CSC 107 – Programming For Science

Today’s Goal  Discuss how to hand data to functions  Review loopholes in variables & scoping rules  Ways to get parameters to do whatever you want  How to use parameters and what they are

Variables  Variable  Variable name location to store data  Only for humans; 0 x 7E8A2410 harder to remember  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  But what if something else modified memory?  Variable's value "updated" without an assignment  But we have no control over these location…

Variables

Variable Scope  Variables not universal, lifetime limited by scope usable  Once declared, variable usable only in braces  Scope defines lifetime where memory location used  Marks memory location "free" when scope ends  Cannot use outside scope as name will not be known

 Variables not universal, lifetime limited by scope usable  Once declared, variable usable only in braces  Scope defines lifetime where memory location used  Marks memory location "free" when scope ends  Cannot use outside scope as name will not be known  Unless we could access memory location directly  Variable is convenience to access computer's memory

 Variables not universal, lifetime limited by scope usable  Once declared, variable usable only in braces  Scope defines lifetime where memory location used  Marks memory location "free" when scope ends  Cannot use outside scope as name will not be known  Unless we could access memory location directly  Variable is convenience to access computer's memory

Variable Scope int findMin(int num) { int temp, min; cin >> min; num = 1; do { cin >> temp; if (temp < min) { min = temp; } num++; } while (temp != ); return min; } int main() { int small, num; small = findMin(num); cout << "In: " << num << ", min= " << small << endl; return 0; }

int findMin(int num) { int temp, min; cin >> min; num = 1; do { cin >> temp; if (temp < min) { min = temp; } num++; } while (temp != ); return min; } int main() { int small, num; small = findMin(num); cout << "In: " << num << ", min= " << small << endl; return 0; } Variable Scope One name -but- two memory locations

int findMin(int num) { int temp, min; cin >> min; num = 1; do { cin >> temp; if (temp < min) { min = temp; } num++; } while (temp != ); return min; } int main() { int small, num; small = findMin(num); cout << "In: " << num << ", min= " << small << endl; return 0; } Variable Scope One name -but- two memory locations

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  Parameters initialized to value in argument  Copies value into memory  Copies value into memory location created for param  Assignments affect memory location for parameter

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  Parameters initialized to value in argument  Copies value into memory  Copies value into memory location created for param  Assignments affect memory location for parameter  Unless new, evil parameter type can be created

Pass-By-Reference Parameters

Passing-By-Reference  Memory location NOT created  Memory location NOT created for parameter assigned memory location of argument  Instead it is assigned memory location of argument  Updates argument's value  Updates argument's value with each assignment  All this works even though argument is out-of-scope!

Passing-By-Reference Caveats  For code to compile argument must be variable  Using argument's memory location, so this required  Since lack location to use, no literals or expression  Data types must match exactly for it to work  Need the memory location sizes to be equal  0 s & 1 s must be interpreted in identical manner  Compiler will enforce rules strictly  Huge security holes can be opened by mistake

Will It Compile?

Passing-By-Reference Usage  Limits how function used now & into future  If more precision needed, cannot change data types  Needs variable for parameter, no matter what MUST  If writing function that MUST return 2+ values  Use return statement for one of the values  Have other values "returned" with pass-by-reference  Much better to rewrite code to avoid this  Pass-by-reference often makes hard to solve bugs

Passing-By-Reference Example

Tracing Example

Your Turn  Get into your groups and try this assignment

For Next Lecture  Read about arrays in Section 10.1 – 10.5  How can we use more than 1 value at a time?  What is meant by the term array?  How are array used and why do we need all these []?  Weekly Assignment #7 out & due next Tuesday  Also do not wait to start Program Assignment #2