What is a Function? What are functions good for?

Transcription

1 Functions What is a Function? What is a Function? Up until this point, every line of code we've shown you has done a simple task, such as performing an arithmetic operation, or checking a boolean condition, or assigning to a variable. Functions allow you to do a whole lot in one line of code. Instead of performing a simple task, a single line of code can display a menu of choices, or compute complicated three-dimensional transformations, or even play Tetris! How is this possible? Functions allow you to group a series of steps under one name. Remember in first session when we were baking chocolate chip cookies? We had to perform the following steps: place two eggs in a bowl add 1.5 c. butter to the eggs... bake cookies for minutes at 375 degrees or until brown In C++, and in most programming languages, you can give a name to a series of steps. Let's say we want to call this procedure "bake cookies". Then, our algorithm for baking cookies becomes: bake cookies What are functions good for? We've just created a function to do the work for us. Don't get too excited - you still need to know how to bake the cookies. You still need to know that the first step is placing two eggs in a bowl, and that the second step is adding butter to the eggs. In this example, what you would do is write a function called bakecookies (C++ won't let you put spaces in the names of functions or variables) that performs the series of steps above, and then whenever you wanted to bake cookies, you would call the function bakecookies, which would execute the lines of code necessary to carry out the procedure. At this point, we've seen one reason why functions are useful. Functions let us create logical groupings of code. If someone is reading your code, and she sees that you call a function bakecookies, she knows immediately that you are baking cookies. If, on the other hand, she sees that your code places eggs in a bowl, then adds butter, etc., it will not be clear right away that you are trying to bake cookies. Lots of recipes start out with putting eggs in a bowl, and lots of recipes add butter to the eggs. By the time she reads the last line, she might realize that you are baking cookies, but only if she is familiar with the recipe. It's possible that she won't realize that you are baking cookies at all! The point is, functions make your code much easier to read. 1

2 There is an even better reason to use functions: they can make your code shorter. Fewer lines of code is not always desirable, but every time you write a line of code, there's the possibility that you are introducing a bug. Functions start to reduce the number of lines of code when you call them repeatedly. Suppose that you want to mail out invitations to eight of your friends for a cocktail party. Let's assume that you need to do the following procedure in order to invite your friend Hank. write Hank's name on the invitation write Hank's name and address on the envelope place the invitation in the envelope seal and stamp the envelope drop the envelope in the mail It takes five lines of pseudo-code to invite one friend, so it takes 40 lines of pseudo-code to invite eight friends. It would look like this: That's a lot of repeated code, and any time you repeat code like this, you are more likely to add a bug to your program. For example, look at what we're doing with Flavio's invitation - we are placing it on, not in, the envelope! We sealed his envelope and dropped it in the mail, but there was no invitation inside. Flavio will receive an empty envelope and he'll be mighty confused. That's a mistake that resulted from having to type the same lines of code over and over again. Functions can substantially reduce the amount of pseudo-code you need to write to invite your eight friends to the party. It seems unlikely that you'd be able to reduce this at all - each of your friends has got to have their own personally addressed invitation, and all of the envelopes have to be sealed and stamped and placed in the mail. How are we going to reduce the number of lines of code? Let's create a function called which does the following procedure: Now that we have this function, we can call it eight times to invite our eight friends: write Hank's name on the invitation write Hank's name and address on the envelope place the invitation in the envelope seal and stamp the envelope drop the envelope in the mail 2

3 You probably noticed a problem with doing it this way. We're inviting Hank eight times, and none of our other friends are going to receive invitations! Hank will get invited eight times because the function invites Hank to the party, and the function is being called eight times. The solution is to modify the function so that it invites friend to the party, where friend can be any of your friends. We'll change our function so that it looks like this: write friend's name on the invitation write friend's name and address on the envelope place the invitation in the envelope seal and stamp the envelope drop the envelope in the mail and then we'll change the way in which we call the function: (friend = Hank) (friend = Ann) (friend = Alicia) (friend = Whitney) (friend = Greg) (friend = Mi Young) (friend = Flavio) (friend = Brian) Now, each time we call the function, friend is a different person, and each of our eight friends will be invited. We've just reduced the number of lines of pseudo-code from 40 to 13 by using a function, and our code became much easier to read. (We also got rid of that bug whereby Flavio received an empty envelope.) All of the examples on these sides were written in pseudo-code, but the next slides describes how to write functions in C++. In other languages called subroutines or procedures. C++ functions all have a type. Sometimes we don t need to have a function return anything in this case the function can have type void. 3

4 C++ Functions (cont.) Sample function C++ functions have a list of parameters. Parameters are the things we give the function to operate on. Each parameter has a type. There can be zero parameters. Return type Function name parameters int add2ints(int a, int b) { return(a+b); Function body Using functions Math Library functions C++ includes a library of Math functions you can use. You have to know how to call these functions before you can use them. You have to know what they return. You don t have to know how they work! double sqrt( double ) When calling sqrt, we have to give it a double. The sqrt function returns a double. We have to give it a double. x = sqrt(y); x = sqrt(100); x = sqrt(y); The stuff we give a function is called the argument(s). Y is the argument here. A C++ function can t change the value of an argument! If y was 100 before we call sqrt, it will always be 100 after we call sqrt. Table of square roots int i; for (i=1;i<10;i++) cout << sqrt(i) << \n ; But I thought we had to give sqrt() a double? C++ does automatic type conversion for you. 4

5 Telling the compiler about sqrt() Other Math Library Functions How does the compiler know about sqrt? ceil floor You have to tell it: #include <math.h> cos sin tan exp log log10 pow fabs fmod Writing a function You have decide on what the function will look like: Return type Name Types of parameters (number of parameters) You have to write the body (the actual code). Function parameters The parameters are local variables inside the body of the function. When the function is called they will have the values passed in. The function gets a copy of the values passed in (we will later see how to pass a reference to a variable). Sample Function int add2nums( int firstnum, int secondnum ) { int sum; sum = firstnum + secondnum; // just to make a point firstnum = 0; secondnum = 0; return(sum); int main(void) { int y,a,b; Testing add2nums cout << "Enter 2 numbers\n"; cin >> a >> b; y = add2nums(a,b); cout << "a is " << a << endl; cout << "b is " << b << endl; cout << "y is " << y << endl; return(0); 5

6 What happens here? int add2nums(int a, int b) { a=a+b; return(a); int a,b,y; y = add2nums(a,b); Local variables Parameters and variables declared inside the definition of a function are local. They only exist inside the function body. Once the function returns, the variables no longer exist! That s fine! We don t need them anymore! Block Variables You can also declare variables that exist only within the body of a compound statement (a block): { int foo; Global variables You can declare variables outside of any function definition these variables are global variables. Any function can access/change global variables. Example: flag that indicates whether debugging information should be printed. Scope The scope of a variable is the portion of a program where the variable has meaning (where it exists). A global variable has global (unlimited) scope. A local variable s scope is restricted to the function that declares the variable. A block variable s scope is restricted to the block in which the variable is declared. A note about Global vs. File scope A variable declared outside of a function is available everywhere, but only the functions that follow it in the file know about it. The book talks about file scope, I m calling it global scope. 6

7 Block Scope int main(void) { int y; { int a = y; cout << a << endl; cout << a << endl; Error a doesn t exist outside the block! 7