7 ms·

Insert on lookup is convenient but encourages either double-lookup patterns (contains + get) or unintentional inserts. I very much like Python's setdefault -- e

Insert on lookup is convenient but encourages either double-lookup patterns (contains + get) or unintentional inserts. I very much like Python's setdefault -- except for its 10-chars name.

gpderetta 4y ago
Well, STL encourages an iterator centric approach, so: if(auto i = x.find(y); i!=x.end()){ // use i->{first,second} } Also ideally you should be able to do this: for(auto [key, value]: x.equal_range(y)) { } But infuriatingly, x.equal_range returns a pair instead of some range view so you need some additional infrastructure (either convert the result of equal range into a range or add some begin/end overloads for pair).
- deredede 4y ago
  My C++ is a bit rusty (no puns intended), I didn't know your first syntax was possible now (another comment states it comes from C++17), that's much nicer than the alternative that pollutes the scope indeed. Having to repeat both x and i is unfortunate, but that's par for the course with C++ iterators. The equal_range solution looks nice from a genericity perspective (look ma I do multimaps!) but is really unintuitive with regular maps imo. Unless I'm mistaken it also does not allow an else branch for when the value is not there, which is often needed.
  - gpderetta 4y ago
    A for-else a-la Python would actually be generally useful! Hopefully one day we will get proper patter matching and destructing in if statements (Herb Sutter has made significant progress on this).
    - deredede 4y ago
      For-else could be useful, but I don't think Python's semantics would actually help here? Python's for-else runs when the for loop did not break (even if nonempty), but what would be needed here would be to run when the for loop was empty, which is a different condition. cpp2 is both interesting and ambitious, I really hope it gets somewhere.
      - gpderetta 4y ago
        Indeed, I misremembered the semantics of for-else! I don't think I have actually used it in python :)
        deredede 4y ago
        Yeah for-else as implemented in Python is... not actually that useful. It's the right tool to inline std::find_if and that's basically it. I think the behavior you had in mind would actually be more intuitive and more useful. OTOH, try...else I find myself missing at times.
sicp-enjoyer 4y ago
You need both access patterns (otherwise you do a double-lookup to insert a default value) and C++ already provides both.
- deredede 4y ago
  Whatever is implemented by operator[] in a given language is going to be implicitly encouraged by the language, and that's what most novices will use.

Insert on lookup is convenient but encourages either double-lookup patterns (contains + get) or unintentional inserts. I very much like Python's setdefault -- except for its 10-chars name.

gpderetta 4y ago

Well, STL encourages an iterator centric approach, so: if(auto i = x.find(y); i!=x.end()){ // use i->{first,second} } Also ideally you should be able to do this: for(auto [key, value]: x.equal_range(y)) { } But infuriatingly, x.equal_range returns a pair instead of some range view so you need some additional infrastructure (either convert the result of equal range into a range or add some begin/end overloads for pair).

deredede 4y ago
My C++ is a bit rusty (no puns intended), I didn't know your first syntax was possible now (another comment states it comes from C++17), that's much nicer than the alternative that pollutes the scope indeed. Having to repeat both x and i is unfortunate, but that's par for the course with C++ iterators. The equal_range solution looks nice from a genericity perspective (look ma I do multimaps!) but is really unintuitive with regular maps imo. Unless I'm mistaken it also does not allow an else branch for when the value is not there, which is often needed.
- gpderetta 4y ago
  A for-else a-la Python would actually be generally useful! Hopefully one day we will get proper patter matching and destructing in if statements (Herb Sutter has made significant progress on this).
  - deredede 4y ago
    For-else could be useful, but I don't think Python's semantics would actually help here? Python's for-else runs when the for loop did not break (even if nonempty), but what would be needed here would be to run when the for loop was empty, which is a different condition. cpp2 is both interesting and ambitious, I really hope it gets somewhere.
    - gpderetta 4y ago
      Indeed, I misremembered the semantics of for-else! I don't think I have actually used it in python :)
      - deredede 4y ago
        Yeah for-else as implemented in Python is... not actually that useful. It's the right tool to inline std::find_if and that's basically it. I think the behavior you had in mind would actually be more intuitive and more useful. OTOH, try...else I find myself missing at times.

sicp-enjoyer 4y ago

You need both access patterns (otherwise you do a double-lookup to insert a default value) and C++ already provides both.

deredede 4y ago
Whatever is implemented by operator[] in a given language is going to be implicitly encouraged by the language, and that's what most novices will use.