Lecture 16 Introduction To Structures

Transcription

1 Lecture 16 Introduction To Structures Learning Objectives: Purpose of Structures Syntax of Structures (Creating Structures) Using Structures Using Structures with functions Pass by Reference 1

2 What is a Structure? It is a user-defined data type (like double or vector) that can hold one or more variables. It s kind of like a miniclass. Vectors come from a class. You declare them like other data types and they have functions associated with them: v.size() give you the size of vector v. Structures will work in a similar way you don t have functions with structs, but they can contain variables. Example: a.x would give you the value of x from the variable a which has been declared as a structure. 2

3 Why use a Structure? Say you are drawing lines and you want to save them in an array or vector so you can redraw them at different times. To do this, you need starting x and y positions and the ending x and y positions. We have no line data type, so you could have arrays or vectors of x1*+, y1*+, x2*+, y2*+, but it s better if you can group these together and make your own type called line. Then you can have just one array of lines. This is what structures allows you to do. Structures also give you a way to return multiple values from a function. Additionally, structures allow you to group variables in a way that make your program easier to follow (encapsulation). 3

4 Creating a Structure (your own data type) If you do this above main then this data type is available to all functions in your program (it s global). If you put it in a particular function, this data type is local to just that function. Example: Structure to hold the endpoints of a line struct Line //the keyword struct then the name of the structure { //any variable that you want grouped into this structure are declared here double x1; //these variable are called fields double y1; double x2; double y2; }; //note the semicolon 4

5 Declaring a Structure Variable First you declare a variable of the new structure s type. Remember: A struct is like a type (like double or int). Example: Line myline; // the name of the struct then the new variable name. Now you have a variable called myline that can hold 4 values, x1, y1, x2, and y2 You could also declare other variables of the line type that could hold different values of x1, y1, x2, and y2. Example: Line anotherline; //note a struct is like a data type (int, double, etc.) 5

6 Using a Structure Variable Use the. just like we did for vectors: Then treat the structure variable like any other variable. Examples: myline.x1 = 2.6; //sets x1 for the variable myline to 2.6 anotherline.y2 = 4.7;//sets y2 for the variable anotherline to 4.7 printf( starting x value = %lf\n, myline.x1); myline.x2 = myline.x2+2.3; scanf( %lf, &anotherline.y1) 6

7 Passing structures into functions A structure variable can be an argument just like any other variable. Just declare it in the function header and function prototype. Structures are passed by value by default (copies are made and the variables involved are not linked to each other). Example: Function to find the length of a line double length(line aline) { double xdist = aline.x2-aline.x1; double ydist = aline.y2 aline.y1; return sqrt(xdist*xdist + ydist*ydist); } Example Call if myline is a line structure: double z = length(myline); 7

8 Returning Structures from functions A structure can be returned just like any other variable. Just declare it in the function header and function prototype before the function s name. This is a way to return more than one value at a time. Examples: 1) Function prototype for a function to move a Line to the right 2 units Line move2(line aline); To call this function: (Assuming you already have a Line structure called myline). You ll need to assign it to a structure. Line movedline = move2(myline); 2) Function prototype for a function to move a Line to the right x units Line movex(struct Line aline, double x); To call this function: (Assuming you already have a Line structure called myline). You ll need to assign it to another structure. myline = movex(myline, 4.2); 8

9 #include<stdio.h> struct Employee { double salary; //note: A struct can contain different data types. int years; Example: Entire }; Employee increase(employee p); Program int main(){ Employee person; //declare person as an employee char ans; do{ printf( \n\nenter Salary and years of service: $ ); scanf( %lf %i, &person.salary, &person.years); person = increase(person); printf( new salary = $%.2lf\nnew years of service = %i person.salary, person.years); printf( \n\nenter y to calculate another: ); scanf( %c, &ans); }while(ans== y ans == Y ); } Employee increase(employee p) { //increase salary by 5% and increase years of service by 1 p.salary = 1.05*p.salary; p.years++; return p; } 9

10 $./structure Output of previous example Enter Salary and years of service: $ new salary = $ new years of service = 3 Enter y to calculate another: y Enter Salary and years of service: $ new salary = $ new years of service = 9 Enter y to calculate another: n 10

11 Review: Passing Vectors By Reference How did you make a vector pass by reference? Put an & between the type and name in the function header and prototype. Example: int myfunction(vector <double> & v); Now, if you call myfunction() like this: x = myfunction(a); What happens when you change v within myfunction()? a also changes 11

12 The & works for all variable types, including structs Example Function Prototype: void movex(line &myline, double x); Say you call movex() like this: movex(a, 3.2); What happens when to x1 of the Line called a when the following line executes from within movex()? myline.x1 = myline.x1 + x; a.x1 increases by

13 The & works for all variable types, including structs Example Function Prototype for Roulette: void winorlose(int spin, double b, int &w, double & acct); Say you call winorlose like this from main(): winorlose(roll, bet, win, account); What happens when w and acct are changed within winorlose()? win and account also change. Why is this handy? You are essentially returning more than one variable from a function. 13