CS 31 Discussion, Week 4

1. CS 31 Discussion, Week 4 Faisal Alquaddoomi, faisal@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 doublebodyMassIndex(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. Functions Review: Calling doublebodyMassIndex(double height, double weight) { return weight/(height*height) * 703.0; } int main() { doublemyBMI = bodyMassIndex(5*12 + 11, 150); cout << “My BMI: “ << myBMI; return 0; } • Functions are calledfrom other code, which executes them and produces a valueof the same typeas the function

5. Functions Review: Arguments 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; } • 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.)