Chapter 7 - Notes User-Defined Functions II

Transcription

1 Chapter 7 - Notes User-Defined Functions II I. VOID Functions ( The use of a void function is done as a stand alone statement.) A. Void Functions without Parameters 1. Syntax: void functionname ( void ) statement 1 ; : statement n ; OR void functionname ( ) statement 1 ; : statement n ; 2. Function Call How void function is called in the main function: void printstars ( ) for ( int i = 1; i <= 30; i++ ) cout << "*" ; cout << endl ; ; function definition function call printstars ( ) ; cout << "******* Your Name Here *******" << endl; printstars ( ) ; system("pause") ; function call return 0 ; Page 1

2 OUTPUT ****************************** ******* Your Name Here ******* ****************************** Press any key to continue... OR IT CAN BE WRITTEN WITH A PROTOTYPE void printstars ( ) ; function prototype function call printstars ( ) ; cout << "******* Your Name Here *******" << endl; printstars ( ) ; system("pause") ; function call return 0 ; void printstars ( ) for ( int i = 1; i <= 30; i++ ) cout << "*" << endl ; cout << endl ; function definition 3. Notice that a void function without parameters does not have any values passed in and it does not pass back a value. C. Void Functions with Parameters 1. Function Definition i. Syntax: void functionname ( formal parameter list ) statement 1 ; : statement n ; Page 2

3 2. Formal Parameter List i. Syntax: ( same as in value returning functions ) 3. Function Call a. For use with all default value parameters ( datatype variable, datatype variable,... ) b. For use with all reference parameters ( datatype & variable, datatype & variable,... ) void printstars ( int num ) ; function prototype function call printstars ( 30 ) ; cout << "******* Your Name Here *******" << endl; printstars ( 30 ) ; system("pause") ; function call return 0 ; void printstars ( int num ) for ( int i = 1; i <= num; i++ ) cout << "*" << endl ; cout << endl ; 4. Actual Parameter List function definition i. Syntax: ( same as in value returning functions ) ( expression or variable, expression or variable,... ) Page 3

4 II. Value vs. Reference Parameters A. Value Parameters: The default passing parameter in the C++ language 1. Passes the value(s) from the actual parameter(s) in the function call to the formal parameters in the function by copying the values and storing them in the temporary variables. Example: int area ( int x_length, int y_width ) x_length y_width return x_length * y_width ; ; 12 5 int main () copy copy int length, width, tot_area ; cout << "Please enter the length and width of a rectangle here: "; cin >> length >> width ; length width tot_area = area( length, width ); 12 5 cout << endl; cout << "The rectangle's area is " << tot_area << " square inches"; cout << endl << endl ; system("pause"); return 0; Assume the user enters the values 12 and 5. Then the value 12 is stored in length and the value 5 is stored in width in the main function. When the function area is called in the main function, the values from length and width in the main function are copied and sent to new variables created in the function called x_length and y_width. x_length and y_width can now be used as variables in the area function. Page 4

5 B. Reference Parameters 1. Passes the address of the actual parameter to the function's formal parameter by using an ampersand (&) between the data type and formal parameter of the function heading (and prototype, if used). In this way, the value in the actual parameter is NOT copied, but a second name for the original variable is created in the function ( This is not what actually happens, but it appears that way.). Any changes to the value in the function for a reference variable are actually made to the variable used as the actual parameter in the calling function. 2. Importance of the ampersand ( & ) i. the ampersand indicates that the following formal parameter is being passed by reference. Example: int area ( int & x_length, int & y_width ); int main () int length, width, tot_area ; cout << "Please enter the length and width of a rectangle here: "; cin >> length >> width ; length width tot_area = area( length, width ); cout << endl; cout << "The rectangle's area is " << tot_area << " square inches"; cout << endl << endl ; system("pause"); return 0; int area ( int & x_length, int & y_width ) x_length y_width x_length += 2 ; y_width += 2 ; return x_length * y_width ; length width Page 5

6 III. Scope of an Identifier A. Definition: The scope of an identifier refers to where in the program the identifier is accessible or visible. ( Where can it legally be used or not used.) B. Local Identifier: An identifier declared within a function (block) and not accessible outside of the function (block). C. Global Identifier: Identifiers declared outside of every function definition. (Remember that the main function is still a function.) D. Main Function as a Block: Consider the main function to be a block of code. Main Function 1. Nested Blocks: One or more lines of code within the main function enclosed with an open curly brace and closed curly brace. Main Function Nested Block a. int X ; X = 5 ; Nested Block int Y ; Y = 3 ; cout << Y << endl << endl ; cout << X << endl << endl ; E. Note that functions can be called from within the definition of other functions or subprograms (as long as they are defined before the function in which they are called.) Page 6

