Indicators

JMU

Indicators
A Programming Pattern (With Examples in Python)

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

The Situation:
- Calculations must be performed that vary based on one or more binary conditions
Terminology:
- Indicator Variables
- Dummy Variables
Common Notation:
- \(\delta\) (often with a subscript identifying the indicator)

An Example

A Model of Birth Weight:
- \(w = -2200 + 148.2 g - 238.6 \delta_s\)
Notation:
- \(w\) denotes the weight of the baby in grams
- \(g\) denotes the length of the gestation period in weeks
- \(\delta_s\) is 1 if the mother smoked during the gestation period and 0 otherwise

The Pattern

Getting Started:
- Define as many 0-1 variables as needed
The Process with One Indicator:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: pattern)
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```

Examples

The Setting:
- The birth weight example
The Code:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: birthweight)
```
        
```

Examples (cont.)

The Setting:
- The fine associated with a first parking ticket is smaller than the fine associated with subsequent parking tickets (specifically, $10.00 for the first ticket and $45.00 for subsequent tickets)
The Code:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: parking)
```
        
```

Examples (cont.)

The Setting:
- A rental car company that charges a base rate $19.95 per day
- There is a surcharge of $5.00 per day if multiple people drive the car
- There is a surcharge of $10.00 per day if any driver is under 25 years of age
The Code:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: rental)
```
        
```

A Warning

What About if Statements?
- They tend to be much more verbose
- They treat the discrete and continuous variables differently for no apparent reason
One Example:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: ifbirthweight)
```
        
```
Another Example:
- pythonexamples/programmingpatterns/Indicators.py (Fragment: ifrental)
```
        
```

A Note About Python

Arithmetic Operations on Booleans:
- Are allowed - True evaluates to 1 and False evaluates to 0
An Implication:
- You needn't use 0-1 variables, you can use Boolean variables for the indicators
Some Advice:
- Many people think it's confusing and dangerous to perform arithmetic operations on non-numbers
- This is very language specific (i.e., idiomatic), so, even if you disagree, it's still important to know the pattern