Notes

Outline

COM1100 Fundamentals of Computer Science –Winter 2000 Lecture 7 01/24/99

Reading material this week Textbook Chapter 6 : Iteration 6.2.1: for loops Chapter 5 : Functions 5.5: Parameters 5.8: Reference Parameters Package 1 – Patterns NameUseFunctions LoopCounting Package 2 – Recitation 3 Package 3 – Lab 3

Picture lab – Picture() void Picture(int x, int y, int size); void Picture(int x, int y, int size) { PaintRect(x, y, x + size, y + 0.6*size); … } Paint a square picture of a given size with the left top corner at coordinates (x,y) Each graphic function (except SetForeColor) should in term of x, y and size Don’t put fixed number, such as PaintRect(100, 100, 150, 150)

Picture lab – BigPicture() void BigPicture(); void UserPicture(); void FourPictures(); // write a function BigPicture that first clear the GW // then display the picture to fill the entire GW void BigPicture() { ClearDrawing(); Picture(0, 0, 300); }

Picture lab – UserPicture() // write a function UserPicture that first clear the GW // then ask user to enter the desired location and size of the picture // Use a default location(100,100) and default size 150 Void UserPicture() { ClearDrawing(); int x = RequestInt(“Enter x (0 ~ 300): “, 100); int y = RequestInt(“Enter y (0 ~ 300): “, 100); int size = RequestInt(“Enter size (0 ~ 300): “, 150); Picture(x, y, size); }

Picture lab – FourPictures() // write a function FourPictures that first clear the GW // then display four pictures of size 150 so that they fill the window Void UserPicture() { ClearDrawing(); Picture(0, 0, 150); Picture(?, ?, 150); Picture(?, ?, 150); Picture(?, ?, 150); }

Recitation 2 Question 23 will use pass parameters to function by reference About MakeChange Similar to Q22 in recitation 1 You may wait Tuesday’s lecture will finish all about function

Previous lectures Functions Function declaration: signature Function implementation Function call Flow of control if statement C++ source file layout C++ programming style

Today’s lecture Functions Parameter passed by value Parameter passed by reference Increment operator ++ Decrement operator --

Functions  -- parameter passed by value void Swap(int x, int y); // function declaration void Swap(int x, int y) { // function implementation int temp;              cout << “ x =  “ << x  << “  y = “ << y << endl; // x=5 y =9 temp = x; x = y; y  = temp; cout << “ x =  “ << x  << “  y = “ << y << endl; // x=9 y =5 } int main() { int m = 5, n = 9; Swap(m, n); // function call cout << “ n =  “ << m  << “  n = “ << n << endl; // m=5 n =9 return 0; }

Actual and Formal arguments – pass by value When main function calls Swap functions First makes copies of actual arguments m and n Use copies to initialize formal argument x and y Any changes made for x and y in Swap have nothing to do with actual argument m and n Because only values of m and n passed into function Swap

Functions  -- parameter passed by reference void Swap(int& x, int& y); // function declaration void Swap(int& x, int& y) { // function implementation int temp;              cout << “ x =  “ << x  << “  y = “ << y << endl; // x=5 y =9 temp = x; x = y; y  = temp; cout << “ x =  “ << x  << “  y = “ << y << endl; // x=9 y =5 } int main() { int m = 5, n = 9; Swap(m, n); // function call cout << “ n =  “ << m  << “  n = “ << n << endl; // m=9 y =5 return 0; }

Reference parameter C++ syntax data-type& reference-name Example float& num1 Declare that num1 is a reference parameters that will be used to store the address of a float

& symbol In C++, & means “the address of” When an & symbol is used within a function declaration, it refers to “the address of” the preceding data type. Declaration such as float& num1 is sometimes more clearly understood if it is read backwards. “num1 is the address of a floating-point value”

Actual and Formal arguments – pass by reference Here actual arguments m,n and formal arguments x, y reference the same data items. The significance of this is that the values in the actual arguments (m, n) can now be altered from within Swap by using the formal names (x, y). x, y do not store copies of the values in m, n, but directly access the locations in memory set aside for these two arguments.

The equivalence of actual and formal arguments when passed by reference

Method 2 – pass by value Question: write a function that increments an integer. Method 1: The user supplies the integer value (that does not need to be stored as an integer variable, but can be computed as the function call is made), by using the value argument. The function returns the incremented value as the function return value.

Method 1 -- Example int Increment(int n); // function declaration int Increment(int n){ // function definition return (n+1); } int main() { int x = 25; x = Increment(x); int y = Increment(x); cout << x << " -- " << y << " -- " << Increment(y)         << Increment(6)  << endl; return 0; } // this will print 26 -- 27 -- 28 -- 7

Method 2 – pass by reference Question: write a function that increments an integer. Method 2 : the user supplies a name of the variable whose value should be incremented. The function code will be able to read the value of this variable and change it. We say that the argument is passed by reference and indicate this in the signature by following the data type with an &.

Method 2 -- Example void Increment(int& n); // function declaration void Increment(int& n){ // function definition n = n+1; }; int main() { int x = 25; cout << x << " ~~~ "; Increment(x); cout << x << endl; return 0; } // this will print 25 ~~~ 26

How many mistakes ? void Increment(int& n); // function declaration      void Increment(int& n){ // function definition n = n+1; };     int main() { int x = 25; cout << x << " ~~~ " << Increment(x) << Increment(6) << endl; return 0; } Increment(x) : the function call does not result in any usable data value and so cannot be used in any computation or in the output stream. Increment(6) : cannot convert parameter 1 from 'const int' to 'int &'

Today’s lecture Functions Parameter passed by value Parameter passed by reference Increment operator ++ Decrement operator --

Increment operator ++ variable = variable + 1 can be replaced by the expression variable++ or ++variable Examples

Prefix vs. Postfix increment operator Prefix increment operator : when ++ appears before a variable Example : ++x; Postfix increment operator : When ++ appears after a variable Example : x++;

Difference between ++x and x++ ++x increment x then use new value x++ use old value of x then increment x

"The distinction between a prefix..." The distinction between a prefix and postfix increment operator is important when the variable being incremented is used in an assignment expression. k = ++n; is equivalent to : n = n + 1;       // increment n first k = n;      // assign n’s value to k k = ++n; is equivalent to : k = n;     // assign n’s value to k first            n = n + 1;   // and then increment n

Examples-- interview questions from Microsoft int n = 5; int k = 7; k = ++n; cout << “ k = “ << k << “ n = “ << n << endl; // will print : k = 6 n = 6 int n = 5; int k = 7; k = n++; cout << “ k = “ << k << “ n = “ << n << endl; // will print : k = 5 n = 6

Decrement operator -- variable = variable - 1 can be replaced by the expression variable-- or --variable Examples

Examples int n = 3; int k = --n; cout << “ k = “ << k << “ n = “ << n << endl; // will print : k = 2 n = 2 int n = 3; k = n--; cout << “ k = “ << k << “ n = “ << n << endl; // will print : k = 3 n =2

Summary -- increment and decrement operators Four possibilities Auto increment/decrement most useful with subscripts, pointers and loop counters May be used in expressions Often used in stand-alone Z = I * I; ++I;

Today’s lecture Functions Parameter passed by value Parameter passed by reference Increment operator ++ Decrement operator --

Lecture Tuesday (01/25) More on function Pass parameters by reference Flow of control Repetition – for statement Format output