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Why do arrays start at 0?

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Re: Why do arrays start at 0?

#531

Post by Aeolun »

TillE wrote: post #372
just_boost_it wrote: post #324

Earlier 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…

You go as deep as you need to. My day job involves writing tons of Javascript and there is zero need for me to dive into the underlying processes until something goes wrong. So far the lowest I’ve ever needed to go was the node source.

Re: Why do arrays start at 0?

#532

Post by shpx »

nousermane wrote: post #505
saalweachter wrote: post #318

Earlier 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…

What if the variable is used for array indexing and something else, like displayed to the user?

Re: Why do arrays start at 0?

#533

Post by nephanth »

javajosh wrote: post #470
ajuc wrote: post #219

Earlier quoted context omitted.

The ground floor is 0. The floor above 0 is 1. The floor below 0 is -1. Anything else is crazy TBH, the whole point of integer numbers is to count things that start at a defined point and go opposite ways. Why shift it and then reinvent weird pseudo-negative numbers like S1? But then again people measure distance in feet and write dates as month day year :)

This is how it should be, but the only country in the world, AFAIK, that does it that way is Germany.

I think the only country i've been which does not do this is the US. It was pretty weird at first

Re: Why do arrays start at 0?

#534

Post by yarskegg »

jbaczuk wrote: post #422
diego_sandoval wrote: post #273

Because otherwise you would be wasting a perfectly good number for no reason, which means you need to use more bits to do the same thing. To write 4 numbers (including zero) you only need two bits 0: 00 1: 01 2: 10 3: 11 To write 4 numbers if you avoid using the number zero, you need three bits 1: 001 2: 010 3: 011 4: 100 If you extrapolate that a little bit, you'll realize that you'll need two bytes (1 Byte + 1 bit…

Yeah, but then couldn't the compiler just swap the 1 with a 0 and 256 with 255?

That gets really dicey if you're storing the value of the index in a variable. Does the programmer need to know that if it's an INDEX they're storing that goes from 1-256 they can fit that into an 8-bit value because the compiler will magic away that last bit? Speaking of which, how will the compiler know which 1's are really 0's and which 256's are really 255's? Will it compile that 1+1=1 if I use that result as an index? Just having a 0th index is much simpler than all of that noise.

Re: Why do arrays start at 0?

#535

Post by Miiko »

systemvoltage wrote: post #255
bregma wrote: post #168

If you ask people which floor of building they're on, it's going to depend on which country they're in. In North America, at least, the first floor you walk into (in a sane city: I understand there are some which do not qualify in this respect due to hills or historic disaster recovery) is the first floor. On other continents, you enter the ground floor and need to take stairs or an elevating device to get to the fir…

I've been living in North America for decades. Every building has G (zero). This is the floor that you walk into and has a lobby. The next floor up is 1 as such is labeled in the elevator. So your initial assumption is incorrect from my experience.

A lot of elevators in US do indeed have G for the ground floor, but that's because they are made by ThyssenKrupp AG, a German company ;).

Re: Why do arrays start at 0?

#536

Post by biztos »

dathinab wrote: post #530
tuatoru wrote: post #516

Earlier quoted context omitted.

I would be fine with it all if 0-based was called "offset", with "index" reserved for 1-based. The element at offset 0 is the first element. Precision in naming is important.

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 all the time and our brains manage just fine.

Re: Why do arrays start at 0?

#537

Post by clhodapp »

blueflow wrote: post #327
Blikkentrekker wrote: post #313

Earlier quoted context omitted.

“machine code” isn't really the zero point that's special nowadays. The code the programmer wrote is compiled to something such as LLVM IR, LLVM IR is further compiled by LLVM to Assembly, this is further compiled by an assembler into machine code, and 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.

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 choice we truly have is which compiler we trust to optimize our program.

Re: Why do arrays start at 0?

#538

Post by joaonmatos »

biztos wrote: post #536
dathinab wrote: post #530

Earlier 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…

Quick complement. Where I’m from (Portugal) we use the literal number 0 for the ground floor.

Re: Why do arrays start at 0?

#539

Post by petilon »

The best source for the development of C language is Bell Labs https://www.bell-labs.com/usr/dmr/www/chist.html

Excerpt:

   By a general rule, in B the expression
      *(V+i)
   adds V and i, and refers to the i-th location after V.
   Both BCPL and B each add special notation to sweeten such 
   array accesses; in B an equivalent expression is V[i]
C was designed to be as close to the machine as possible. Prior to C the only programming language for systems programming was assembly. If the compiler-generated code subtracted 1 from the index every time you indexed an array that would have been seen as not as efficient as assembly.

Re: Why do arrays start at 0?

#540

Post by wizofaus »

btilly wrote: post #428
wizofaus wrote: post #387

Earlier quoted context omitted.

I'd argue 0-base is a source of bugs too! Ideally we'd be able to catch more array indexing bugs at compile time - there are definitely cases where it should be possible to determine that arrays are being incorrectly indexed via static analysis.

The problem is that libraries which assume 0-base break when you have a 1-based array. And vice versa. Trying to combine libraries with different conventions becomes impossible. Therefore changing the base leads to more bugs than either base alone. That said, the more you can just use a foreach to not worry about the index at all, the better. Of 0-based and 1-based, the only data point I have is a side comment of Dij…

I'd expect the compiler not to let you to pass a 0-based array to a library function expecting a 1-based array. I'm pretty sure that's how it worked with Visual Basic, which was the only language I ever used such a feature in.
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