Defining an array size with a string length?

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hi all,
is it possible to use the length of a string to set a size parameter for an array?
I'm working on building the message for an LED array
Something like this...(which doesn't work)

char displayText[] ="enter text here"; 
const int msgLen = 6*(strlen(displayText)-1);   // miss off the null character from end of char array
                                   // 6 x columns of LEDs per character
boolean message [msgLen][7];

This gives an error "Array bound is not a integer constant"

Many thanks

2

a little bit of experimentation and further reading...
I see that I don't need the -1 to take the null character into account as strlen will deal with that for me.

So in simple terms, I want to.

set a Char array.
find its length
define a 2D boolean array using the length.

I've included other failed attempts in this code sample..

char displayText[] ="123456789"; 
// #define msgLen strlen(displayText);
//const int msgLen = strlen(displayText);
const int msgLen = int(strlen(displayText));
boolean message[msgLen][7];

Unless I set the msgLen manually as a number I get the "array bound is not an integer constant" every time :frowning:

3

If you declare the variables with local scope rather than the global scope I assume you're giving them, there is no reason this should not work.

4

I'm declaring this above the Void loop as I wanted to use a function call (first time I've had a go at one) to populate the Array. Assumed I needed to make it global to keep the contents?

thanks for responding.

5

You could declare the string with global scope, and put the boolean inside a function.

char displayText[] ="123456789";

void setup ()
{
  // H/w setup in here
  // then...
  myFn (displayText);
}

void loop ()
{
}

void myFn (char* str)
{
  const int msgLen = strlen(str);
  boolean message[msgLen][7]; 
  // whatever else you want to do.
  // can be done here, put yur own infinite loop
  //here and "loop ()" will never be called
}
6

Thanks again for your time.

Can you tell me why this can't be set as a global array?

My plan WAS to have the array configured globally.
Then use a function to fill it with binary depending on the letters in the message. Called from within setup loop.
Then use the loop to repeat the display on an LED matrix.

I can set it all up in a function or in the loop itself but I was trying to be more structured so would be interested to know where I'm going wrong.

Many thanks!

7

It's a good question and one to which I've forgotten the exact answer.
Globals are static so have to have a predetermined size. 'strlen' is a function so can't run outside of another function.

8

And yes, I do realise that the compiler ought to be able to determine the size of a static string
i guess it boils down to the size of the static allocation must be fixed..

9 
char displayText[]      = { 'e', 'n', 't', 'e', 'r', ' ', 't', 'e', 'x', 't', ' ', 'h', 'e', 'r', 'e' };
const int msgLen = sizeof(displayText);

bool message [msgLen][7];

If 'displayText' was an array of anything larger than a byte 'msgLen' would need to changed to:

const int msgLen = sizeof(displayText)/sizeof(displayText[0]);
10

at the risk of looking foolish....I have no idea what you mean I'm afraid lloyddean.