Array of pointers to strings

Question

I have the following;

const CHAR string_1[] PROGMEM = "String 1";
const CHAR string_2[] PROGMEM = "String 2";
const CHAR string_3[] PROGMEM = "String 3";
const CHAR string_4[] PROGMEM = "String 4";
const CHAR string_5[] PROGMEM = "String 5";

const CHAR *string_table[] PROGMEM  = 
{
    string_1,
    string_2,
    string_3,
    string_4,
    string_5
};

How would I save this address of string_table so I could call it in a function;

CHAR acBuffer[20];
UCHAR ucSelectedString = 2; // get string number 3
//
    pcStringTable = string_table ...?? What is the proper line here??
//
strcpy_P(acBuffer, (char*)pgm_read_byte(&(pcStringTable[ucSelectedString])))

Based on the comments below, I changed by structure too;

typedef struct
{
...
CHAR **pasOptions;

I then tried to assign string_table to it;

stMenuBar.pasOptions = string_table;

The compiler throws this warning;

warning: assignment from incompatible pointer type

Any more thoughts?

@MahmoudAl-Qudsi I don't follow. Haven't I declared it in the struct? — mriksman
– mriksman, Commented May 4, 2012 at 6:55

Mahmoud Al-Qudsi · Accepted Answer · 2012-05-04 07:07:38Z

3

string_table is an array of pointers to strings. An array can decay to a (one-dimensional, because that's the only kind) pointer just fine.

So an array to arrays of strings can be represented as a pointer [think: array] to (pointers of chars [think: strings]).

const char **pcStringTable = string_table;

Which you can then access as any other one-dimensional array:

printf("%s", pcStringTable[2]);

edited May 4, 2012 at 7:07

answered May 4, 2012 at 6:08

Mahmoud Al-Qudsi

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5 Comments

Lundin Over a year ago

The code in the OPs example is not a two-dimensional array, it is an array of pointers, which can decay into a pointer-to-pointer. When you downcast a pointer-to-pointer into a single pointer, you end up with a single pointer.

mriksman Over a year ago

@MahmoudAl-Qudsi I tried your suggestions, but to no avail. I have edited my original question.

Mahmoud Al-Qudsi Over a year ago

That's just a const correctness warning. Since your strings are defined as const char[], the string table variable must be const char ** - I've fixed my answer to reflect that.

mriksman Over a year ago

Thanks - that did it. Is that even necessary when I declared them as PROGMEM?

Mahmoud Al-Qudsi Over a year ago

That's an arduino-specific compiler command, it tells the compiler to store the variables in the flash instead of the main memory. It has nothing to do with the C code.

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