LECTURE 17:FUNCTION RETURN

CSC 107 – Programming For Science

Today’s Goal

Discuss writing functions that return values return statement’s meaning and how it

works When and why we use the return

statement Why skipping return statements are bad

ideas

Why We Use Functions

Simplify code Replace copies of code by placing in single

location Locate commonly-used computations &

actions Good to find code, but can only return a

single value Input & output performed in these

functions Will discuss ways to change parameters’

values

Functions

Already been programming with functions Built-in functions like pow, exp, & log Writing & using programmer-defined

function: main All functions similar

Will be using same process to call function Handling return result same for all functions Process is same for variables, scoping,

passing data

Functions

All functions similar, differ only in who wrote it

Built-in Function WriterUser-Defined Function Writer

Function Names

Rules over legal names identical to variables Letter or underscore start name for it to be

legal Can use any letters, numbers, or

underscore after Names should be unique (including

variables) Function names rely upon relaxed style

rules Start with a letter & make name

meaningful Names should only use words you would

say to:

Function Declaration

List prototypes for functions near top of file For multiple files, simplifies finding what is in each

file After #includes and using statements in file If function call before it is defined in file, it is

required Return type, name, & parameters listed only

After these are listed, end with semi-colon Information compiler needs to know calls are legalfloat squareNumber(float x);double computeAvg(int x, int y);int promptAndReadInput();bool noParams(void);

Return Type

Functions must specify their return type Type of value returned by the functionfloat abs(float x);double pow(double x, double t);int main();

Use special return type, void, if no value is returned void goodSoldier();void doSomethingWithX(int x);

Function Definitions

Eventually must actually write the function Specifies code to run when function is called Not a part of main: must be outside other

functionsvoid nothingReturned(int x) { // Code goes here, skipped for space}

float notTitanic(int men, int women) { // Code goes here, skipped for space}

int main() { // Code goes here, skipped for space}

return Statement

Used by function to specify value to return Value must be appropriate for function’s

return type Since not returning data, void functions

cannot use Statement written as: return value;

Value could be a literal, variable, or expression

Must be of appropriate type; no limits otherwisereturn 4;return varX;return varX * 2.1 + 45 == 0 && 4 < y;return pow(varX, 2.52);

return Examples

void printText() { cout << “Nothing to see here” << endl; return 23;}

float getNumber() { double val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

return Examples

void printText() { cout << “Nothing to see here” << endl; return 23;}

float getNumber() { double val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

return Examples

void printText() { cout << “Nothing to see here” << endl;}

float getNumber() { double val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

return Examples

void printText() { cout << “Nothing to see here” << endl;}

float getNumber() { double val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

return Examples

void printText() { cout << “Nothing to see here” << endl;}

float getNumber() { float val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

return Examples

void printText() { cout << “Nothing to see here” << endl;}

float getNumber() { float val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch; ch += 3;}

Function ends when return is executed Any and all code after return will be

ignored Calling function will resume its execution There are no errors or warnings for this

common bug

return Statement

return Examples

void printText() { cout << “Nothing to see here” << endl;}

float getNumber() { float val; cout << “Enter a number: ” << endl; cin >> val; return val;}

char getEncryptedLetter() { char ch; cout << “Enter a letter: ” << endl; cin >> ch; return ch + 3;}

Multiple return Statements

Multiple returns possible in a single function Each time function is called, only one is executed Gives greater flexibility by not tying code down

bool userTypedOddNumber() { int num; cout << “What’s your number?” << endl; cin >> num; if ( (num % 2) == 1) { return true; } else { return false; }}

Your Turn

Get into your groups and try this assignment

For Next Lecture

Read about parameters in Section 9.4 – 9.4.3 How do we pass values to a function? How does a function take in those values? What can a function do to those values?

Weekly Assignment #6 out & due next Tues. Avoid the rush by start working on it now

Programming Assignment #2 now on Angel This is a larger assignment and due in 3

weeks