Function Compile Error

[kidko]

Hi... I'm working on a small, text-based game, compiling under Kubuntu 6.10 (Edgy Eft) with G++ 4.1.2. Up until I added this function (and its corresponding call in main.cpp), it would compile correctly. Could anybody tell me why I'm getting the error "functions.h:163: error: invalid conversion from ‘int (*)[1]’ to ‘int'"?

int initcreate() {
    int initiative[(MAXUNITS-1)][1];
    int unitcount = 0;
    for (int i = 0; i < (BOARDX-1); i++) {
        for (int j = 0; j < (BOARDY-1); j++) {
            if (forces[i][j] != 0) {
                initiative[unitcount][0] = i;
                initiative[unitcount][1] = j;
                unitcount++;
            }
        }
    }
    return initiative;
}

[tavianator]

This code should me correct (note the [2] in the second line, replacing [1]), as long as forces is a two-dimensional array. Where specifically, in the function, is the error?

int initcreate() {
    int initiative[(MAXUNITS-1)][2];
    int unitcount = 0;
    for (int i = 0; i < (BOARDX-1); i++) {
        for (int j = 0; j < (BOARDY-1); j++) {
            if (forces[i][j] != 0) {
                initiative[unitcount][0] = i;
                initiative[unitcount][1] = j;
                unitcount++;
            }
        }
    }
    return initiative;
}

[/quote]

Hi... I'm working on a small, text-based game, compiling under Kubuntu 6.10 (Edgy Eft) with G++ 4.1.2. Up until I added this function (and its corresponding call in main.cpp), it would compile correctly. Could anybody tell me why I'm getting the error "functions.h:163: error: invalid conversion from ‘int (*)[1]’ to ‘int'"?

int initcreate() {
    int initiative[(MAXUNITS-1)][1];
    int unitcount = 0;
    for (int i = 0; i < (BOARDX-1); i++) {
        for (int j = 0; j < (BOARDY-1); j++) {
            if (forces[i][j] != 0) {
                initiative[unitcount][0] = i;
                initiative[unitcount][1] = j;
                unitcount++;
            }
        }
    }
    return initiative;
}

[nmadct]

Your function is declared as type int, but you are returning a pointer to a 2-dimensional array of int. Also, the array you're returning a pointer to is on the stack, so it won't exist anymore after the function returns!

[kidko]

The error message points to the last line, return initiative;.

How would I return the contents of the array itself, if in my main.cpp I had something like

int initiativelist[(MAXUNITS-1)][2]; // MAXUNITS is declared outside of all functions in functions.h
initiativelist = initcreate();

Should I change my code to something resembling this:

// MAIN.CPP
int *initiativelist[(MAXUNITS-1)][2]; // MAXUNITS is declared outside of all functions in functions.h
initiativelist = initcreate(initiativelist);
 
// FUNCTIONS.H
int* initcreate(int *array) {
    int unitcount = 0;
    for (int i = 0; i < (BOARDX-1); i++) {
        for (int j = 0; j < (BOARDY-1); j++) {
            if (forces[i][j] != 0) {
                array[unitcount][0] = i;
                array[unitcount][1] = j;
                unitcount++;
            }
        }
    }
    return *array;
}

[nmadct]

Yes, passing an existing array to the function as an argument is probably the best approach. This gives the calling function the most flexibility and control. However, what you're doing in this latest code is passing in a 1-dimensional array and using it as a 2-dimensional array. That won't work.

Since you have only 2 dimensions, an alternative way to set up the data structure would be to create a struct with 2 members, and pass an array of that struct. Something like this:

struct twoelements { int x; int y; };
...
   struct twoelements myarray[ARRAY_LEN];
   dostuff(myarray);
...
void dostuff(struct twoelements * myargument) {
   ...
   myargument[2].x = 4;
   myargument[2].y = 8;
   ...
}
 

If you decide to use multi-dimensional arrays you have to be very careful about how you declare them when you're passing them around as function arguments, so often it's best to just avoid them altogether.

[kidko]

Structs... I totally forgot about those, I think they'll work very well. Thank you very much for your help in answering my question!

Yes, passing an existing array to the function as an argument is probably the best approach. This gives the calling function the most flexibility and control. However, what you're doing in this latest code is passing in a 1-dimensional array and using it as a 2-dimensional array. That won't work.

Since you have only 2 dimensions, an alternative way to set up the data structure would be to create a struct with 2 members, and pass an array of that struct. Something like this:

struct twoelements { int x; int y; };
...
   struct twoelements myarray[ARRAY_LEN];
   dostuff(myarray);
...
void dostuff(struct twoelements * myargument) {
   ...
   myargument[2].x = 4;
   myargument[2].y = 8;
   ...
}
 

If you decide to use multi-dimensional arrays you have to be very careful about how you declare them when you're passing them around as function arguments, so often it's best to just avoid them altogether.

[nmadct]

Great, I'm glad that works for you!