NonProgrammer's Tutorial for Python 3/Decisions
If statement[edit]
As always, I believe I should start each chapter with a warmup typing exercise, so here is a short program to compute the absolute value of an integer:
n = int(input("Number? "))
if n < 0:
print("The absolute value of", n, "is", n)
else:
print("The absolute value of", n, "is", n)
Here is the output from the two times that I ran this program:
Number? 34 The absolute value of 34 is 34
Number? 1 The absolute value of 1 is 1
So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = int(input("Number? "))
". Next it reads the line "if n < 0:
". If n
is less than zero Python runs the line "print("The absolute value of", n, "is", n)
". Otherwise it runs the line "print("The absolute value of", n, "is", n)
".
More formally Python looks at whether the expression n < 0
is true or false. An if
statement is followed by an indented block of statements that are run when the expression is true. Optionally after the if
statement is an else
statement and another indented block of statements. This second block of statements is run if the expression is false.
There are a number of different tests that an expression can have. Here is a table of all of them:
operator  function 

<

less than 
<=

less than or equal to 
>

greater than 
>=

greater than or equal to 
==

equal 
!=

not equal 
Another feature of the if
command is the elif
statement. It stands for else if and means if the original if
statement is false but the elif
part is true, then do the elif
part. And if neither the if
or elif
expressions are true, then do what's in the else
block. Here's an example:
a = 0
while a < 10:
a = a + 1
if a > 5:
print(a, ">", 5)
elif a <= 3:
print(a, "<=", 3)
else:
print("Neither test was true")
and the output:
1 <= 3 2 <= 3 3 <= 3 Neither test was true Neither test was true 6 > 5 7 > 5 8 > 5 9 > 5 10 > 5
Notice how the elif a <= 3
is only tested when the if
statement fails to be true. There can be more than one elif
expression, allowing multiple tests to be done in a single if
statement.
Examples[edit]
# This Program Demonstrates the use of the == operator
# using numbers
print(5 == 6)
# Using variables
x = 5
y = 8
print(x == y)
And the output
False False
high_low.py
# Plays the guessing game higher or lower
# This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter. (Extra Credit, modify it to be random
# after the Modules chapter)
number = 7
guess = 1
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
Sample run:
Guess the number! Is it... 2 It's bigger... Is it... 5 It's bigger... Is it... 10 It's not so big. Is it... 7 Hooray! You guessed it right!
even.py
# Asks for a number.
# Prints if it is even or odd
number = float(input("Tell me a number: "))
if number % 2 == 0:
print(int(number), "is even.")
elif number % 2 == 1:
print(int(number), "is odd.")
else:
print(number, "is very strange.")
Sample runs:
Tell me a number: 3 3 is odd.
Tell me a number: 2 2 is even.
Tell me a number: 3.4895 3.4895 is very strange.
average1.py
# keeps asking for numbers until 0 is entered.
# Prints the average value.
count = 0
sum = 0.0
number = 1 # set to something that will not exit the while loop immediately.
print("Enter 0 to exit the loop")
while number != 0:
number = float(input("Enter a number: "))
if number != 0:
count = count + 1
sum = sum + number
if number == 0:
print("The average was:", sum / count)
Sample runs:
Enter 0 to exit the loop Enter a number: 3 Enter a number: 5 Enter a number: 0 The average was: 4.0
Enter 0 to exit the loop Enter a number: 1 Enter a number: 4 Enter a number: 3 Enter a number: 0 The average was: 2.66666666667
average2.py
# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
#Notice that we use an integer to keep track of how many numbers,
# but floating point numbers for the input of each number
sum = 0.0
print("This program will take several numbers then average them")
count = int(input("How many numbers would you like to average: "))
current_count = 0
while current_count < count:
current_count = current_count + 1
print("Number", current_count)
number = float(input("Enter a number: "))
sum = sum + number
print("The average was:", sum / count)
Sample runs:
This program will take several numbers then average them How many numbers would you like to average: 2 Number 1 Enter a number: 3 Number 2 Enter a number: 5 The average was: 4.0
This program will take several numbers then average them How many numbers would you like to average: 3 Number 1 Enter a number: 1 Number 2 Enter a number: 4 Number 3 Enter a number: 3 The average was: 2.66666666667
Exercises[edit]
Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."
name = input('Your name: ')
if name == 'Bryn':
print('That is a nice name.')
elif name == 'John Cleese':
print('... some funny text.')
elif name == 'Michael Palin':
print('... some funny text.')
else:
print('You have a nice name.')
Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." at the end, otherwise print "Good job!"
number = 7
guess = 1
count = 0
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
count = count + 1
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
if count > 3:
print("That must have been complicated.")
else:
print("Good job!")
Write a program that asks for two numbers. If the sum of the numbers
is greater than 100, print "That is a big number."
number1 = float(input('1st number: '))
number2 = float(input('2nd number: '))
if number1 + number2 > 100:
print('That is a big number.')