Misunderstood topic - c

Question

Define a function CoordTransform() that transforms its first two input
parameters xVal and yVal into two output parameters xValNew and
yValNew. The function returns void. The transformation is new = (old +
1) * 2. Ex: If xVal = 3 and yVal = 4, then xValNew is 8 and yValNew is
10.
Question: I have to make this code, it is for homework in a text book. I'm not quite understanding the whole section. So, I'm asking for help and an explanation. Thank you!
My code is as followed:
#include <stdio.h>
/* Your solution goes here */
void CoordTransform( int* xValNew, int* yValNew)
{
*xValNew = ((xValNew +1) * 2);
*yValNew = ((yValNew +1) * 2);
return;
}
int main(void) {
int xValNew = 0;
int yValNew = 0;
CoordTransform(3, 4, &xValNew, &yValNew);
printf("(3, 4) becomes (%d, %d)\n", xValNew, yValNew);
return 0;
}

Answer 1

the code for the CoordTransform() should be like:
#include <stdio.h>
void CoordTransform(int xVal, int yVal, int *xValNew, int *yValNew) {
*xValNew = ((xVal +1) * 2);
*yValNew = ((yVal +1) * 2);
}
int main(void) {
int xValNew;
int yValNew;
CoordTransform(3, 4, &xValNew, &yValNew);
printf("(3,4) becomes (%d, %d)\n", xValNew, yValNew);
return 0;
}
The main purpose for this exercice is to distinguish between passing parameters by value like xVal and yVal in order to use the values without changing them, and passing parameters by address or reference like xValNew and yValNew if you intend to change the value of these output parameters.

Answer 2

You specs say "... transforms its first two input parameters xVal and yVal into two output parameters xValNew and yValNew" so your function should have four parameters, not two:
/* Your solution goes here */
void CoordTransform( int xVal, int yVal, int* xValNew, int* yValNew)
{
*xValNew = ((xVal +1) * 2);
*yValNew = ((yVal +1) * 2);
}
int main(void) {
int xValNew;
int yValNew;
CoordTransform(3, 4, &xValNew, &yValNew);
printf("(3, 4) becomes (%d, %d)\n", xValNew, yValNew);
return 0;
}

Answer 3

you are giving 4 parameters to a function that accepts only 2, and in your function CoordTransform, you should dereference the pointers to get the values
/* Your solution goes here */
void CoordTransform( int* xValNew, int* yValNew)
{
*xValNew = ((*xValNew +1) * 2);
*yValNew = ((*yValNew +1) * 2);
return;
}
( the return statement is not needed since the type of your function is void )

Answer 4

I encountered the same problem in my c++ class...
However I haven't learned C, so perhaps it doesn't allow '&' referencing? And only uses '*' for pointers?
My solution:
#include <iostream>
using namespace std;
/* Your solution goes here */
void CoordTransform(int xVal, int yVal, int& xValNew, int& yValNew) {
xValNew = (xVal + 1) * 2;
yValNew = (yVal + 1) * 2;
}
int main() {
int xValNew;
int yValNew;
int xValUser;
int yValUser;
cin >> xValUser;
cin >> yValUser;
CoordTransform(xValUser, yValUser, xValNew, yValNew);
cout << "(" << xValUser << ", " << yValUser << ") becomes (" << xValNew << ", " << yValNew << ")" << endl;
return 0;
}
I used a previous example as reference:
#include <iostream>
using namespace std;
void ConvHrMin(int timeVal, int& hrVal, int& minVal) {
hrVal = timeVal / 60;
minVal = timeVal % 60;
}
int main() {
int totTime;
int usrHr;
int usrMin;
totTime = 0;
usrHr = 0;
usrMin = 0;
cout << "Enter total minutes: ";
cin >> totTime;
ConvHrMin(totTime, usrHr, usrMin);
cout << "Equals: ";
cout << usrHr << " hrs ";
cout << usrMin << " min" << endl;
return 0;
}
which outputs this (with 156 input):
Enter total minutes: 156
Equals: 2 hrs 36 min