7 Example Main Function VAR X, Y Function One VAR A, B Function Two VAR J, K Example Code for Above Diagram #include <cmath> const double PI = ; Global Variable int squared ( int J) return J * J ; ; int pythag ( int A, int B ) Calling function int sum; sum = squared (A) + squared ( B) ; return sum; ; int X, Y ; int c_squared ; Page 7

8 cout << Please enter 2 integers for the bases of a right triangle. ; cin >> X >> Y ; c_squared = pythag ( X, Y ) ; Calling function cout << endl << endl ; cout << The hypotenuse is: << sqrt( double ( c_squared )) << endl ; cout << endl ; system( pause ) ; return 0 ; F. Rules For Accessing Identifiers 1. Global Identifiers: (such as variables) are accessible by a function or a block if... a. The identifier is declared before the function definition (block). b. The function name is different from the identifier. c. All parameters of the function have names different than the name of the identifier. d. All local identifiers (such as local variables) have names different than the name of the identifier. 2. Nested Block An identifier declared within a block is accessible... a. Only within the block from the point at which it is declared until the end of the block. b. By those blocks that are nested within that block if the nested block does not have an identifier with the same name as that of the outside block (the block that encloses the nested block). 3. Functions (sub-programs) a. The scope of a function name is similar to the scope of an identifier declared outside any block. That is, the scope of a function name is the same as the scope of a global variable. Page 8

9 G. Scope Resolution Operator :: ( two colons, side by side before an identifier) 1. The scope resolution operator is used when the programmer wants to access an identifier immediately outside its block with the same name as an identifier declared inside the block. Example: int X, Y, Z ; X = 5 ; Y = 3 ; Z = X + Y ; cout << Z << endl ; int X, Z; X = 6 ; Z = ::Z + X ; cout << Z << endl ; cout << Z << endl << endl ; refers to this Z system( pause ) ; return 0 ; H. External Variable Declaration 1. A way in C++ to access a global variable declared after the definition of a function. Syntax: extern int variable_name ; NOTE: that extern is a reserved word in the C++ language and the global variable name you want to access is not already being used in the calling block or function. Example: Page 9

10 const int age = 19 ; int subtract_it ( int X, int Y) extern int MilYear ; ; return X - ( Y - MilYear ) ; int MilYear = 2000 ; int curage, curyear ; cout << Please enter your current age and the current year: ; cin >> curage >> curyear ; cout << endl ; cout << Your age at the turn of the century was ; cout << subtract_it( curage, curyear) << endl << endl ; cout << It s been << curage - age ; cout << years since you were a teenager. << endl << endl ; system( pause ) ; return 0 ; IV. Global Variable Cautions A. Any function that uses global variables is not independent from the rest of the program. For example - you cannot unplug the function and insert another and maintain program coherency. B. If more than one function uses the same global variable and the program has a logical error, it may be difficult to determine where the error occurred. C. It is recommended that a programmer avoid using global variables unless absolutely necessary. Page 10

11 V. Static Variables A. The variables discussed so far have been automatic variables and have followed two simple rules Memory for global variables remains allocated as long as the program executes. 2. Memory for a variable declared within a block is allocated at block entry and deallocated at block exit. For example, memory for the formal parameters and local variables of a function is allocated when the function is called and deallocated when the function exits. B. Automatic Variable 1. A variable for which memory is allocated at block entry and deallocated at block exit. 2. Variables declared within a block ( main, nested, function ) are automatic variables. C. Static Variables 1. A variable for which memory remains allocated as long as the program executes. 2. Global variables are static variables. 3. You can declare a static variable from within a block by using the reserved word static. a. Syntax: static datatype identifier ; Example: void test ( ) ; int count; for ( count = 1 ; count <= 5 ; count++ ) Page 11

12 test( ) ; system( pause ) ; return 0; void test ( ) static int x = 0 ; int y = 10 ; x = x + 2 ; y = y + 1 ; cout << Inside test x = << x << and y = << y << endl ; OUTPUT : Inside test x = 2 and y = 11 Inside test x = 4 and y = 11 Inside test x = 6 and y = 11 Inside test x = 8 and y = 11 Inside test x = 10 and y = 11 Press any key to continue Scope of static variables: Static variables declared within a block are local to the block, and their scope is the same as that of any other local identifier of that block. Page 12