1. CS 31 Discussion, Week 4 Faisal Alquaddoomi, faisal@cs.ucla.edufaisal@cs.ucla.edu Office Hours: BH 2432, MW 4:30-6:30pm, F 12:30-1:30pm (today)

2. What is this? double bodyMassIndex(double height, double weight) { return weight/(height*height); }

3. Functions Review: Parts double bodyMassIndex(double height, double weight) { return weight/(height*height) * 703.0; } A function has a name, parameters, and a return type The return value of the function must be the same type as the return type

4. double bodyMassIndex(double height, double weight) { return weight/(height*height) * 703.0; } int main() { double myBMI = bodyMassIndex(5*12 + 11, 150); cout << “My BMI: “ << myBMI; return 0; } Functions Review: Calling Functions are called from other code, which executes them and produces a value of the same type as the function

5. double bodyMassIndex(double height, double weight) { return weight/(height*height) * 703.0; } int main() { double myBMI = bodyMassIndex(5*12 + 11, 150); cout << “My BMI: “ << myBMI; return 0; } Functions Review: Arguments When called, the values passed to the function are called arguments Each argument must match the type of its corresponding parameter

6. Functions Calling Functions double bmiMetric(double height, double weight) { return weight/(height*height); } double bmiEnglish(double height, double weight) { double weightKg = weight * 0.453592; double heightM = height * 0.0254; return bmiMetric(weightKg, heightM); } Note that they are defined separately, even though bmiEnglish() calls bmiMetric() What’s the advantage of having one call the other?

7. Functions and Modularity What’s wrong with having giant do-all functions? They’re not modular Modularity is the property of being reusable – Achieved by being self-contained and operating for a variety of inputs The many advantages to writing modular code: – Easier to reuse existing code – Easier to understand what code does – Easier to test A bug in a module can usually be constrained to just that module

8. No Modularity int main() { double height, weight; cout << “Enter your height(m), weight (kg): “ cin >> height >> weight; cout << “BMI : “ << weight/(height*height) << endl; cout << “Enter your height(in), weight (lbs): “ cin >> height >> weight; weight *= 0.453592; height *= 0.0254; cout << “BMI (from English): “; cout << weight/(height*height) * 703.0 << endl; }

9. Poor Modularity void bmiMetric(double height, double weight) { cout << “BMI : “ << weight/(height*height); } void bmiEnglish(double height, double weight) { weight *= 0.453592; height *= 0.0254; cout << “BMI: “; cout << weight/(height*height) * 703.0; } The functions have redundant code They’re also not self-contained: they print to the screen, which assumes something about the caller (e.g. that they want stuff on the screen)

10. Good Modularity double bmiMetric(double height, double weight) { return weight/(height*height); } double bmiEnglish(double height, double weight) { double weightKg = weight * 0.453592; double heightM = height * 0.0254; return bmiMetric(weightKg, heightM) * 703.0; } They share the same basic calculation They’re appropriately named and can be used in any program that requires BMI calculation They return a value versus printing to the screen; the value could be used for anything, not just printing (storing to a file, comparisons, etc.)