Assigning an empty array row to reference [duplicate]

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Assigning an element of a multidimensional array to a second array (2 answers)

Closed 5 years ago.

int colors[][3] = {
 { 255, 0, 0 }, 
 { 0, 255, 0 }, 
 { 0, 0, 255 }, 
 { 253, 7, 210 }
};
int (*EXCLUSIVE_COLOR)[3];

I have a two-dimensional array to store some color values.
I have a 2nd array, which I am using to assign an "EXCLUSIVE_COLOR" from the first colors array.

What is the best method to assign a null or empty value to the "EXCLUSIVE_COLOR" array to be used upon initialization and when the "EXCLUSIVE_COLOR" is unset?

Initially I tried just assigning a default value to the array upon initialization:

int (*EXCLUSIVE_COLOR)[3] = {0,0,0};
//Produces an error: scalar object ‘EXCLUSIVE_COLOR’ requires one element in initializer
int (*EXCLUSIVE_COLOR)[3] = {0};
//assigns non-zero values to all three elements

I know that I can add a "dummy" record to the colors array to reference, when there isn't an "EXCLUSIVE_COLOR" set.

int colors[][3] = {
 { 255, 0, 0 }, 
 { 0, 255, 0 }, 
 { 0, 0, 255 }, 
 { 253, 7, 210 }, 
 { 0, 0, 0, } //dummy record
};
int (*EXCLUSIVE_COLOR)[3] = &colors[4];

However I wanted to know if there was a cleaner way to be able to assign an empty value to reference when a color is not set, and when the array is initialized.

• Juraj: use the pointer as I recommended in you previous question
• VerySeriousSoftwareEndeavours: I am currently using a pointer to a 3 int array, as suggested in the previous question. However that only specifies how to assign an array element by reference from the colors array, which is why I'm currently using the "dummy record" approach to set a default value. I was wondering if there was a better way to assign a default value to the pointer, other than having it point to a dummy array element in the colors array.
• Juraj: int* EXCLUSIVE_COLOR = {0,0,0}; read my comment or the edit at the end of Edgar's answer

Answer 1

If you're using a pointer then there's no limit to what that pointer can point to.

The way I see it you have two sensible choices:

1. Point to a default value

This doesn't have to be in your array. It can be anything that is an int[3], since that is what your array consists of.

int[3] nothing = {0, 0, 0};
EXCLUSIVE_COLOR = nothing;

This means that it's actually pointing to something valid, in this case it's actually (I assume) the colour black. That can make it easy to deal with, since EXCLUSIVE_COLOR[0] will be a valid value.

2. Point to nothing

You can point your pointer to NULL and it will be pointing to "nothing". This means that EXCLUSIVE_COLOR[0] is no longer valid, which can make handling it somewhat harder. However it does allow you to distinguish between "nothing" and "black", unlike option 1.

For example:

EXCLUSIVE_COLOR = colors[2]; // {0, 0, 255}
...
EXCLUSIVE_COLOR = NULL; // Not pointing anywhere

To test if it's pointing somewhere you can simply use if:

if (EXCLUSIVE_COLOR) { 
 // you have a valid entry
}