Can use variable to assign array size?

Projects Programming
1

Hi,
Can I used:

int a=8;
int Array[a];

or not?
Thanks
Adam

2

Hello

Yes, of course, try it

3

Thanks.
int Array[‘a’];

4

That’s not your original question…

‘a’ is not a.

Try doing what you asked. Tell us it worked. Show the code where it did, and we all learn something.

a7

5

Yup, that will make you an array of 97 int(s).

image
image1425×827 237 KB

6

Thanks for detail.
the [a] got compiling error:
'a' compiling passed.
I have a picture upload failed.

int a = 8;
int arraya[a];

void setup() {
  // put your setup code here, to run once:

Serial.begin(9600);
  Serial.println("xxx_setup!");
  Serial.print("File   : "), Serial.println(__FILE__);
  Serial.print("Date   : "), Serial.println(__DATE__);
  
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 0; i < a; i++)
  {
    arraya[i] = i;
    Serial.print(arraya[i]);
  }
  delay(1000);
}

ERROR:

Arduino: 1.8.16 (Windows 7), Board: "Arduino Mega or Mega 2560, ATmega2560 (Mega 2560)"

arraya_test:3:13: error: array bound is not an integer constant before ']' token

 int arraya[a];

             ^

C:\Users\HUA.DELLV-PC\Documents\Arduino\arraya_test\arraya_test.ino: In function 'void loop()':

arraya_test:19:5: error: 'arraya' was not declared in this scope

     arraya[i] = i;

     ^~~~~~

exit status 1

array bound is not an integer constant before ']' token



This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.
7

Yeah, because you can’t do that at file scope.

But why do you need or want to do that?

a7

8

Chances are you'll never want an array of NEGATIVE size. So, 'int' is not the best choice. Any unsigned type would be better. But, there's actually a specific type for this purpose 'size_t'.

Also, the compiler error is quite clear. It wants the array size specified with a constant. So:

const size_t a = 8;
int Array[a];
9

Thanks.
do you mean why do I use ['a']?
because I have few of array[] with the same size, and I need modify the size when to do the test.

10

Sure you can:

const size_t a = 8;
int Array[a];
const size_t arraySize = sizeof(Array)/sizeof(Array[0]);

void setup() {
  Serial.begin(115200);
  delay(1000);
  Serial.println(arraySize);
}

void loop() {
}
11

Great!
Thank you gfvalvo.

12

You added, and even pointed out the necessity of doing, the const.

a7