Functions

1. Functions Referring to a name or naming a reference

2. Why? • Structure • Partition a larger problem into smaller more manageable chunks • The partition must be “logical” • A good function can be described with a single simple sentence or phrase • Benefits • Development • Debugging and maintenance • Re-use cs113 Gene Itkis

3. Picture of a function OUT IN cs113 Gene Itkis

4. Example c=(a2+b2) a b c = bsroot(a*a + b*b); • Somewhere before that… float bsroot(float x){ …\\ function definition return …;} Input type Output type cs113 Gene Itkis

5. Example (from H) • Absolute value double abs(double x) { if (x>= 0) return x; else return –x; } cs113 Gene Itkis

6. Good header comments • Purpose/description • Compute square root • Inputs • x – number to compute the square root of (type: float) • Returns • Square root of x (type: float) • Remarks • Uses binary search method cs113 Gene Itkis

7. YAE: yet another example (from H) int max(int x, int y) /* receives two integers x,y returns the larger of the two */ { if (x>y) return x; else return y; } cs113 Gene Itkis

8. The void • Type of what is not there • No return • No input void donothingforever(void) { for(;;); } cs113 Gene Itkis

9. Functions (C++) vs. “Procedure/Subroutines” (others) • Return • Function returns a value (unless void) • Procedures return nothing • procedure  void function • Side effects • Print • Write (e.g. to database), etc. • Parameter value change cs113 Gene Itkis

10. Call by Value • C++ “default” • E.g. examples above are by value • Input parameters immediately before function call = immediately after function return • Changes to the parameters are visible only inside the function cs113 Gene Itkis

11. Call by Value: example int x=0, y=10; int func(int a) { a++; return a }; y = func(x); //x=0, y=1 cs113 Gene Itkis

12. Call by Value: example 2 (from H) Employee Harry; … give_raise(Harry.salary, 25); // Harry should get a 25% raise • Void raise or raise with no value void give_raise(salary s, float by) { s= s*(1+by/100.0); } // Harry’s fooled cs113 Gene Itkis

13. Call by Reference • Side effect: change of parameter value persists after the function return int x=0, y=10; int func(int& a) { a++; return a }; y = func(x); \\ x=1, y=1 cs113 Gene Itkis

14. Call by Reference • Reference parameter must be a variable (not an expression) int x=0, y=10; int fV(int a) { a++; return a }; int fR(int& a) { a++; return a }; y = fV(x+1); // This is OK. x=0, y=2 y = fR(x+1); // But this is illegal cs113 Gene Itkis

15. Scope • Scope of a variable x – where x is defined • From its definition to the end of the block { … float x; … { … int x; … {…} … } … } cs113 Gene Itkis

16. Global variables • Declared outside function, but used inside { int x=5, y; int addx(int a) {return a+x;} y=addx(2); //y=7 } • Bad style cs113 Gene Itkis

17. Discussion • Call by value vs. call by reference • Efficiency, style, “testing”; pointers • Stubs and small functions • From pseudo-code to code • Looks like a function, … • rand/srand -a function or an object? • “stateful” cs113 Gene Itkis

18. Recursion • Factorial • n! = n * (n-1)! • Binary search • Divide interval • Binary search one sub-interval • Others • Termination conditions!!! cs113 Gene Itkis

19. Functions: advanced issues

20. Other topics covered in class • More scope • Scope of function declaration • Example: double recursion • Two f-ns calling each other from their bodies • Prototypes • Namespaces • Static: persistent variables • Typical mistakes • Forgotten returns • [Pointers to] non-persistent variables • Style: bad side-effects examples • Printing error msgs from a function • Terminating programs from a function • Const ref cs113 Gene Itkis