FUNCTIONS - II Chapter 9 and 10

1. FUNCTIONS - IIChapter 9 and 10

2. Today we will continue with functions covering…. • Passing by Reference  • Scope • Sorting variables • Function Structure Charts (Ch10)

3. This function changes its parameter Q: What does x hold after fn call? void countdown(float n) { while (n>0) { cout <<n; n=n-1;} cout << "...ZERO!!!“<<endl; } void main( ) { float x=5; countdown(x); cout<<“after countdown(), x is “<<x; }

4. Program Output 54321…ZERO!!! after countdown(), x is 5 (WHY?) • Pass-by-Value: • Only a copy of the argument x is given to the function • Even though n changes, x retains original value • Sometimes you need the function to change the argument. Thus we also have …

5. Passing by Reference • This is done in situations where a function needs to change the value of the argument passed to it • To pass an argument by reference, simply append an ampersand ‘&’, to the type specifier in the function’s parameter list

6. Passing by Reference (contd..) • Then the local variable is only a reference to the argument passed to it • The argument is read-write instead of read-only(as in the case of pass by value) • Any change to the local variable inside the function will cause the same change to the argument that was passed to it

7. This function uses PBR Q: What does x hold after fn call? void countdown(float& n) { while (n>0) { Added Only 1 Character! cout <<n; n=n-1;} cout << "...ZERO!!!“<<endl; } void main( ) { float x=5; countdown(x); cout<<“after countdown(), x is “<<x; }

8. New Program Output 54321…ZERO!!! after countdown(), x is 0 (WHY?) • Pass-by-Reference: • Direct access to argument x is given to the function (Read/Write) • When it changes “n”, it really changes x • Here n is another name for the argument, x • Often called a reference or “alias” to x

9. Demonstrate politic.cpp in Ch09 • PBR Functions can “return” more than 1 value • Reference parameters can be same name or different name than arguments • Draw lines between arguments and parameters • Arguments to Reference parameters must be variables of the same type (no constants) • Arguments to Value parameters can be constants, variables, or expressions, and type is more lenient • Illustrate by modifying politic.cpp

10. Demonstrate circle.cpp, ch09 • PBR Functions can calculate 2 or more quantities and return them • Some parameters are Value, some are Reference • Draw lines between arguments and parameters

11. Typical Test Question….output? void f(float x, float& y) { // changes reference argument to 99: x = 88; y = 99; } void main() { float a = 22, b = 44; cout<< "a = “<< a << ", b = "<< b << endl; f(a,b); cout<< "a = "<< a << ", b = "<< b << endl; }

12. The call f(a,b) passes a by value to x and it passes b by reference to y • x is a local variable that is assigned a’s value of 22 • y is an alias for the variable b whose value is 44 • The function assigns 88 to x but that has no effect on a • When it assigns 99 to yit is really assigning 99 to b, because y is an alias for b • Finally, a still has its original value 22 (read-only), while b has the new value 99(read-write)

13. Summing up…

14. Your Turn What does this program display? void func1 (int &x, int &y, int z); void main() { int a=2, b=2, c=2; func1 ( a, b, c); cout <<"a="<<a<<"b="<<b<<"c="<< c <<endl; } void func1 (int &x, int &y, int z) { x=x+1; y=y-1; z=z+2; }

15. How do you know when to use &? • Don’t overdo it because you are not sure • Ask…will the function need to initialize or update an argument you are giving it? • YES…then use a reference parameter • NO…then use a value parameter • See circle.cpp again to study the value and reference parameters

16. Today we will continue with functions covering…. • Passing by Reference • Scope  • Sorting variables • Function Structure Charts (Ch10)

17. Remember scope ? • The scope of a name consists of that part of the program where it can be used • It begins where the name is declared • The scope extends to the end of the innermost block that contains the declaration

18. void f(); // f() is global void g(); // g() is global float x = 11; // this x is global void main() { float x = 22; { float x = 33; cout<<"In block inside main(), x = "<<x<<endl; } // end scope of internal block cout<<"In main(), x = "<<x<< endl; cout<<"In main(), ::x = "<<::x<<endl; //global x f(); g(); } // end scope of main() void f() { float x = 44; cout << "In f(), x = " << x << endl; } // end scope of f() void g() { cout << "In g(), x = " << x << endl; } // end scope of g()

