Conditionals
Overview
Teaching: 10 min
Exercises: 15 minQuestions
How can programs do different things for different data?
Objectives
Correctly write programs that use if and else statements and simple Boolean expressions (without logical operators).
Trace the execution of unnested conditionals and conditionals inside loops.
Use if
statements to control whether or not a block of code is executed.
- An
if
statement (more properly called a conditional statement) controls whether some block of code is executed or not. - Structure is similar to a
for
statement:- First line opens with
if
and ends with a colon - Body containing one or more statements is indented (usually by 4 spaces)
- First line opens with
mass = 3.54
if mass > 3.0:
print(mass, 'is large')
mass = 2.07
if mass > 3.0:
print (mass, 'is large')
3.54 is large
Conditionals are often used inside loops.
- Not much point using a conditional when we know the value (as above).
- But useful when we have a collection to process.
masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
if m > 3.0:
print(m, 'is large')
3.54 is large
9.22 is large
Use else
to execute a block of code when an if
condition is not true.
else
can be used following anif
.- Allows us to specify an alternative to execute when the
if
branch isn’t taken.
masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
if m > 3.0:
print(m, 'is large')
else:
print(m, 'is small')
3.54 is large
2.07 is small
9.22 is large
1.86 is small
1.71 is small
Use elif
to specify additional tests.
- May want to provide several alternative choices, each with its own test.
- Use
elif
(short for “else if”) and a condition to specify these. - Always associated with an
if
. - Must come before the
else
(which is the “catch all”).
masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
if m > 9.0:
print(m, 'is HUGE')
elif m > 3.0:
print(m, 'is large')
else:
print(m, 'is small')
3.54 is large
2.07 is small
9.22 is HUGE
1.86 is small
1.71 is small
Conditions are tested once, in order.
- Python steps through the branches of the conditional in order, testing each in turn.
- So ordering matters.
grade = 85
if grade >= 70:
print('grade is C')
elif grade >= 80:
print('grade is B')
elif grade >= 90:
print('grade is A')
grade is C
- Does not automatically go back and re-evaluate if values change.
velocity = 10.0
if velocity > 20.0:
print('moving too fast')
else:
print('adjusting velocity')
velocity = 50.0
adjusting velocity
- Often use conditionals in a loop to “evolve” the values of variables.
velocity = 10.0
for i in range(5): # execute the loop 5 times
print(i, ':', velocity)
if velocity > 20.0:
print('moving too fast')
velocity = velocity - 5.0
else:
print('moving too slow')
velocity = velocity + 10.0
print('final velocity:', velocity)
0 : 10.0
moving too slow
1 : 20.0
moving too slow
2 : 30.0
moving too fast
3 : 25.0
moving too fast
4 : 20.0
moving too slow
final velocity: 30.0
Create a table showing variables’ values to trace a program’s execution.
i | 0 | . | 1 | . | 2 | . | 3 | . | 4 | . |
velocity | 10.0 | 20.0 | . | 30.0 | . | 25.0 | . | 20.0 | . | 30.0 |
- The program must have a
print
statement outside the body of the loop to show the final value ofvelocity
, since its value is updated by the last iteration of the loop.
Compound Relations Using
and
,or
, and ParenthesesOften, you want some combination of things to be true. You can combine relations within a conditional using
and
andor
. Continuing the example above, suppose you havemass = [ 3.54, 2.07, 9.22, 1.86, 1.71] velocity = [10.00, 20.00, 30.00, 25.00, 20.00] i = 0 for i in range(5): if mass[i] > 5 and velocity[i] > 20: print("Fast heavy object. Duck!") elif mass[i] > 2 and mass[i] <= 5 and velocity[i] <= 20: print("Normal traffic") elif mass[i] <= 2 and velocity <= 20: print("Slow light object. Ignore it") else: print("Whoa! Something is up with the data. Check it")
Just like with arithmetic, you can and should use parentheses whenever there is possible ambiguity. A good general rule is to always use parentheses when mixing
and
andor
in the same condition. That is, instead of:if mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:
write one of these:
if (mass[i] <= 2 or mass[i] >= 5) and velocity[i] > 20: if mass[i] <= 2 or (mass[i] >= 5 and velocity[i] > 20):
so it is perfectly clear to a reader (and to Python) what you really mean.
Tracing Execution
What does this program print?
pressure = 71.9 if pressure > 50.0: pressure = 25.0 elif pressure <= 50.0: pressure = 0.0 print(pressure)
Trimming Values
Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the origina list’s values were positive.
original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4] result = ____ for value in original: if ____: result.append(0) else: ____ print(result)
[0, 1, 1, 1, 0, 1]
Processing Small Files
Modify this program so that it only processes files with fewer than 50 records.
import glob import pandas for filename in glob.glob('data/*.csv'): contents = pandas.read_csv(filename) ____: print(filename, len(contents))
Initializing
Modify this program so that it finds the largest and smallest values in the list no matter what the range of values originally is.
values = [...some test data...] smallest, largest = None, None for v in values: if ____: smallest, largest = v, v ____: smallest = min(____, v) largest = max(____, v) print(smallest, largest)
What are the advantages and disadvantages of using this method to find the range of the data?
Using Functions With Conditionals in Pandas
Functions will often contain conditionals. Here is a short example that will indicate which quartile the argument is in based on hand-coded values for the quartile cut points.
def calculate_life_quartile(exp): if exp < 58.41: # This observation is in the first quartile return 1 elif exp >= 58.41 and exp < 67.05: # This observation is in the second quartile return 2 elif exp >= 67.05 and exp < 71.70: # This observation is in the third quartile return 3 elif exp >= 71.70: # This observation is in the fourth quartile return 4 else: # This observation has bad data return None calculate_life_quartile(62.5)
2
That function would typically be used within a
for
loop, but Pandas has a different, more efficient way of doing the same thing, and that is by applying a function to a dataframe or a portion of a dataframe. Here is an example, using the definition above.data = pd.read_csv('Americas-data.csv') data['life_qrtl'] = data['lifeExp'].apply(calculate_life_quartile)
There is a lot in that second line, so let’s take it piece by piece. On the right side of the
=
we start withdata['lifeExp']
, which is the column in the dataframe calleddata
labeledlifExp
. We use theapply()
to do what it says, apply thecalculate_life_quartile
to the value of this column for every row in the dataframe.
Key Points
Use
if
statements to control whether or not a block of code is executed.Conditionals are often used inside loops.
Use
else
to execute a block of code when anif
condition is not true.Use
elif
to specify additional tests.Conditions are tested once, in order.
Create a table showing variables’ values to trace a program’s execution.