10. If Statements¶
So far we’ve learned to create expressions, use variables, and make our own functions.
Our next step is to learn how to write code that will only run based on a certain condition. For example, we may want to draw “Game Over” text on the screen–but only if the player has run out of lives.
To do this we need to learn how to use an
if statement is also known as a
conditional statement. (You can use the term “conditional statement” when you
want to impress everyone how smart you are.) The
if statement allows a computer
to make a decision. Is it hot outside? Has the spaceship reached the edge of the
screen? Has too much money been withdrawn from the account? A program can
decide which block of code to run based on this statement.
10.1. Basic Comparisons¶
Here are two example
As you can probably guess, the statements see which variable is smaller,
and which is larger.
1# Variables used in the example ``if`` statements 2a = 4 3b = 5 4 5# Basic comparisons 6if a < b: 7 print("a is less than b") 8 9if a > b: 10 print("a is greater than b") 11 12print("Done")
Try running the code. You should get output that looks like this:
a is less than b Done
a is less than
b, the first statement will print out if this code
is run. Flip the values, and the second statement will run.
If both variables
b were equal to 4, then neither of
the two statements would print because 4 is not greater than 4.
Play around with the program by adjusting the values to verify the code continues to do what we expect.
To show the flow of a program a flowchart may be used. Most people can follow a
flowchart even without an introduction to programming. See how the flowchart below
matches with the code we just ran.
if statements go in diamonds and have two
possible paths out of them. Regular statements go in rectangles, and have only one
This book skips an in-depth look at flow-charting, but if you are curious and want to learn more, Wikipedia has a great article introducing how it works:
10.3. More Comparison Types¶
The prior example checked to see if variables were “greater than” or “less than.”
Numbers that were equal would not pass the test. To check if values are greater than or equal,
we use the two characters
>= right next to each other, with no space between.
Let’s adjust our prior code sample:
1if a <= b: 2 print("a is less than or equal to b") 3 4if a >= b: 5 print("a is greater than or equal to b")
>= symbols must be used in order, and there may not be a
space between them. For example,
=< will not work, nor will
When writing these statements out on actual paper, some new programmers like to use the
symbol, because that’s what we learned in math. For example:
if a ≤ b:
≤ symbol doesn’t actually work in a program. Plus most people don’t
know how to type it on the keyboard. (Just in case you are curious,
to type it hold down the ‘alt’ key while typing 243 on the number pad.) So when
writing out code, particularly on paper, remember to use the two characters
and not the one character
≤ symbol. If you encounter a written test or quiz,
make sure to write it out correctly and not lose points.
The next set of code checks to see if two items are equal or not. If we want to
check for equality, we use two equals signs in a row:
The operator for not equal is
!=. Here they are in action.
Add them to the bottom of the other code we were working on, and try them out.
1# Equal 2if a == b: 3 print("a is equal to b") 4 5# Not equal 6if a != b: 7 print("a and b are not equal")
Learn when to use = and ==.
It is very easy to mix up when to use
== if you
are asking if they are equal, use
= if you are assigning a value.
The two most common mistakes in mixing the
== operators are
1# This is wrong 2a == 1 3 4# This is also wrong 5if a = 1: 6 print("A is one")
Stop! Please take a moment to go back and carefully study the last two code
examples. Save time later by making sure you understand when to use
==. Don’t guess.
Indentation matters. Each line under the
if statement that is indented will
only be executed
if the statement is
True. In this example, the three lines
indented under the
if statement will run only if true, the fourth print statement will
always run because it is not indented under an
1if a == 1: 2 print("If a is one, this will print.") 3 print("So will this.") 4 print("And this.") 5 6print("This will always print because it is not indented.")
The indentation amount must be the same. This wandering code doesn’t work.
1if a == 1: 2 print("Indented two spaces.") 3 print("Indented four. This will generate an error.") 4 print("The computer will want you to make up your mind.")
if statement has been finished, it is not possible to re-indent to
go back to it. The test has to be performed again. So this code example doesn’t
work at all:
1if a == 1: 2 print("If a is one, this will print.") 3 print("So will this.") 4 5print("This will always print because it is not indented.") 6 print("This will generate an error. Why it is indented?")
