Function with Output Parameters

1. Function with Output Parameters • We have seen that functions can return a single value or no value (void return type) • It is quite often useful to be able to return more than one value • In this case, we use output parameters to pass back the additional information • The argument in this case must specify a location to put the value in, not a value

2. Function with Output Parameters • When we want to specify the location to store (e.g.) an integer, we must declare a pointer to an integer void separate(double num, char *signp, int *wholep, double *fracp) { double magnitude; if (num < 0) *signp = ‘-’; else if (num == 0) *signp = ‘0’; else *signp = ‘+’; magnitude = fabs(num); *wholep = floor(magnitude); *fracp = magnitude - *wholep; }

3. Function with Output Parameters • Now, if we want to call this function, we have to supply a value for the first argument and variables for the second, third, and fourth int main(void) { double value; char sn; int whl; double fr; printf(“Enter a value to analyze> “); scanf(“%lf”, &value); separate(value, &sn, &whl, &fr);

4. Function with Output Parameters • Notice how we specify the address of a variable - with the & operator (as in scanf) • In the called function, we must use *var in expressions - otherwise we will be calculating with the address of the variable, not the value! • A declaration such as int *var declares var as a pointer to an integer variable (which is declared somewhere else)

5. Function with Output Parameters • Note that we can pass a number (e.g. 5.24) as the first argument to the function, but we can’t pass numbers (or characters) for the other arguments since they expect addresses not values • What happens if we omit the & operator when calling the function?

6. Meaning of the * Symbol • The * symbol has three separate meanings in C • The simple one is the binary multiplication operator: var1 * var2 • In a declaration it means that the variable is a pointer to an element of the given type: char *signp • In the body of a function (e.g. in an expression), it means follow the pointer: *signp = ‘-’; myvar = *ptrvar + 1;

7. Arguments Used for Both I/O • We have seen arguments used for input or for output • We can also use a single argument for both input (pass information to the called function) and output (return information to the calling function) • To do this we must use a pointer to a variable • Let’s write a function to add switch the values of two variables

8. Arguments Used for Both I/O void switch(int *first, int *second) { int holder; holder = *first; *first = *second; *second = holder; } int main(void) { int one = 1, two = 2; /* Initialized! */ printf(“One %d Two %d\n”, one, two); switch(&one, &two); printf(“One %d Two %d\n”, one, two); }

9. Scope of Names • The scope of a name refers to the region of a program where a particular meaning of a name is visible (can be referenced) • We need to understand the scope of functions, variables, and constants • For a constant - #define PI 3.14 - we can use the constant only in the file in which it is declared • Arguments are visible only in the function in which they are declared

10. Scope of Names • Local variables (variables defined in a function) are visible only in that function • Global variables (variables defined outside of and before all functions) are visible to all functions in the file • Functions are visible to all other functions in the file • Arguments and local variables can be declared with the same name as global functions or variables. In this case, they hide the globals

11. Scope of Names • Functions cannot be nested in C (in many other languages they can, resulting in more complicated scope rules) • More complications arise when we use more than one file to implement a program