Non-Programmer's Tutorial for Python 3/Decisions

If statement
As always, I believe I should start each chapter with a warm-up typing exercise, so here is a short program to compute the absolute value of an integer:

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 " ". Next it reads the line " ". If  is less than zero Python runs the line " ". Otherwise it runs the line " ".

More formally Python looks at whether the expression  is true or false. An  statement is followed by an indented block of statements that are run when the expression is true. Optionally after the  statement is an   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:

Another feature of the  command is the   statement. It stands for else if and means if the original  statement is false but the   part is true, then do the   part. And if neither the  or   expressions are true, then do what's in the   block. Here's an example:

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  is only tested when the   statement fails to be true. There can be more than one  expression, allowing multiple tests to be done in a single   statement.

Examples
And the output

False False

high_low.py

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

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

average2.py

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
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."

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!"

Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."