UserDefined Functions contd Reference Parameters CSCE 106 Outline

User-Defined Functions (cont’d) Reference Parameters CSCE 106

Outline § Exercise § Passing by value § Scope of variables § Reference parameters (section 6. 1) CSCE 106 2

Tracing Exercise #include <iostream> using namespace std; Execution 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 ) { int yellow; function CSCE 106 } // parameters passed by value // local variable in Blend cout << “enter Blend “ << red <<‘ ‘ << green << ‘n’; yellow = red + green; cout << “leave Blend “ << red <<‘ ‘ << green << ‘n’; return (yellow + 1); 3

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 106 4

Tracing Exercise (cont’d) main red blue 5 3 9 15 Blend (1 st call) red green yellow 3 5 8 Output: enter Blend 3 5 leave Blend 3 5 59 enter Blend 5 9 leave Blend 5 9 5 15 Blend (2 nd call) red green yellow 5 9 14 CSCE 106 5

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 u 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 106 6

Scope of Variables (cont’d) u 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 u must be used with care u CSCE 106 7

Listing 3. 15 Outline of program for studying scope of variables CSCE 106 8

Scope of Variables (cont’d) § Global variables MAX & LIMIT are visible to main, one, fun. Two functions § Local variable local. Var in main function visible only to main function § Local variables an. Arg, second & one. Local in one function are visible only to one function § Local variables one, an. Arg & local. Var in fun. Two function are visible only to fun. Two function CSCE 106 9

Passing by Reference Example #include <iostream> 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 106 10

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 106 11

Passing by Reference Example main (cont’d) red blue 5 3 9 15 Blend (1 st call) red green yellow 5 8 Output: enter Blend 3 5 leave Blend 3 5 59 enter Blend 5 9 leave Blend 5 9 15 9 Blend (2 nd call) red green yellow 9 14 CSCE 106 12

User Defined Functions Can: § return no value u type void § return exactly one value function type u return statement u § return more than one value type void or function type u reference parameters u CSCE 106 13

Reference Parameters § Formal parameter data type directly followed by & indicate a parameter passed by reference CSCE 106 14

Listing 6. 1 CSCE 106 Function to compute sum and average 15

Listing 6. 1 Function to compute sum and average (continued) CSCE 106 16

User-Defined Function compute. Sum. Ave § Two function input parameters u num 1, num 2 § Two function output parameters u sum, average § & indicates function output parameters § Function call compute. Sum. Ave(x, y, sum, mean); CSCE 106 17

Argument Correspondence Actual Argument x y sum mean CSCE 106 Corresponds to Formal Argument num 1 (fn. input) num 2 (fn. input) sum (fn. output) average (fn. output) 18

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 argumentpassing mechanism CSCE 106 19

Figure 6. 1 Data areas after call to compute. Sum. Ave (before execution) CSCE 106 20

Figure 6. 2 Data areas after execution of compute. Sum. Ave CSCE 106 21

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 compute. Sum. Ave(float, float&, float&); § Note that this is a void function CSCE 106 22

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 106 23

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 106 24

Next lecture we will talk more about Value and Reference Parameters CSCE 106 25