Indicators

Indicators
A Programming Pattern

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 (each of which can be an int or a double) as needed
The Process with One Indicator:
- javaexamples/programmingpatterns/Indicators.java (Fragment: pattern)
```
                total = base + (indicator * adjustment);
        
```

Examples

The Setting:
- The birth weight example

The Code:

javaexamples/programmingpatterns/Indicators.java

(Fragment: birthweight)

                w = -2200.0 + (148.2 * g) - (238.6 * delta_s);

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:

javaexamples/programmingpatterns/Indicators.java

(Fragment: parking)

                baseFine = 10.00;
        repeatOffenderPenalty = 35.00;        
        totalFine = baseFine + (ticketedIndicator * repeatOffenderPenalty);

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:

javaexamples/programmingpatterns/Indicators.java

(Fragment: rental)

                baseRate =  19.95;
        ageSurcharge =  10.00;        
        multiSurcharge = 5.00;
        
        rate = baseRate + (multiIndicator * multiSurcharge)
                        + (youngIndicator * ageSurcharge);

Some Warnings

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:

javaexamples/programmingpatterns/Indicators.java

(Fragment: ifbirthweight)

                w = -2200.0 + (148.2 * g);
        if (smoker) {
            w -= 238.6;
        }

Another Example:

javaexamples/programmingpatterns/Indicators.java

(Fragment: ifrental)

                baseRate =  19.95;
        ageSurcharge =  10.00;        
        multiSurcharge = 5.00;
        
        rate = baseRate;
        if (areMultipleDrivers) {
            rate += multiSurcharge;
        }
        if (areYoung) {
            rate += ageSurcharge;
        }

Some Warnings (cont.)

What About ternary operators?
- Some people think they are more concise then if statements but they are still more verbose than indicators

An Example:

javaexamples/programmingpatterns/Indicators.java

(Fragment: ternaryrental2)

                baseRate =  19.95;
        ageSurcharge =  10.00;        
        multiSurcharge = 5.00;
        
        rate = baseRate;
        rate += areMultipleDrivers ? multiSurcharge : 0;        
        rate += areYoung ? ageSurcharge : 0;