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Comment by jschveibinz | Hacker News Reader

I have always assumed that is just one less operation required to resolve the absolute memory address.

The mental model of being an offset to a memory address is, I think, part of it.

I'd be surprised if the use of zero vs one actually made any difference, from a number of operations point of view -- from the compiler's point of view, the first element in the array is the first element in the array, no matter what we call it.

I mean in an extreme edge case maybe if you are computing an index, and it happens to be zero, then in your math maybe some identity related to zero could be exploited (go to element simple_integer*complicated_function(), where simple_number might be zero) but that seems a bit silly.

How would it not make a difference? If you calculate an index at runtime, to get access to the element, in 0 based would be pointer + index. In 1 based it is however pointer + index - 1 clearly there is an extra subtraction there?

In x86 you could probably hide it in the addressing but that does not mean it does not to be computed

If you’re calculating indexes at runtime, you’re probably not worried about that level of optimization.

One option, at least, would be for malloc and friends to return pointer-1, rather than pointer. That said, I'm sure a compilers expert could come up with a much better option.

This is primarily the actual answer. While most languages force you to see the array as an array, in C/C++ (in C++ I'm only referring to the memory allocated variable type, not an "array class") you can see it as a pointer and do your own math to address any part of it that you want. And the calculation for that memory address is idx * sizeof(the thing in your array). So the real answer here is that array semantics needed to match memory address semantics in the languages that provide that.

There's no reason why memory addresses had to start at 0.

No.

1. Optimizing compilers exist.

2. Addressing at fixed offsets is cheap (a single instruction): https://en.wikipedia.org/wiki/Addressing_mode

0-based indexing is superior (as a default) because it simplifies a lot of common math, as discussed in TFA.

3. Less buggy for small systems since the Base Pointer address points directly to a record.

4. Macro expansion is often used to store indexes and other 'magic numbers' in Assembler, and this pattern originated either when Assembly was the primary programming language, or even earlier when humans directly punched out cards with machine instructions. Compilers in any remote sense of the luxury we have today did not exist or were not common.

That is true for vectors, but not matrices, right?

  char at(char matrix[10][10], char i, char j) {
    return matrix[i][j];
  }

still has more computation on 1-indexed than on 0-indexed, I believe.

Well you've either got:

memory_address + array_width*i + j

memory_address + array_width*(i-1) + (j-1) = (memory_address-array_width-1)+ array_width*i + j

so you can just absorb the extra computation into the pointer.

TFA = Thanks For Asking?

The "Friendly" Article

https://mtsknn.fi/blog/tfa-stands-for-the-featured-article/

> 0-based indexing is superior (as a default) because it simplifies a lot of common math, as discussed in TFA.

If that was true, Fortran, Julia, R and Matlab would use 0 instead of 1-based.

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