User-Defined Functions (cont’d) - Reference Parameters

CSCE 1062 Outline  Exercise  Passing by value  Scope of variables  Reference parameters (section 6.1)

CSCE 1063 Tracing Exercise #include using namespace std; int Blend( int red, int green ); // prototype void main() { int red = 5, blue = 3; blue = Blend(blue, red); cout << red << ' ' << blue << '\n'; blue = Blend(red, blue); cout << red << ' ' << blue << '\n'; } int Blend( int red, int green ) // parameters passed by value { int yellow;// local variable in Blend function cout << “enter Blend “ << red <<‘ ‘ << green << ‘\n’; yellow = red + green; cout << “leave Blend “ << red <<‘ ‘ << green << ‘\n’; return (yellow + 1); } Execution

CSCE 1064 Passing by Value main function data area actual arguments: red 5 blue 3 Blend function data area (1 st call) formal arguments: red 3 green 5 Local arguments: yellow 8

CSCE 1065 Tracing Exercise (cont’d) main Blend (1 st call) Blend (2 nd call) redblue Output: enter Blend 3 5 leave Blend enter Blend 5 9 leave Blend redgreenyellow 358 redgreenyellow 5914

CSCE 1066 Scope of Variables  Scope - where a particular meaning of a variable identifier is visible or can be referenced  Local - can be referred to only within a program segment or function  In a program segment (localized declarations of variables) for (int i = 1; i <= 10; ++i) cout << “*”; Commonly used for loop control variables Declared at point of first reference Value has meaning (i.e. can be referenced) only inside loop segment.

CSCE 1067 Scope of Variables (cont’d)  In a function this applies to formal argument names constants and variables declared within the function  Global - can be referred to within all functions  useful especially for constants  must be used with care

CSCE 1068 Listing 3.15 Outline of program for studying scope of variables

CSCE 1069 Scope of Variables (cont’d)  Global variables MAX & LIMIT are visible to main, one, funTwo functions  Local variable localVar in main function visible only to main function  Local variables anArg, second & oneLocal in one function are visible only to one function  Local variables one, anArg & localVar in funTwo function are visible only to funTwo function

CSCE Passing by Reference Example #include using namespace std; int Blend( int& red, int green ); // prototype void main() { int red = 5, blue = 3; Blend(blue, red); cout << red << ' ' << blue << '\n'; Blend(red, blue); cout << red << ' ' << blue << '\n'; } void Blend(int& red, int green) // green parameter passed by value, // while red parameter passed by reference { int yellow;// local variable in Blend function cout << “enter Blend “ << red <<‘ ‘ << green << ‘\n’; yellow = red + green; cout << “leave Blend “ << red <<‘ ‘ << green << ‘\n’; red = yellow + 1; }

CSCE Passing by Reference main function data area actual arguments: red 5 blue 3 Blend function data area (1 st call) formal arguments: red address of blue green 5 Local arguments: yellow 8

CSCE Passing by Reference Example (cont’d) main Blend (1 st call) Blend (2 nd call) redblue Output: enter Blend 3 5 leave Blend enter Blend 5 9 leave Blend redgreenyellow 58 redgreenyellow 914

CSCE User Defined Functions Can:  return no value  type void  return exactly one value  function type  return statement  return more than one value  type void or function type  reference parameters

CSCE Reference Parameters  Formal parameter data type directly followed by & indicate a parameter passed by reference

CSCE Listing 6.1 Function to compute sum and average

CSCE Listing 6.1 Function to compute sum and average (continued)

CSCE User-Defined Function computeSumAve  Two function input parameters  num1, num2  Two function output parameters  sum, average  & indicates function output parameters  Function call computeSumAve(x, y, sum, mean);

CSCE Argument Correspondence Actual Argument x y sum mean Corresponds to Formal Argument num1 (fn. input) num2 (fn. input) sum (fn. output) average (fn. output)

CSCE Call-by-Value and Call-by- Reference Parameters  & between type and identifier defines a parameter as function output mode (pass by reference)  no & in a parameter’s declaration identifies parameter as fuction input mode (pass by value)  Compiler uses information in parameter declaration list to set up correct argument-passing mechanism

CSCE Figure 6.1 Data areas after call to computeSumAve (before execution)

CSCE Figure 6.2 Data areas after execution of computeSumAve

CSCE Notes on Call-by-Reference  Place the & only in the formal parameter list - not in the actual parameter list  Place the & also in the prototype: void computeSumAve(float, float, float&, float&);  Note that this is a void function

CSCE When to Use a Reference or a Value Parameter  If information is to be passed into a function and doesn’t have to be returned or passed out of the function, then the formal parameter representing that information should be a value parameter (function input parameter).  If information is to be returned to the calling function through a parameter, then the formal parameter representing that information must be a reference parameter (function output parameter).

CSCE When to Use a Reference or a Value Parameter (cont’d)  If information is to be passed into a function, perhaps modified, and a new value returned, then the formal parameter representing that information must be a reference parameter (input/output parameter)

CSCE Next lecture we will talk more about Value and Reference Parameters