How to increment index and print the right string with pointer? - c

Question

The code uses a struct ( int x,y) to print message when c->x increase the value by 1,
but unlike c->x , ptr1 pointer "forgets" its address.
How can I create a pointer to string array without "forgetting" its address?
#include <stdio.h>
#define msgROW 5
#define msgLEN 16
struct cursxy{
unsigned int x;
unsigned int y;
};
void cursor(struct cursxy *c,char (*ptr1)[msgLEN])
{
c->x++;
printf("%s \n", *ptr1++);
}
int main()
{
struct cursxy cursxy = {0,0};
char messages[msgROW][msgLEN] =
{
"Set Duty Cycle",
"Set Frequency",
"Set Hours",
"Set Minutes",
"Set Current"
};
char (*ptrMsg)[msgLEN] = messages;
//c->x = 1 , prints first message
cursor(&cursxy,ptrMsg); //ptrMsg point to first message
//c->x = 2 , prints again the first message
cursor(&cursxy,ptrMsg); //ptrMsg Didn't point to second message <------------
// and so on
}

Answer 1

The difference between the expressions
c->x++
and
*ptr1++
is that the latter modifies a function argument, whereas the former does not.
In C, functions have their own copy of the values of the function arguments that were used to call the function. As a consequence, modifying these arguments inside the function will not modify the original variable.
Therefore, in the function cursor, any changes to the function argument ptr1 will not change the variable ptrMsg in the function main.
The simplest solution to your problem would be to increment ptrMsg inside the function main instead of inside cursor.
However, if you insist on changing ptrMsg from inside cursor, then you will have to pass the variable ptrMsg by reference instead of by value. This means that you will have to instead pass the address of ptrMsg to the function cursor, like this:
cursor(&cursxy,&ptrMsg);
You will also have to change the prototype of the function cursor, so that the second parameter has an additional layer of indirection. Additionally, you will have to add a * dereference operator in order to access the original variable via the pointer. Afterwards, your function will look like this:
void cursor(struct cursxy *c,char (**ptr1)[msgLEN])
{
c->x++;
printf("%s \n", *( (*ptr1)++ ) );
}

Answer 2

this is what you're looking for.
same as cursxy, use double pointer.
#include <stdio.h>
#define msgROW 5
#define msgLEN 16
struct cursxy{
unsigned int x;
unsigned int y;
};
void cursor(struct cursxy *c, char (**ptr1)[msgLEN])
{
c->x++;
printf("%s \n", *ptr1);
*ptr1 = ++*ptr1;
}
int main()
{
struct cursxy cursxy = {0,0};
char messages[msgROW][msgLEN] =
{
"Set Duty Cycle",
"Set Frequency",
"Set Hours",
"Set Minutes",
"Set Current"
};
char (*ptrMsg)[msgLEN] = messages;
char (**ptr)[msgLEN] = &ptrMsg;
//c->x = 1 , prints first message
cursor(&cursxy, ptr); //ptrMsg point to first message
//c->x = 2 , prints second message
cursor(&cursxy, ptr); //ptrMsg Didn't point to second message <------------
cursor(&cursxy, ptr); //ptrMsg Didn't point to second message <------------
// and so on
}