If you try running it, you’ll get the following
File "c:/my_code/test.py", line 8 print("This will generate an error. Why it is indented?") ^ IndentationError: unexpected indent
10.5. Using And/Or¶
if statement can check multiple conditions by chaining together
or. These are also considered to be
operators just like the
- characters are.
Go ahead and create a third variable
c and try using these two new operators
1# And 2if a < b and a < c: 3 print("a is less than b and c") 4 5# Non-exclusive or 6if a < b or a < c: 7 print("a is less than either b or c (or both)")
Repeat yourself please.
A common mistake is to omit a variable when checking it against multiple
conditions. You cannot write
if a < b or c. Neither will
if a < b or < c work.
The computer does not know
what to check against the variable
c. It will not assume to check it
a. You need the full
if a < b or a < c for it to work correctly.
1# This is not correct 2if a < b or < c: 3 print("a is less than b and c")
10.6. Boolean Variables¶
Python supports Boolean variables. What are Boolean variables? Boolean variables
can store a value of either
False. Boolean algebra was
developed by George Boole back in 1854, well before electronic computers.
Yet his work forms the basis of how computer logic works.
If only he knew how important his work would become!
if statement needs an expression to evaluate to
may seem odd is that it does not actually need to do any comparisons if a
variable already evaluates to
For example, in this code we set the variable
does’t need a comparison. If
a is True it will execute the statement, if it is
it will not.
1# Boolean data type. This is legal! 2a = True 3if a: 4 print("a is true")
Another operator we can use is the
not operator. You can take any statement or boolean
and flip between True/False. The statement
if a runs if
True, the statement
if not a runs if the statement is
1# How to use the not function 2if not a: 3 print("a is false")
It is also possible to use Boolean variables with the
1a = True 2b = False 3 4if a and b: 5 print("a and b are both true")
Who knew True/False could be hard?
It is also possible to assign a variable to the result of a comparison. In the
code below, the variables
b are compared. If they are equal,
c will be
1a = 3 2b = 3 3 4# This next line is strange-looking, but legal. 5# c will be true or false, depending if 6# a and b are equal. 7c = a == b 8 9# Prints value of c, in this case True 10print(c)
Zero means False. Everything else is True.
It is possible to create an
if statement with a condition that does not
False. Because this is confusing, we don’t use this
fact a lot, but it is
important to understand how the computer handles these values when searching for
problems. These two examples will run, and will cause the text to be printed out
because the values in the
if statement are non-zero:
1if 1: 2 print("1") 3if "A": 4 print("A")
The code below will not print out anything because the value in the
statement is zero. The value zero is treated as
Any value other than zero (like 2, -1, 600, or even “Fred”) is considered
1if 0: 2 print("Zero")
Rather than hard-coding values into our program, we can use the
function to ask the user to type something in. The
is reasonably simple to use:
1temperature = input("What is the temperature in Fahrenheit? ") 2print("You said the temperature was " + temperature + ".")
As a parameter to
input, just give it text to use as a prompt. Whatever
the user types in, is stored in the variable on the left.
Note that there is a question mark and a space at the end of that string. If you didn’t have this, what the user types in will run right up next to the prompt. That looks terrible. The user is tempted to type a space as the first part of their input, which complicates things for us later.
There’s one more thing we have to learn. We can’t take what the user types in and compare it to a number. This program:
1temperature = input("What is the temperature in Fahrenheit? ") 2if temperature > 90: 3 print("It is hot outside.")
…will fail with a
TypeError: '>' not supported between instances of 'str' and 'int'
Whatever the user types in is stored as text. We also call text a “string” because to the computer it is just a string of characters. This is different than a number, and the computer does not know how to compare them.
Therefore, we need to convert the input into a number. We can do with with
int for integers. If we have floating point numbers, use the
In this example we get the input, convert it to an integer, and then we can
use it in an
1# Get input from the user 2temperature = input("What is the temperature in Fahrenheit? ") 3 4# Convert the input to an integer 5temperature = int(temperature) 6 7# Do our comparison 8if temperature > 90: 9 print("It is hot outside.")
