Comp 255: Lab 02
Parameter Passing
pass by value
pass by reference
In, out, in/out and array parameters
int readInt()// reads/inputs unsigned integer
{
char str[80];
int i;
int number;
i = 0;
str[i] = getchar();
A1: if (str[i] == '\n')
goto A2;
i++;
str[i] = getchar();
goto A1;
A2: str[i] = '\0';
// convert to integer via Horner's Method
number = 0;
i = 0;
A3: if (str[i] == '\0')
goto A4;
number = number * 10 + (str[i] - 48);
i++;
goto A3;
A4: return(number);
}
int printInt(int number)// prints unsigned integer
{
char str[80]; int i;
i = 0;
C1: str[i] = (number % 10) + 48;
number = number / 10;
i++;
if (number > 0)// bottom test loop
goto C1;
// print out string backwards
C3: if (i == 0)
goto C4;
i--;
putchar(str[i]);
goto C3;
C4: return(0);
}
int printStr(char s[])
{
int i = 0;
P1: if (s[i] == '\0')
goto P2;
putchar(s[i]);
i++;
goto P1;
P2: return(0);
}
// prints cstring
Pass By Reference in C
In pass by reference the address of a variable is passed; not the
value of a variable
In the definition of the function this is indicated by prefixing a *,
the pointer operator, to the formal parameter
int fubar(int *x, int *y)
So in the call of the function you must explicitly pass the address
of the variable using the & operator
err = fubar(&a, &b)
Pass by Reference in C
In the definition of the function you must dereference all pass by
reference parameters to obtain the value of the parameter using the *
dereference operator.
int fubar(int *x, int *y)
{
int q, r;
if (*y == 0)
goto F1;
q = *x / *y;
r = *x % *y;
*x = q;
*y = r;
return(0);
F1: return(1); // error!
}
max01 function example
pass by value; function returns a value
int max01(int a, int b)
{
if (a > b)
return a;
else
return b;
}
Function definition
int main()
{
int a, b, c;
c = max01(a, b); // function call
max02 function example
int max02(int a, int b, int *c)
{
if (a > b)
c* = a;
else
c* = b;
}
int main()
{
int a, b, c;
max02(a, b, &c); //function call
Function definition
Four Logical Types of Parameters
in parameters: used to pass values into a function; call by value
out parameters: used to return values from a function; call by
reference
in/out parameters: parameters will are passed in to a function,
modified and returned to the calling program; call by reference
array parameters: parameter is an array type; syntactically pass
by value
Notes