Joe's Home Robot

@jpgeorgia wrote:

"One problem is the Arduino's limitation to one-dimensional arrays."

Are you sure? I can compile two dimensional arrays with the Mega board selected.

I once built a medium sized robot that used 24volt windscreen wiper motors. One of the motors failed (oil leak into motor) so it is now scrapped. You can see the two little sealed lead acid batteries I used.

To enlarge an image, right click image and select Open link in new window.

@jpgeorgia Here is a link to a tutorial on multi-dimension arrays. https://www.tutorialspoint.com/arduino/arduino_multi_dimensional_arrays.htm

Make sure your lead acid batteries are of the sealed type, or better still use LiFePO4, much safer, better all round other than up front cost but lifetime cost is less.

@robotbuilder Wow, you are correct, and I am quite surprised. I tried a two-dimensional array on the online Arduino Editor, and the sketch compiled in a "check and save". In fact, after declaring int Test [2] [2], I was able to assign values to Test [4] [5], and the compiler accepted it. I guess it redimensioned my array. The only explanation I can come with is that I had used an old downloaded IDE app, but I am not sure. At the time, I did confer with another Arduino hobbyist, and he had the same problem. He had written and used a sketch function to simulate a two-dimensional array. It was too complicated for me to follow, so I designed my own. It was longer than his, but it was easy to follow. Thanks for your help.

@zander UPS batteries are sealed lead acid batteries. Otherwise, they would leak during shipping. Another reason I chose those batteries is cost.

Thanks for your advice. Joe.

Hi @jpgeorgia,

I think multi-dimensional arrays are likely to always have been possible, but I was concerned about your supplementary 'discovery'.

re: after declaring int Test [2] [2],

I was able to assign values to Test [4] [5], and the compiler accepted it.

I assume you are referring to C/C++ .... now I admit I am out of date with compiler changes, so I appeal to others to correct me if there are any 'clever' tricks I am unaware of,

but I would interpret 'compiler accepted it' means 'compiler didn't care or check'

When a compiler allocates memory for an array, it simply figures out how many bytes are required for the original size requested, and saves that amount of space.

When an assignment for a given position in the array, the code figures out a memory address for that array member, based on the start address of the array and the array indices as offsets into that array.

In many cases, the compiler does not check that that address actually falls within the block of memory allocated for the array, and obviously if it is outside, the assignment can still be attempted, but will result in something else being overwritten, possibly even program code, depending upon the memory map.

Of course, protection methods to prevent this happening are possible ... but in many cases they are omitted, at least from the final run time code, because they require extra resources, computations and so on.

But my guess, on an Arduino system, it will either:

• still work, because the overwrite happened to a bit of spare memory

or

• break, because the rightful contents of the memory that got overwritten, have been changed

And if you are really unlucky, both of the above can apply, so that it works every time you test it, but fails when someone else tries to use it.

.....

Of course, this is related to one of the common ways of hacking into a system ... feed in some data that is larger than expected, and in the process, overwrite the program with some alternate code of the hacker's choosing.

----

Moral ..

If a compiler refuses some code, you definitely have a problem.

If a compiler accepts some code, you are in with a chance of success, but in most cases, your problems haven't been identified....yet!

Best wishes and happy coding, Dave

@davee I go along with your notion that the compiler did not check the declaration and the sketch will likely crash during execution.

Joe

@jpgeorgia FYI @davee Multi-dimensional arrays are a standard part of C and C++ and Arduino.

Here is a tutorial on the subject. https://www.tutorialspoint.com/arduino/arduino_multi_dimensional_arrays.htm

Posted by: @jpgeorgia

↑

 In fact, after declaring int Test [2] [2], I was able to assign values to Test [4] [5],

You are standing in the middle of a minefield. You can declare [2][2] then store in [4][5] but if you did not declare[4][5] C will happily store there putting data on top of other data. It then goes BANG.

To be honest I'm not sure if this is what you mean but the number of times I've shot myself in the foot with arrays is distressing.

int x[2];

x[2] = 123;

BANG.

C is very cunning, it waits for the worst possible moment to go BANG as in "Hey look at this, impressive !". Then BANG.

@hilldweller I forget what it's called, but there is a switch you can turn on to detect out-of-bounds conditions. It will affect performance, but at least it won't go bang.

@hilldweller There may even be a #pragma that makes the bounds checking overhead only affect those statements that MIGHT go bang.