Indicator Methods

Indicator Methods
A Programming Pattern

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

The Idea:
- Problems in which an indicator’s value must be calculated before it can be used in another expression
An Example:
- "The show must go on" (i.e., the number of seats sold for a particular show time must be used to calculate an indicator, setting it to 0 when no tickets have been sold and setting it to 1 otherwise)

Review

A Specific Solution that Leads to Code Duplication:
- javaexamples/programmingpatterns/IndicatorMethods.java (Fragment: review0)
```
        if (sold >= 1) {
            shows = 1;
        } else {
            shows = 0;
        }
        
```

A Better Specific Solution:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: review1)

    /**
     * Calculate the number of shows from the number of tickets
     * sold for a particular show time.
     *
     * @param  sold  The number of tickets sold for a particular show time
     * @return 0 if sold is 0; 1 otherwise
     */
    public static int shows(int sold) {
        if (sold >= 1) {
            return 1;
        } else {
            return 0;
        }
    }

The Pattern

A General Solution for Threshold Indicators:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: indicator)

    /**
     * Calculate a threshold indicator from a value and threshold.
     *
     * @param value      The value of interest
     * @param threshold  The threshold value
     * @return 0 if the value is less than the threshold; 1 otherwise
     */
    public static int indicator(int value, int threshold) {
        if (value >= threshold) {
            return 1;
        } else {
            return 0;
        }
    }

Other General Solutions:
- The parameters of the method and the logic in the body of the method vary with the problem

Examples of Threshold Indicators

The Show Must Go On:
- javaexamples/programmingpatterns/IndicatorMethods.java (Fragment: shows)
```
        shows = indicator(sold, 1);
        
```

Parking Tickets:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: parking)

        baseFine = 10.00;
        repeatOffenderPenalty = 35.00;        
        totalFine = baseFine + (indicator(priors, 1) * repeatOffenderPenalty);

Another Example

Basic Metabolic Rate (BMR):
- \(b = 5.00 + 10.00 m + 6.25 h - 5.00 a - 161.00 \delta_f \)

The Indicator Method:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: female)

    /**
     * Return 1 if the person's sex is female and 0 otherwise.
     *
     * @param sex  The person's sex ('F' or 'M')
     * @return     1 if the person is female, and 0 otherwise
     */
    public static int indicator(char sex) {
        if ((sex == 'F') || (sex == 'f')) {
            return 1;
        } else {
            return 0;
        }
    }

Using the Indicator Method:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: bmr)

        b = 5.00 + 10.00 * m + 6.35 * h - 5.00 * a - 161.00 * indicator(s);

A Warning

Don't Overgeneralize:
- This pattern is often not appropriate for boolean methods

An Inappropriate Use of the Pattern:

javaexamples/programmingpatterns/IndicatorMethods.java

(Fragment: old0)

        if (age >= DRINKING_AGE) {
            isLegal = true;
        } else {
            isLegal = false;
        }

The Appropriate Solution:
- javaexamples/programmingpatterns/IndicatorMethods.java (Fragment: old1)
```
        isLegal = (age >= DRINKING_AGE);        
        
```