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Conditionals

Overview

Teaching: 60 min
Exercises: 30 min
Questions

  • How can I make my program make choices?

Objectives

  • Understand the syntax and indentation of an if statement

  • Know when to use if, elif, and else

  • Be able to explain which statements in the if statement block will be executed

Conditionals

Conditionals are another powerful tool to use in our computer programs: they allow the program to make choices and run part of the code only if a specific condition or conditions are met.

The syntax of in if statement looks like this. Note that the indentation is very important!

if <condition>:
	do this if condition 1 is true
else:
	do this if the condition is not true
	there can be multiple statements nested under the if or else statement
		
this is not part of the if/else block and will always be executed

For example, we could write code to check the values of variables and take an action if they are in a specific range

a= 17

if a < 10:
	print("the number is less than 10")
else:
	print("the number is 10 or greater")

print("this will always get printed")

We can use elif (short for “else if”) to check multiple conditions. Python will check each condition one at a time until it finds one that is true. It will then execute the code under that first true statement, and then be done with the for loop. If no true statement is found, it will execute the code under the else statement

number = 29
if number < 10:
	print("the number is less than 10")

elif number < 20:
	print("the number is between 10 and 20")

elif number < 30:
	print("the number is between 20 and 30")
	
else:
	print('It is a big number')
    
print('We are done checking if/else conditions')

When you run this code, only the statment under the second elif (elif number < 30) is printed. Once a true condition is encountered, the code under that condition is executed (in this case, print('the number is between 20 and 30')). Python skips over the remaining else statement and then proceeds to the last print statement, which is not part of the if/elif/else block.

Even if multiple conditions are true, Python will only execute the code under the first true if/elif statement it encounters:

number = 15
if number < 10:
	print("the number is less than 10")

elif number < 20:
	print("the number is between 10 and 20")

elif number < 30:
	print("the number is between 20 and 30")

elif number == 15:
	print("the number is exactly 15")
	
else:
	print('It is a big number')
    
print('We are done checking if/else conditions')

Notice that when you run this code, only the statement under the first elif is printed (‘the number is between 10 and 20’), even though the next two elif statements are also true!

“How many paths”

Consider the code below. Which statement(s) would be printed? Why?

count = 207
if count < 30:
    print("count is too small")
elif count < 300:
    print("Plate is countable")
else:
    print("count is too large")

Solution

Only the second statement ‘Plate is countable’ should be printed. The first statement (count < 30) is not true. The second statement (count < 300) is true, so the print statement underneath it will be executed. The final else statement is not checked because one of the preceding statements was true.

Comparison operators (==, <, >, <=, >=, !=)

We have already seen the comparison operators < (less than) and == (used for checking equality, not the same as the = assignment operator) Other possible operators are:

Booleans

True and False are special words in Python called booleans which represent true and false statements. However, they aren’t the only values in Python that are true and false. In fact, any value can be used in an if or elif statement.

What values are True?

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')

Solution/Explanation

In general, the value 1 is considered to be true and 0 is false. Empty objects, such as the empty string and empty list above are considered false.

Checking multiple conditions with one statement

We can combine and check multiple conditions in one statement, using the keywords and and or.

number = 10
if number > 5 and number < 20:
    print("number is medium")
else:
    print("the number is small or large")
    
if number < 5 or number > 20:
    print("number is large or small")
else:
    print("number is medium")

number is medium
number is medium

Combining conditionals

Previously, we saw code to check whether or not a plate count was in the “countable” range, between 30 or 300 colonies:

count = 207
if count < 30:
    print("count is too small")
elif count < 300:
    print("Plate is countable")
else:
    print("count is too large")

Modify this code to check countability using a single if statement.

Solution

There are a couple of ways you could tackle this. One is to use an and statement:

count = 207
if count > 30 and count < 300:
    print("Plate is countable")
else:
    print("count is too small or too large")

You could also use an or statement:

count = 207
if count < 30 or count > 300:
    print("count is too small or too large")
else:
    print("Plate is countable")

Which parts do I need?