19. In the previous example… • f() and g() and the first x are global – their scope includes the entire file • The second x is declared inside main() , it is accessible only from within the main() • The third x is declared inside an internal block, so its scope is restricted to that internal block

20. PROGRAM OUTPUT for Previous Example In block inside main(), x = 33 In main(), x = 22 In main(), ::x = 11 In f(), x = 44 In g(), x = 11

21. Scope and You: the Rules!! • Don’t use global variables for now. You don’t need them. • Lazy programmers try to skip writing parameter lists by making all function variables global—0 points if you do!! • Best is to declare all variables needed at beginning of main/other function • Advice—use different names for function call arguments and function parameters

22. Today we will continue with functions covering…. • Passing by Reference • Scope • Sorting variables  • Function Structure Charts (Ch10)

23. Sorting Data • Arranging data in increasing order • A real “bread and butter” CS/IS task • Sorted data are easier to search • Usually a slow process for long lists of data • Efficiency of advanced sort methods can result in tremendous gains in processing time (CSIS 10B)

24. Sort works by using SwapOne of the most useful algorithms • Swap two variables: float x=3, y=5, temp; temp=x; x=y; y=temp; • Used so much we want to make a function

25. void swap(float& x, float& y) { // exchanges the values of x , y: float temp = x; x = y; y = temp; } void main() { float a = 22.2, b = 44.4; cout<<"a = "<<a<<",b = "<<b<<endl; swap(a,b); cout<<"a = "<<a<<",b = "<<b<<endl; }

26. Let’s understand that program.. • The formal parameters x and y are declared as reference variables : float& x, float& y • The reference operator&makes x and y synonyms for the arguments passed to the function

27. Basic Sort • If two adjacent data are out of order, swap float a, b; cin>>a>>b; if (a>b) swap(a, b);

28. Extending Sort for 3 variables cin>>a>>b>>c; • If any two adjacent data are out of order, swap if (a>b) swap(a, b); if (b>c) swap(b, c); • Recheck 1st two in case c was smallest if (a>b) swap(a, b);

29. Demonstrate alpha3.cpp, ch09 • Modify to simplify, only swap w1 and w2 and show changes • What if you want to sort integers? • Function overloading… • make another swap(int &x, int &y) • Compiler can tell which one to go to during execution by looking at the arguments you are using

30. Making a Sort3( ) function…revA //Sort3( ) Sorts three params in numeric order //IN/OUT: a, b, and c are returned in numeric order // currently only a stub void Sort3(float &a, float &b, float &c) { cout<<“values passed to Sort():”<<endl; cout<< a <<“ ”<< b <<“ ”<< c << endl; a=1.0; b=2.0; c=3.0; } void main( ) { float r, s, t; cin>>r>>s>>t; Sort3(r, s, t); cout<<“in order:” << r <<“ ”<< s <<“ ”<< t << endl; } This is called a “function stub” It just tests parameter passing

31. Making a sort3( ) function…revB // Add swap definition here //Sort3( ) Sorts three params in numeric order //IN/OUT: a, b, and c are returned in numeric order void Sort3(float &a, float &b, float &c) { if (a>b) swap(a, b); if (b>c) swap(b, c); if (a>b) swap(a, b); } void main( ) { float r, s, t; cin>>r>>s>>t; Sort3(r, s, t); cout<<“in order:” << r <<“ ”<< s <<“ ”<< t << endl; }

32. Today we will continue with functions covering…. • Passing by Reference • Scope • Sorting variables • Function Structure Charts (Ch10) 

33. Structure Charts (see p226 +) • Show the large scale layout of your program • Each block is a function • Linkage between functions shown • value and reference parameters • Demonstrate structure charts for • Politic.cpp • Circle.cpp • Class.cpp (ch10 folder) • Will help you visualize Project 2 ATM

34. Demonstrate class.cpp • Notice the documentation of in and out parameters • Also notice the passing of a filestream to find_mark function • ofstream& fout • This will be important later in Project 2

35. THE END !!!