that work like this:
PYTHON
x = 1 # original value
x += 1 # add one to x, assigning result back to x
x *= 3 # multiply x by 3
print(x)
OUTPUT
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?
PYTHON
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, large data sets are stored in files
whose names start with “inflammation-” and small data sets – in files
whose names start with “small-”. We also have some other files that we
do not care about at this point. We’d like to break all these files into
three lists called large_files
, small_files
,
and other_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
starts with the string passed as an argument, that is:
OUTPUT
True
But
OUTPUT
False
Use the following Python code as your starting point:
PYTHON
filenames = ['inflammation-01.csv',
'myscript.py',
'inflammation-02.csv',
'small-01.csv',
'small-02.csv']
large_files = []
small_files = []
other_files = []
Your solution should:
- loop over the names of the files
- figure out which group each filename belongs in
- append the filename to that list
In the end the three lists should be:
PYTHON
for filename in filenames:
if filename.startswith('inflammation-'):
large_files.append(filename)
elif filename.startswith('small-'):
small_files.append(filename)
else:
other_files.append(filename)
print('large_files:', large_files)
print('small_files:', small_files)
print('other_files:', other_files)
Counting Vowels
- Write a loop that counts the number of vowels in a character string.
- Test it on a few individual words and full sentences.
- 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)?
Key Points
- Use
if condition
to start a conditional statement,elif condition
to provide additional tests, andelse
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 bothX
andY
are true. -
X or Y
is true if eitherX
orY
, 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
andFalse
represent truth values.