Updating

Updating
A Programming Pattern (With Examples in Python)

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Frequently:
- We need keep track of something that is changing
- We want to refer to thing that is changing using one name
Examples:
- Age changes over time
- Temperature changes over time
- Position changes over space

Thinking About the Problem

The Assignment Operator:
- Takes the result of evaluating the expression on its right and puts it in the memory location identified by the variable on its left
Some Examples:
- pythonexamples/programmingpatterns/Updating.py (Fragment: additionage)
```
        
```

Thinking About the Problem (cont.)

An Observation:
- These three assignment statements are particularly easy to understand because the expression on the right side of the assignment operator does not involve the variable on the left side
A Different Situation:
- pythonexamples/programmingpatterns/Updating.py (Fragment: update1)
```
        
```

Thinking About the Problem (cont.)

To the Non-Programmer:
- This statement looks like a mistake
To the Programmer:
- Nothing about the syntax of assignment statements prevents this

The Pattern

The Idea:
- Use the same variable on the left and right sides of the assignment operator in order to change its value
An Abstract Version:
- value = value operator adjustment
Why It Works:
- The assignment operator has the lowest precedence so it is executed last
- The assignment operator places the value on the right side into the variable on the left side

A Gradebook Example

pythonexamples/programmingpatterns/Updating.py

(Fragment: grade)

A Retail Sales Example

pythonexamples/programmingpatterns/Updating.py

(Fragment: sales)

A Banking Example

pythonexamples/programmingpatterns/Updating.py

(Fragment: banking)

Compound Assignment Operators

The Operators:
- +=
- *=
- -=
- /=
- %=
Examples:
- pythonexamples/programmingpatterns/Updating.py (Fragment: compound)
```
        
```

A Warning