Arrays of Arrays (2009)

Post by sesuximo » Sun, Dec 20, 2020, 2:38 PM UTC

mhh__ wrote: post #18

sesuximo wrote: post #5

Earlier quoted context omitted.

Imo foo[x][y] is better for many reasons: - compiler can break it apart so foo[x] is evaluated once for many ys just like any other function - you can make types that are unaware of their rank eg vector doesn’t know if it’s a 1D vector of int or a 2D vector of vectors - “seamlessly” handles jagged arrays

They'd probably be lowered to exactly the same code, so the compiler argument isn't very good.

Post by anonymoushn » Sun, Dec 20, 2020, 3:20 PM UTC

thomasahle wrote: post #15

Animats wrote: post #3

Over a decade ago I was trying to get the C++ committee to support multidimensional arrays via overloading, so we could have 2D collections. In C++, you can overload "operator[]". But it can't take more than one argument. Why? Because of the C "comma operator". In C, the comma operator evaluates both operands and discards the first result. It's an ancient hack used mostly with preprocessor defines. In C and C++, foo(…

> In C, the comma operator evaluates both operands and discards the first result. Sounds like how the semicolon operator works in modern languages, like Rust.

  fn main() {
      println!("ok"; "Hello, world!");
  }

Post by Animats » Sun, Dec 20, 2020, 6:01 PM UTC

tbrgmnn wrote: post #9

Animats wrote: post #3

Over a decade ago I was trying to get the C++ committee to support multidimensional arrays via overloading, so we could have 2D collections. In C++, you can overload "operator[]". But it can't take more than one argument. Why? Because of the C "comma operator". In C, the comma operator evaluates both operands and discards the first result. It's an ancient hack used mostly with preprocessor defines. In C and C++, foo(…

With C++20, use of comma expressions as second argument of a subscript operator got deprecated.

Post by Animats » Sun, Dec 20, 2020, 11:25 PM UTC

gpderetta wrote: post #26

jiggawatts wrote: post #6

Earlier quoted context omitted.

Which reminds me of an insightful observation that the array indexing operator [] and the function call operator () are basically doing the same thing: evaluating a function and returning a value. So what you've just described is basically Currying, or partial function evaluation. It's a bit like defining "al(i)" as array lookup for an index 'i' instead of "[i]". Then the following are equivalent: something[i][j] som…

In C++ you can create a simple generic reusable currying wrapper. Boost as a few. The reality is that the vast majority of C++ programmers don't see the need.

Post by gpderetta » Tue, Dec 22, 2020, 2:39 PM UTC

Animats wrote: post #27

gpderetta wrote: post #26

Earlier quoted context omitted.

In C++ you can create a simple generic reusable currying wrapper. Boost as a few. The reality is that the vast majority of C++ programmers don't see the need.

Trying to do this this through Currying turns a simple 2D array access into a quite complicated operation. One that a compiler has to be really smart to turn into a gheap 2D array indexing operation again. This really matters in number-crunching code, where it seems that almost everything is about multiplying arrays.