Wow, we are still talking about this, are we? Honestly, linear algebra texts should just give in and adopt zero based indexing for matrices at this point.
Why do arrays start at 0?
601–610 of 702 posts
Re: Why do arrays start at 0?
#602Earlier quoted context omitted.
For constant addressing; for arr[i] = 2, you'll still need to subtract 1 from i with 1-based addressing when converting to machine instructions.
> for arr[i] = 2, you'll still need to subtract 1 You can, but don't need to. Just have compiler store array pointer constant as (arr-1) instead of arr, et voila, zero runtime overhead. Or, for modern(-ish) ISAs, often you can add/substract a small constant at runtime, with no extra cycles taken. For example, for x86_64: # rbp contains "true" pointer to arr # rax contains 1-based array index mov rax, QWORD PTR [rbp-8…
It's basically only x86 among modern ISAs that lets you do base + literal + register * literal, aarch64 only gives you base + register shifted by a literal, and I believe RISC-V is similar: https://gcc.godbolt.org/z/dz768768z
Re: Why do arrays start at 0?
#603Earlier quoted context omitted.
Since as you say, C doesn't carry size information, you would actually need to do it each time a pointer is created from an object. In C you can have arrays of arrays (not pointers) or arrays in structs, where there is no pointer on creation: // 11 arrays are created here int foo[10][10]; It doesn't make sense to create pointers for the 10 inner arrays, so the subtraction would presumably happen when referring to eac…
when I count apples I happily start with one. it's "natural" to label the first thing "1". or how would you count, say, 3 Apples? or three legionaries?
Counter-example:
You have to remove all desks from #20 to #30, how many desks are there?
Oh, it's 11 desks, I see. This is because we used INCLUSIVE indexing, instead of semi-open interval.
If we use semi-open intervals, we don't include the last item, so that we can write
match index {
0..10 => println!("first ten"),
10..20 => println!("next ten"),
_ => println!("something else"),
}
Re: Why do arrays start at 0?
#604Earlier quoted context omitted.
We're 20 years past the point where you can expect everyone in tech to trace every instruction down to machine instructions. Higher level languages abstract away the need for it, and for the most part, we can rely on the authors of those languages to make many of the decisions that impact performance. The rest of us learn about these details on posts like this. It's not a sad fact, in fact it's probably one of the mo…
I barely touch assembly in my day to day work, but I do understand on a fairly deep level how a computer works , which I feel is extremely important and very frequently influences how I write high-level code. Certainly one can bang out code their entire career without ever having a clue how machine code works, but I really wouldn't advise it. At worst it leads to total ignorance, and at best you accumulate a disconne…
When I did web dev it was more important to know how the WEB BROWSER works, not how machine code works
it's not useful to know how the browser allocates memory or garbage collects it, it's more useful to know that it has web sockets. Don't get the idea that things you know are relevant to other people.
Re: Why do arrays start at 0?
#605Earlier quoted context omitted.
I barely touch assembly in my day to day work, but I do understand on a fairly deep level how a computer works , which I feel is extremely important and very frequently influences how I write high-level code. Certainly one can bang out code their entire career without ever having a clue how machine code works, but I really wouldn't advise it. At worst it leads to total ignorance, and at best you accumulate a disconne…
What do you consider "high-level code" and in what way does deeply understanding how a computer works influence your approach?
In college, I optimized the array removal routine from O(N^2) to O(N) by using INVARIANTS
But it probably ran in like 100us anyway
Re: Why do arrays start at 0?
#606Earlier quoted context omitted.
Machine code is special because its the only thing the CPU can actually execute.
... then the c.p.u. further compiles this to it's internal code as it executes it. “machine code” really is no more special in this chain of events than, say, LLVM IR. I think this is the core point. Even though you are mechanically feeding machine code into the CPU, modern CPUs deeply transform your program before (or even while) running it, with potentially deep performance implications. With modern CPUs, the only…
Sometimes it executes it exactly how you think it will execute. But it's still slower because the "less optimized code" fit in its cache.
Re: Why do arrays start at 0?
#607Earlier quoted context omitted.
other important and day to day tools like rulers, clocks and speedometers also start at zero. So it's not exactly "bending humans to microprocessors weird alien ways". It all makes perfect sense in the context of measuring.
The analogy is slightly off though. The first position of an array occupies 1 unit of memory. 0 is not a possible measurement in the world of arrays. Maybe a null terminated string could have length 0 if you don't count the terminator. But that 0 is a property of the "string" abstraction. The actual "array" would still be length 1.
An array of length 0 starts at 0 and ends with 0
Re: Why do arrays start at 0?
#608Earlier quoted context omitted.
With those other things zero is zero, but with arrays the zero index is the first item. The disconnect really starts when you get the size of an array and then have to minus it by one to have the last index. To take your ruler example and apply array logic to to. Say you are measuring a 2cm thing. If the ruler follows the index then it would show 1cm (the last index), and so you would have to add + to get the size/le…
it's a good idea to call the 0th element of an array the zeroth element, else you encourage off-by-one errors. The size of the array, the "count", is that which your index must be less than. I'm not saying it has to be this way and it can't start with 1, I'm saying what you are pointing out as a flaw is actually the mixing of systems. in computer programming teams it's best to adopt a standard unambiguous language to…
No, it's the first element, if it's a 32 bit int it still occupies bytes
0..4
the second int occupies bytes
4..8
you can see the array up to the second int occupies the contiguous space 0..8 which 0..4*2 and it has TWO elements
Re: Why do arrays start at 0?
#609Earlier quoted context omitted.
the "differently wired circuit" would be an extra stage of logic computing a carry all the way from lsb to msb (an ALU outside the ALU?) and would contribute a fair bit of extra time. Easier to just use the ALU to do it, which is inserting an extra instruction, also a time waster.
If you're already doing base index addressing (eg, [SI + DI]) you're already using the ALU to compute your memory address, and it's not _that_ much extra wiring to have the ALU support either a constant power-of-two displacement or even a three-way addition; a constant displacement for a basic ripple adder is a XOR and an AND NOT, eg. The x86 family does three-way addition (and a shift) to support "[ESI] [EBX * scale…
It's not just the extra wiring, but the extra time; each additional computation layer in an integrated circuit introduced delays that would made the operation noticeably slower.
Re: Why do arrays start at 0?
#610Earlier quoted context omitted.
Defining index as the element wise offset is a precise well formed definition/name. It's also a definition commonly used in math, through not always. It depends a lot of the area of mathematics and even the cultural context. The only reason we sometimes feel 0-index is wrong IMHO is because the english language describes entities in a sequence as 1st, 2nd, 3rd etc. But I wouldn't be surprised if there is some human l…
Which floor is the first floor in your building? Depends which country your building is in, and more! My apartment building is 1,2… but my mall is G,M,1,2… (same country different architects). For many years I lived in Europe where it’s usually G,1,2… (where G can also be E or Fsz or whatever) and when I was in the US I had to remember that 1 is G, though L,2,3… is also common. City people live with index ambiguity a…
Not only that but which floor is the 13th (in the West) or 14th (in China)? Often those numbers are skipped because "luck", "feng shui", or other superstitious nonsense.
Do you count the machine floors in high rise buildings?
In AU, the ground floor is effectively "zero", the first floor is the one above ground. Depending on the design of the building, there might be a "mezzanine" floor between ground and 1st.
Basements usually follow the same pattern, so B1 is the first floor below ground etc.