Getting the Computer to Make Decisions

Introduction to Decisions Logic

Single Selection Decisions

Without the ability to get the computer to make decisions, a computer is little more than a fancy calculator. The ability to make decisions and select and execute appropriate behavior accordingly is what makes computers so powerful. It is the reason, for example, that computers can drive autonomous vehicles, and beat humans at chess, and process online transactions, and so much more.

The simplest type of decision that we can get the computer to make is a decision to execute a set of instructions or skip that set of instructions. Such a decision is called a single selection decision, because we are selecting from a single choice, do this or don't do this.

5.7 Decision.png

Single Selection Decision Structure

Here is an example, deciding whether or not to display a bunny.

01  user_answer = input('Do you like bunnies (yes or no)? ')
02 
03  if user_answer == 'yes':
04  print(' (\\(\\ ')
05  print(' (-.-) ')
06  print(' O_(")(") ')

Lines 03-06 in the example above are a single selection decision structure. It instructs the computer to either print the bunny or skip those lines. The decision that the computer makes (to print the bunny or skip printing the bunny) is based on a condition user_answer == 'yes' on line 03. The condition for a decision is always an expression that evaluates to a boolean value: True or False. If the conditional expression evaluates to True, then the instructions that follow it will be executed. If the conditional expression evaluates to False, then the instructions that follow it will be skipped.

Note that a single selection decision is often simply called an if statement.

All single selection decisions look the same in Python. In other words, they all follow the same rules of syntax. Here, as an example, is an even simpler single selection decision.

if user_answer == 'yes':
    print('Hello!')

This code instructs the computer to make a decision based on the value of the expression user_answer == 'yes'. If the expression user_answer == 'yes' evaluates True, then the computer will execute the print('Hello!') statement. If the expression user_answer == 'yes' evaluates to False, then the computer will skip the print('Hello!') statement.

We can use a block diagram to illustrate the syntax of a single selection decision.

5.8 Decision.png

Note that a single selection decision always has two main parts - a decision header, and a body. The decision header is always the keyword if, which is followed by a boolean expression called the condition, which is followed by a colon (:). We can use a railroad diagram to illustrate the parts of a decision header.

5.9 Decision.png

The decision body is always an indented list of statements (instructions). The body can be any sequence of statements.

We can also use a block diagram to illustrate the fact that the same syntax is followed for original example:

if user_answer == 'yes':
    print(' (\\(\\ ')
    print(' (-.-) ')
    print(' O_(")(") ')

5.10 Decision.png

Note that the Python language is very strict when it comes to indenting a body. All of the statements (lines) that form the body must be indented the same. This means that, if the first statement is indented by four spaces, then all other statements in the body must also be indented by exactly four spaces. If the first statement is indented by one tab, then all other statements in the body must also be indented by exactly one tab. I recommend that you always use four spaces.

Here is example for you to experiment with. Be sure to run this example several times. Try each of the following inputs: yes, no, Yes, No, YES, NO, maybe, sure.

Note that this program will only print a bunny when you respond to the input prompt with the exact text 'yes'. This is because the condition expression user_answer == 'yes' will only evaluate to True when the value stored in user_answer is 'yes'.

Use Python Tutor to trace and step through the example code below.

01  height = 5
02  width = 7
03 
04  # Make Landscape - wider than tall.
05  if height > width:
06       #rotate & swap dimensions
07       temp = height
08       height = width
09       width = temp
10 
11  print("Final Picture Height: ", height)
12  print("Final Picture Width: ", width)

Notice that lines 07-09 are skipped - because the expression height > width evaluates to False.

Now use Python Tutor to trace and step through the modified example code below. In this modified example, we have changed the value of the variable height, so that it is greater than the value of the variable width.


01  height = 15
02  width = 7
03 
04  # Make Landscape - wider than tall.
05  if height > width:
06      #rotate & swap dimensions
07      temp = height
08      height = width
09      width = temp
10 
11 print("Final Picture Height: ", height)
12 print("Final Picture Width: ", width)

Notice that lines 07-09 are not skipped - because the expression height > width evaluates to True.