Functions. CS111 Lab Queens College, CUNY Instructor: Kent Chin

Transcription

1 Functions CS111 Lab Queens College, CUNY Instructor: Kent Chin

2 Functions They're everywhere! Input: x Function: f Output: f(x) Input: Sheets of Paper Function: Staple Output: Stapled Sheets of Paper

3 C++ Function Prototypes The Syntax The data-type of the return value or variable. If the function doesn't return anything, it has a special return type called void. Name of your function. Comma-separated list of variables which serve as function inputs; the parameter list can be empty. RETURN_TYPE FUNCTION_NAME(PARAMETER(S)); Function prototypes are function title-lines terminated with a semi-colon. They merely give the compiler a description of your function (i.e. input types, output types, etc).

4 C++ Functions The Syntax RETURN_TYPE FUNCTION_NAME(PARAMETER(S)) { //...FUNCTION BODY CODE... //DO SOME WITH THE PARAMETER(S) //AND RETURN A VALUE It's not enough to give the compiler a description of your function, you have to define what it does by filling in the body code! We usually want to process the parameter(s) in some way and return a value with a data-type which matches the return type.

5 sqrt function (in <cmath>) double sqrt(double x); x (type: double) sqrt type: double Sample Usage //must #include <cmath> double y = sqrt(9); cout << y; //prints 3.0 double sqrt(double x); sqrt(9) cout << sqrt(9); //or call and print directly!

6 abs function (in <cstdlib>) int abs(int x); x (type: int) Function: abs type: int Sample Usage //must #include <cstdlib> int y = abs(-18); cout << y; //prints 18 int abs(int x); abs(-18) cout << abs(-18); //or call and print directly!

7 Function Writing Example num (type: int) square type: int We're returning the value of the variable ans, and ans is an int! int square(int num) { int ans = num*num; return ans; We process the parameter(s) and give back a value via the return keyword. //returns square of num Even Simpler int square(int num) { return num*num;

8 Another Function Example a (type: int), b (type: int) add type: int We're returning the value of the variable ans, and ans is an int! int add(int a, int b) { int ans = a + b; return ans; We process the parameter(s) and give back a value via the return keyword. //returns sum of a, b Even Simpler int add(int a, int b) { return a + b;

9 Complete Program Example #include <iostream> using namespace std; int square(int num) { return num*num; Defining a function to square a number. Return type: int Function name: square Parameter(s): num (int) int main() { int n; cout << Enter a number to be squared: ; cin >> n; argument int square(int num); int val = square(n); square(n) cout << Squared: << val << endl; return 0;

10 Another Complete Example #include <iostream> using namespace std; int square(int num); Prototype here, above main. int main() { //main is a function too :D int n; cout << Enter a number to be squared: ; cin >> n; int val = square(n); cout << Squared: << val << endl; return 0; int square(int num) { return num*num; Function fully defined here, below main.

11 Function Scope Example #include <iostream> using namespace std; string get_name(); int main() { string name = get_name(); cout << Your name is << name << endl; return 0; These two variables live in different {scopes/blocks, so these variable names don't conflict! You should still try to use different variable names, though. string get_name() { string name; cout << Enter your name: ; cin >> name; return name; Defining a function to get and return a user's name. Return type: string Function name: get_name Parameter(s): None Nothing get_name type: string

12 void functions Functions with a return type of void do not return actual values. void say_hi() { cout << Hi << endl; return; The return keyword in a void function simply exits the function; void functions exit upon seeing the last curlybrace ( ) anyway, so you can drop the return keyword: void say_hi() { cout << Hi << endl;

13 Void Function Example #include <iostream> using namespace std; void print_row(int n); int main() { int n; cout << How many stars do you want? ; cin >> n; print_row(n); //calling print_row! Though these variable names are the same, they live in different scopes, so therefore they do not conflict! You should still try to use different variable names, though. return 0; void print_row(int n) { for (int c = 1; c <= n; ++c) cout << * ; cout << endl; Defining a function which prints a row of n stars. Return type: void Function name: print_row Parameter(s): n (int)