In general all if statement blocks must start with an if statement! However, the other parts aren’t always needed.

  • An if statement is not required be followed by an else. However, else must always be preceded by an if!
  • You may have as many elif statements as you like following your if statement. elif indicates that these are alternatives to each other, and only the code under the first true if or elif will be executed.
  • In general, an else statement is not required for if/elif code blocks. If an else is present, code underneath the else statement will only be executed if none of the preceding if or elif statements are true.
  • If you wish to check multiple conditions that are not alternatives to each other (i.e. each statement should be checked regardless of the previous statement) you may stack up multiple if statements. If you use if/if , each if statement will be checked for truth.

Which path?

Look at the code below. Which print statements will be executed and why? If you are not sure, try running it!

number = 5

if number <= 10:
    print('number is smaller than 10')
if number <= 20:
    print('number is smaller than 20')
if number <= 40:
    print('number is smaller than 40')
elif number > 30 and  number < 100:
    print('number is medium-sized')
else:
    print('the number is really big')

Solution

The code above should generate the following output:

number is smaller than 10
number is smaller than 20
number is smaller than 40

The first three if statements are independent of each other. They will all be checked, regardless of whether or not a previous if statement was true. The elif and else statements go with the third if statement. elif will only be checked if the third if is not true, and else will only be checked if the the third if and the elif are not true.

What value of number would cause only the statement under elif to be printed? What would cause only the else statement to be printed?

Combining loops and conditionals part 1: Checking plate counts

We can combine for loops and conditionals to check a condition for each item in a list. For example, the code below will print each fruit in the list of fruits that has exactly six letters:

fruits = ['apple', 'cherry', 'peach', 'orange', 'banana', 'mango']
for fruit in fruits:
    if len(fruit) == 6:
        print(fruit)

cherry
orange
banana

In a previous example, you wrote code to check whether or not a plate count was countable (between 30 and 300 colonies). Modify that code to check countability for each count in a list of plate counts. Use the list below:
plateCounts = [43, 207, 6, 1247]

Solution

We can simply nest our if statement code underneath the for loop:

plateCounts = [43, 207, 6, 1247]    
for count in plateCounts:
    if count > 30 and count < 300:
        print("Plate is countable")
    else:
        print("count is too small or too large")

Combining loops and conditionals part 2: Sorting image files into bins

Imagine that we have a list of image files that are different types of images (you will learn more about the different image formats later in the week).
You have the following list of image names and three empty lists.

images = ['image1.png', 'image2.jpg', 'image3.bmp', 'image4.jpg', 'image5.png']

bmps = []
pngs = []
jpgs = []

Write code to put each of the image names in images into the appropriate list. Your code should

  1. Loop through all of the images.
  2. For each image, check the file extension (the part after the “.”), and append the filename to the appropriate list.
  3. Print the final lists. They should have these values:

bmps = [‘image3.bmp’]
jpgs = [‘image2.jpg’, ‘image4.jpg’]
pngs = [‘image1.png’, ‘image5.png’]

Solution

images = ['image1.png', 'image2.jpg', 'image3.bmp', 'image4.jpg', 'image5.png']

#create the empty lists that we will add file names into
bmps = []
pngs = []
jpgs = []

for filename in images:
    if filename[-3:] == 'bmp': # [-3:] looks at the last 3 characters of the filename
            bmps.append(filename)
    elif filename[-3:] == 'png':
            pngs.append(filename)
    elif filename[-3:] == 'jpg':
            jpgs.append(filename)
    else:
            print(filename, 'is not a jpg, bmp, or png file')

#print final lists
print('bmps: ', bmps)
print('pngs: ', pngs)
print('jpgs: ', jpgs)

Key Points

  • If/elif/else statements allow your program to do different things if different conditions are met

  • If/elif/else blocks require careful attention to indentation

  • In an if/elif/else block, the code underneath the first condition that is true will be executed

  • Code under other conditions will not be executed, even if they are also true

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