Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 1Winter Quarter User-Written Functions Lecture 12

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 2Winter Quarter User-Written Functions So far, we have written only one function ourselves. That is function is called "main ( )". The syntax used has been as follows: void main ( ) { /* Declarations */ /* Statements */ } The word void means that the function is not expected to return any values to another function.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 3Winter Quarter User-Written Functions The parentheses, ( ), both indicate that main is a function, AND provide a place for any variable names (arguments) that could be sent as values from another function. Since main is the main program function, it will normally not have a parameter list. (Who calls the main function anyway?) Most, but not all, other functions have parameter lists. Some such, as rand ( ), do not since they need no data or values from the calling function.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 4Winter Quarter Writing User-Written Functions There are typically three different types of statements needed to properly set-up and make use of a user-written function. They are: 1. The function prototype statement. 2. The first line of the function definition. 3. The calling statement.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 5Winter Quarter 1. The Function Prototype Statement. The function prototype's format is (note semicolon): return-value-type function-name (parameter-list) ; The return-value-type may be any legal data type such as int, long, float, double, char, or it may be void. The parameter list is a list of data-types for the arguments and, optionally, the names of arguments. Example: float sum ( int, int, int ) ; /* OR */ float sum ( int a, int b, int c ) ;

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 6Winter Quarter 2. The First Line of the Function Definition. The first line of the function definition is the same as the examples of the function prototype except that the SEMICOLON IS ELIMINATED and variable or argument NAMES ARE REQUIRED. Example: float sum ( int a, int b, int c ) Here, the function sum is of type float and has three calling parameters, all of which are of type int. The variables a, b, and c take on the values of the variables or constants used in the calling statement.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 7Winter Quarter The Body of the Function A complete function consists of the the function definition and the body of the function in the { }. Example: float sum ( int a, int b, int c) { float total; total = a + b + c ; return total; } Note: The function has return statement because it has a return type.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 8Winter Quarter The Body of the Function float sum ( int a, int b, int c) { int total; total = a + b + c ; return total; } The names a, b, and c are known only inside sum. Likewise, any variables, like total, declared within the function are local variables, and they are known only inside the function in which they are defined.

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 9Winter Quarter 3. The Calling Statement. The function call (or invocation) is like the first line of the function definition except NO DATA TYPES are shown and constants may be used instead of variable names. The valued returned by a function is normally assigned to a variable in the calling function. Example: value = sum ( i, j, k ) ; or value = sum ( 5, 6.73, 2 ) ;

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 10Winter Quarter Factorial Function Main Program #include long factorial ( int x ) ; void main ( ) { int k; for ( k=0 ; k<=10 ; k++ ) { printf ("%2d factorial is %ld\n", k, factorial (k) ) ; }

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 11Winter Quarter Factorial Function long factorial ( int x ) { long fact = 1; int k; if ( x < 0 ) return 0; else if ( x==0 || x==1 ) return 1; else { for ( k = 2 ; k <= x ; k++ ) fact = fact * k ; return fact ; }

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 12Winter Quarter Factorial Program Output 0 factorial is 1 1 factorial is 1 2 factorial is 2 3 factorial is 6 4 factorial is 24 5 factorial is factorial is factorial is factorial is factorial is factorial is

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 13Winter Quarter User-Written Functions (example): #include void swap ( int a, int b ) ; void main ( ) { int a = 5, b = 6; printf ("a=%d b=%d\n",a,b); swap (a, b); printf ("a=%d b=%d\n",a,b); } void swap ( int a, int b ) { int temp; temp = a; a = b; b = temp; printf ("a=%d b=%d\n", a, b ); } a=5 b=6 a=6 b=5 a=5 b=6

Engineering H192 - Computer Programming Gateway Engineering Education Coalition Lect 12P. 14Winter Quarter Call By Value vs. Call By Reference Two ways in which variables are passed: –Call by value –Call by reference "Call by value" means function only gets a copy of the value of the calling argument. The called function can not change the value back in the calling routine. This is normally how it is in C. "Call by reference" means the function gets the original argument, and thus the called function can change the value of the variable back in the calling routine. Must use pointers in C to do this.