You can chain the
int functions on one line if you like, as shown here:
1# Get input from the user 2temperature = int(input("What is the temperature in Fahrenheit? ")) 3 4# Do our comparison 5if temperature > 90: 6 print("It is hot outside.")
10.8. Else and Else If¶
So far, we’ve used simple
if statements to run a block of code, or not
run it. What if we want to select between two blocks of code? What if
we want to run one of four different blocks?
To do this, we need “else” and “else if” logic.
Let’s start with a simple
if statement that prints if it is hot outside:
1temperature = int(input("What is the temperature in Fahrenheit? ")) 2if temperature > 90: 3 print("It is hot outside") 4print("Done")
If the programmer wants code to be executed if it is not hot, she can use the
else statement. As for indentation, notice how the
else is lined up
i in the
statement. Also notice how
else is followed by a colon just like the
Try running this code. The
if statement will always run one of the two
blocks, but never both.
1temperature = int(input("What is the temperature in Fahrenheit? ")) 2if temperature > 90: 3 print("It is hot outside") 4else: 5 print("It is not hot outside") 6print("Done")
In the case of an if…else statement, one block of code will always be
executed. The first block will be executed
if the statement evaluates to
True, the second block if it evaluates to
It is possible to chain several
if statements together using the else…if
statement. Python abbreviates this as
elif. In this example we add a
third possible combination using
1temperature = int(input("What is the temperature in Fahrenheit? ")) 2if temperature > 90: 3 print("It is hot outside") 4elif temperature < 30: 5 print("It is cold outside") 6else: 7 print("It is not hot outside") 8print("Done")
In this next example, we’ve made a mistake. The program will output “It is hot outside” even if the user types in 120 degrees. Why? How can the code be fixed?
1temperature = int(input("What is the temperature in Fahrenheit? ")) 2if temperature > 90: 3 print("It is hot outside") 4elif temperature > 110: 5 print("Oh man, you could fry eggs on the pavement!") 6elif temperature < 30: 7 print("It is cold outside") 8else: 9 print("It is ok outside") 10print("Done")
The computer doesn’t do a “best-fit”, it looks at the
if statement in order.
As 120 is greater than 90, it will run the first condition and skip the rest.
To fix this code, we need to reorder it.
1temperature = int(input("What is the temperature in Fahrenheit? ")) 2if temperature > 110: 3 print("Oh man, you could fry eggs on the pavement!") 4elif temperature > 90: 5 print("It is hot outside") 6elif temperature < 30: 7 print("It is cold outside") 8else: 9 print("It is ok outside") 10print("Done")
10.9. Text Comparisons¶
It is possible to use an
if statement to check text, as shown in this example.
1user_name = input("What is your name? ") 2if user_name == "Paul": 3 print("You have a nice name.") 4else: 5 print("Your name is ok.")
The prior example will only match if the user enters “Paul” with a capital P. It will not work if the user enters lower-case “paul” or upper-case “PAUL”.
A common mistake is to forget the quotes around the string being compared. In the example below, the computer will think that Paul is a variable that stores a value. It will flag an error because it has no idea what is stored in the variable Paul.
1user_name = input("What is your name? ") 2if user_name == Paul: # This does not work because quotes are missing 3 print("You have a nice name.") 4else: 5 print("Your name is ok.")
10.9.1. Multiple Text Possibilities¶
When comparing a variable to multiple possible strings of text, it is important to remember that the comparison must include the variable. For example:
# This does not work! It will always be true if user_name == "Paul" or "Mary":
Instead, the code should read like this:
# This does work if user_name == "Paul" or user_name == "Mary":
Confusingly, the first example will run, but it will always trigger the
if statement even if the name isn’t Paul or Mary.
This is because for any value other than zero, the computer assumes to mean
True. “Mary” isn’t zero.
So to the computer “Mary” is the same thing as
True and so it
will run the code in the
10.9.2. Case-Insensitive Comparisons¶
If the program needs to match text, and we don’t care if it is upper or
lower case, we need a case-insensitive comparison.
The easiest way to do that is to convert everything to lower-case, then compare.
