Conditionals

Last updated on 2025-11-12 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • 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)

PYTHON 

mass = 3.54
if mass > 3.0:
    print(mass, 'is large')

mass = 2.07
if mass > 3.0:
    print (mass, 'is large')

OUTPUT 

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.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is large')

OUTPUT 

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 an if.
  • Allows us to specify an alternative to execute when the if branch isn’t taken.

PYTHON 

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')

OUTPUT 

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”).

PYTHON 

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')

OUTPUT 

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.

PYTHON 

grade = 85
if grade >= 90:
    print('grade is A')
elif grade >= 80:
    print('grade is B')
elif grade >= 70:
    print('grade is C')

OUTPUT 

grade is B
  • Does not automatically go back and re-evaluate if values change.

PYTHON 

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

OUTPUT 

adjusting velocity
  • Often use conditionals in a loop to “evolve” the values of variables.

PYTHON 

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)

OUTPUT 

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 of velocity, since its value is updated by the last iteration of the loop.
Callout

Compound Relations Using and, or, and Parentheses

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or. Continuing the example above, suppose you have

PYTHON 

mass     = [ 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[i] <= 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 and or in the same condition. That is, instead of:

PYTHON 

if mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:

write one of these:

PYTHON 

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.

Challenge

Tracing Execution

What does this program print?

PYTHON 

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

OUTPUT 

25.0
Challenge

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 original list’s values were positive.

PYTHON 

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)

OUTPUT 

[0, 1, 1, 1, 0, 1]

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
    if value < 0.0:
        result.append(0)
    else:
        result.append(1)
print(result)
Challenge

Processing Small Files

Modify this program so that it only processes files with fewer than 50 records.

PYTHON 

import glob
import pandas as pd
for filename in glob.glob('data/*.csv'):
    contents = pd.read_csv(filename)
    ____:
        print(filename, len(contents))

PYTHON 

import glob
import pandas as pd
for filename in glob.glob('data/*.csv'):
    contents = pd.read_csv(filename)
    if len(contents) < 50:
        print(filename, len(contents))
Challenge

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.

PYTHON 

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?

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest, largest = None, None
for v in values:
    if smallest is None and largest is None:
        smallest, largest = v, v
    else:
        smallest = min(smallest, v)
        largest = max(largest, v)
print(smallest, largest)

If you wrote == None instead of is None, that works too, but Python programmers always write is None because of the special way None works in the language.

It can be argued that an advantage of using this method would be to make the code more readable. However, a disadvantage is that this code is not efficient because within each iteration of the for loop statement, there are two more loops that run over two numbers each (the min and max functions). It would be more efficient to iterate over each number just once:

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest, largest = None, None
for v in values:
    if smallest is None or v < smallest:
        smallest = v
    if largest is None or v > largest:
        largest = v
print(smallest, largest)

Now we have one loop, but four comparison tests. There are two ways we could improve it further: either use fewer comparisons in each iteration, or use two loops that each contain only one comparison test. The simplest solution is often the best:

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest = min(values)
largest = max(values)
print(smallest, largest)
Discussion

Conditions and Loops

Solve the Rosalind exercise INI4.

Discussion

Slicing or loops?

Oftentimes, the solution to a problem is to keep certain elements from a sequence. If these elements are regular (e.g. every second/third/tenth element) then slicing can be enough. If the selection criteria are more complex though (e.g. each word that starts with the letter “F”) then a for-loop is needed. Try to solve the INI5 Rosalind exercise using a for-loop and list slicing.

Discussion

Practice!

We previously solved the RNA Rosalind exercise using string built-ins. Can we also solve it with a loop and conditionals?

Challenge

Complementing a strand of DNA

Solve the Rosalind exercise REVC.

The problem has two components - reversing, and complementing. Try solving them one by one!

Reversing can be achieved as a special case of slicing.

Given a base X, make a flowchart of complementation rules. Try to translate that to code!

How could we apply the complementation rules to all positions of a sequence? Does this sound like a repetition of the same task? What does Python use to handle repetitions of the same task?

Challenge

Counting point mutations

Solve the Rosalind exercise HAMM.

By looping over both strings at once, we can count mismatches:

PYTHON 

a = "string1"
b = "string2"
n = len(a)
no_mismatches = 0
for i in range(n):
    if a[i] != b[i]:
        no_mismatches += 1
print(no_mismatches)
Challenge

Complementing a strand of DNA #2

Solve the Rosalind exercise REVC, this time without loops; it should be possible through creative use of string built-in methods.

You probably want to use replace() - that is a good instinct! However, if we use replace() sequentially, we have to be careful to not introduce characters that we will be replace()ing in the future. How can we avoid this?

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 an if 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.