CSC 107 – Programming For Science

Today’s Goal  Write functions that take & return values  How parameters declared and how we call functions  What it means when we pass a value as a parameter  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

Return Type  Functions must specify their return type  Type of value returned by the function float 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

return Statement

return Examples

 Function ends when return is executed  Any and all code after return will be ignored  Calling function will resume its execution common  There are no errors or warnings for this common bug return Statement

return Examples

Multiple return Statements  Multiple return s possible in a single function  Each time function is called, only one is executed  Gives greater flexibility by not tying code down bool getNumber() { int num; cout > num; if ( (num % 2) == 1) { return true; } else { return false; } }

Variables  Variable  Variable names location to store data  Memory location's initial value is unknown  Assignments update memory location with new value  Memory location updated by assignment ONLY  When variable is used in program…  …uses current value at that memory location

Variable Scope  Scoping rules specify variables' lifetimes  Variables not universal  Variables not universal, but have specific lifetimes  Variable usable only in braces in which declared  For this copy of variable, scope defines its lifetime  Variable "dies" with end of scope in which declared  At start of scope, new copy created  Cannot use outside scope: error for bad variable  Must have unique names within a scope  Can reuse names between scopes – meaning is clear

Variable Scope void readNumber(int len) { int num = 0; for (int i = 0; i > ch; num *= 10; num += ch – '0'; } cout << num << endl; } int main() { int num = 3; readNumber(num); readNumber(5); return 0; }

Variable Scope void readNumber(int len) { int num = 0; for (int i = 0; i > ch; num *= 10; num += ch – '0'; } cout << num << endl; } int main() { int num = 3; readNumber(num); readNumber(5); return 0; } One name -but- two memory locations One name -but- two memory locations

Global Variables  Global variables are evil  Name for variables declared outside of any function  Exists throughout the entire program  Since they are global, can be used in any function  Functions can make variable with name of global  Within this function, would use local variable  Good luck figuring code out; prayers for debugging it

Global Variable Scope void readNumber(int len) { int num = 0; for (int i = 0; i > ch; num *= 10; num += ch – '0'; } cout << num << endl; } int num = 3; int main() { readNumber(num); readNumber(5); return 0; } Scopes overlap -but- two memory locations Scopes overlap -but- two memory locations

Global Variables  Evil idea sold by suits like this guy

Functions' Parameters

Parameters are Variables  Just like variables, they name memory location  Get new location each time function is called  Value stored at location changed by assignments  Unrelated to other variables even if names overlap assignments copy value  Like Xerox machine, assignments copy value

Calling Function w/Arguments  No different than we have been doing forever  Need name & parentheses like all function calls  Arguments specify value for each parameter  Must  Must have equal number of arguments & parameters  Argument that is 1 st in parens is value of 1 st parameter  2 nd parameter uses 2 nd argument in function call, etc.

Will It Compile?

Parameters Get Initialized  Assigned value of argument at start of function  Argument could be literal, variable or expression  Works like normal assignment; no connection made WILL NOT  Assigning parameter WILL NOT update argument  Like Xerox machine, assignments copy value

Sample Trace double powerLoss(double wattage) { double current = wattage / 110; wattage = current * current; return wattage; } int main() { double watt = 1100; double printed = powerLoss(watt); cout << printed << " " << watt << endl; double wattage = 0; printed = powerLoss(11.0); cout << printed << " " << wattage << endl; }

Your Turn  Get into your groups and try this assignment

For Next Lecture  Read about parameters in Section 9.4.4, 9.6  How do we pass references to a function?  What does it mean to pass-by-reference?  What is difference with what we discussed today?  Weekly Assignment #7 out & due on Tues.  Avoid the rush by start working on it now  Project #2 available so start working on it now  Project solution relies on topics up through today