This can be done with the
Learn to be insensitive.
The example below will take whatever the user enters, convert it to lower-case, and then do the comparison. Important: Don’t convert to lower-case and compare it against a string that has uppercase! Then you’ll never match.
1user_name = input("What is your name? ") 2if user_name.lower() == "paul": 3 print("You have a nice name.") 4else: 5 print("Your name is ok.")
With the introduction of
if statements we’ve learned how to run code
only if a condition is true. By adding in
we can run one of multiple possible code blocks depending upon a condition.
We’ve learned that we write conditions using comparison operators like
>. We’ve learned that
== is the comparison operator
for asking if two items are equal,
= is the assignment operator used to set a variable’s value.
We can reverse logic using the
Finally, we were introduced to Boolean values, where variables can store
if statements can operate based on those values.
10.10.1. Example if Statements¶
The next set of example code below runs through all the concepts talked about earlier.
1# Sample Python/Pygame Programs 2# Simpson College Computer Science 3# http://programarcadegames.com/ 4# http://simpson.edu/computer-science/ 5 6# Explanation video: http://youtu.be/pDpNSck2aXQ 7 8# Variables used in the example if statements 9a = 4 10b = 5 11c = 6 12 13# Basic comparisons 14if a < b: 15 print("a is less than b") 16 17if a > b: 18 print("a is greater than than b") 19 20if a <= b: 21 print("a is less than or equal to b") 22 23if a >= b: 24 print("a is greater than or equal to b") 25 26# NOTE: It is very easy to mix when to use == and =. 27# Use == if you are asking if they are equal, use = 28# if you are assigning a value. 29if a == b: 30 print("a is equal to b") 31 32# Not equal 33if a != b: 34 print("a and b are not equal") 35 36# And 37if a < b and a < c: 38 print("a is less than b and c") 39 40# Non-exclusive or 41if a < b or a < c: 42 print("a is less than either a or b (or both)") 43 44 45# Boolean data type. This is legal! 46a = True 47if a: 48 print("a is true") 49 50if not a: 51 print("a is false") 52 53a = True 54b = False 55 56if a and b: 57 print("a and b are both true") 58 59a = 3 60b = 3 61c = a == b 62print(c) 63 64# These are also legal and will trigger as being true because 65# the values are not zero: 66if 1: 67 print("1") 68if "A": 69 print("A") 70 71# This will not trigger as true because it is zero. 72if 0: 73 print("Zero") 74 75# Comparing variables to multiple values. 76# The first if statement appears to work, but it will always 77# trigger as true even if the variable a is not equal to b. 78# This is because "b" by itself is considered true. 79a = "c" 80if a == "B" or "b": 81 print("a is equal to b. Maybe.") 82 83# This is the proper way to do the if statement. 84if a == "B" or a == "b": 85 print("a is equal to b.") 86 87# Example 1: If statement 88temperature = int(input("What is the temperature in Fahrenheit? ")) 89if temperature > 90: 90 print("It is hot outside") 91print("Done") 92 93# Example 2: Else statement 94temperature = int(input("What is the temperature in Fahrenheit? ")) 95if temperature > 90: 96 print("It is hot outside") 97else: 98 print("It is not hot outside") 99print("Done") 100 101# Example 3: Else if statement 102temperature = int(input("What is the temperature in Fahrenheit? ")) 103if temperature > 90: 104 print("It is hot outside") 105elif temperature < 30: 106 print("It is cold outside") 107else: 108 print("It is not hot outside") 109print("Done") 110 111# Example 4: Ordering of statements 112# Something with this is wrong. What? 113temperature = int(input("What is the temperature in Fahrenheit? ")) 114if temperature > 90: 115 print("It is hot outside") 116elif temperature > 110: 117 print("Oh man, you could fry eggs on the pavement!") 118elif temperature < 30: 119 print("It is cold outside") 120else: 121 print("It is ok outside") 122print("Done") 123 124# Comparisons using string/text 125# The input statement will ask the user for input and put it in user_name. 126user_name = input("What is your name? ") 127if user_name == "Paul": 128 print("You have a nice name.") 129else: 130 print("Your name is ok.")