Accessing part of an array that doesn't exist

piguy101

Suppose that I had an array of ten bytes called memory. So: byte memory[10]. What would happen if part of the code tried to set or read parts of the array that do not exist such as memory[11]:=0 or x:=memory[11]? My guess is that the first code would not do anything and x in the second code would be set to zero. Is this correct?

Duane Degn

memory[11] := 1 would set the memory location two places after the last element in the array to one. If you had:

VAR
  byte memory[10], x, y, z

Then

memory[11] := 1

would have the same effect as

y := 1

Remember with ten elements in the array, the highest index is 9 (since they start with zero).

So:

memory[10] := 1

would have the same effect as

x := 1

If you use

memory[13] := 1

Then you're in danger of overwriting your program.

piguy101

Thanks. I made a simple game of simon with an array of 255 elements and wondered what would happen if I could memorize that many in a row. I just tested the code, and it does simply read the next variable after the array rather than crashing. This would have caused a huge headache if the Prop had less RAM.

kwinn

If it is possible that the array index (the i part of memory(i)) could be beyond the end of the data array you would want to make sure it is in bounds before using it by comparing i to the maximum value.

David Betz

kwinn wrote: »

If it is possible that the array index (the i part of memory(i)) could be beyond the end of the data array you would want to make sure it is in bounds before using it by comparing i to the maximum value.

Good advice and keep in mind that if your array is declared like this:

VAR
  byte memory[10], x, y, z

Then you want to check that your index is >= 0 and < 10. So memory[10] is outside of the bounds of this array since the index must be in the range of 0 to 9.