Container initialization in C++98

Question

I have to construct an ordered container (which must be iterable) with the following rule:

If the condition is true, the container is {1,0}, else it's {0,1}

I have the following code, but I don't find it "elegant":

   vector<int> orderedSides;
   if (condition) 
   {
       orderedSides.push_back(1);
       orderedSides.push_back(0);
   }
   else            
   {
       orderedSides.push_back(0);
       orderedSides.push_back(1);
   }

Is there a better way to do this (from concision and performance point of view)?

You can make a call to reserve before you implement it. Ultimately, the std::initializer_list constructors aren't better from a performance standpoint, as they have to copy data into the container. — Alex Huszagh, Jun 30 '17 at 08:02
@Someprogrammerdude For C++ noobs: what would be the way to do this in C++11? — Right leg, Jun 30 '17 at 13:23
@Rightleg `vector orderedSided = { condition, !condition }`. *Iff* `condition` is a `bool` expression. If `condition` is otherwise convertible to `int` (or of course is an `int`) but the result might not be `1` or `0`, then `{ !!condition, !condition }` — Some programmer dude, Jun 30 '17 at 14:59
There's nothing wrong with this code. All the answers make it more complicated for nothing. — isanae, Jun 30 '17 at 17:36

score 16 · Accepted Answer · edited Jun 30 '17 at 12:17

16

You might implement something like this:

vector<int> orderedSides(2, 0);
(condition ? orderedSides.front() : orderedSides.back()) = 1;

which is a little bit shorter than explicit if clauses.

As @Deduplicator mentioned below, we might rewrite the second line in a more concise way:

orderedSides[!condition] = 1;

edited Jun 30 '17 at 12:17

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answered Jun 30 '17 at 08:06

Edgar Rokjān

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`orderedSides[!condition] = 1;` as the second line is even shorter and relies less on the optimizer. – Deduplicator Jun 30 '17 at 10:27
@Deduplicator Exactly, perfect note! – Edgar Rokjān Jun 30 '17 at 10:41
3

I find this code much harder to read and the intent is very unclear. I don't really like the bool cast either, but that ones just my opinion. The OP should steer clear of this. No down-voting, as the OP did say brevity was the most important thing, it might even be for code golf for example? – Nathan Cooper Jun 30 '17 at 11:11
The OP also mentioned performance, and this looks bad performance-wise. You have a input-dependent address, which always aliases one of the two prior stores. That's the sort of thing that an OOO CPU dislikes. @Some programmer dude's answer (in the comments, boo!) looks better as it has two definitely unrelated stores. – MSalters Jun 30 '17 at 15:53
if the condition is true, the array should be in descending order. maybe `[!condition]` would read nicer with a different name, say `[!descending]` – Mic Jun 30 '17 at 16:13

score 5 · Answer 2 · answered Jun 30 '17 at 08:07

5

vector<int> orderedSides;
orderedSides.push_back(condition ? 1 : 0);
orderedSides.push_back(condition ? 0 : 1);

I don't think it's more performant but I find it more elegant.

answered Jun 30 '17 at 08:07

Antoine Thiry

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Mic · Answer 3 · 2017-06-30T15:17:25.770

1

You could compromise between efficiency and avoiding repetition, initialise the first with the condition and the second from the first.

vector<int> orderedSides(1, bool(condition)) ;
orderedSides.push_back(!orderedSides.back());

edited Jun 30 '17 at 15:17

answered Jun 30 '17 at 08:33

Mic

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`= { bool(condition) }` is not C++98 as requested. – aschepler Jun 30 '17 at 12:23

mattn · Answer 4 · 2017-06-30T08:31:25.023

0

orderedSides.push_back(0);
orderedSides.push_back(1);
if (condition)
  std::iter_swap(orderedSides.begin(), orderedSides.begin()+1);

I know this take bits cost. As one of candidates.

edited Jun 30 '17 at 08:31

answered Jun 30 '17 at 08:13

mattn

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That'd be `std::swap`, since `front` and `back` return references and not iterators. – Quentin Jun 30 '17 at 08:23

moooeeeep · Answer 5 · 2017-06-30T12:33:58.613

0

You can populate a std::vector from an array, even in C++98.

Here's an example:

#include <iostream>
#include <vector>

int main() {
    bool condition = false;
    std::cout << "condition is: " << std::boolalpha << condition << '\n';

    int arr[][2] = {{0,1}, {1,0}};
    int index = condition;
    std::vector<int> v(arr[index], arr[index]+2);

    for (int i = 0; i < v.size(); i++)
        std::cout << v[i] << ' ';
    std::cout << '\n';
}

The output is:

$ g++ tt.cc && ./a.out
condition is: false
0 1

For reference:

http://en.cppreference.com/w/cpp/container/vector/vector

edited Jun 30 '17 at 12:33

answered Jun 30 '17 at 12:27

moooeeeep

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How is that constructor implemented in a typical C++03 standard library implementation? Is it just doing a `for` loop over the array internally? – Cody Gray - on strike Jun 30 '17 at 16:11
@CodyGray I'd hope that the implementation would resize/reserve enough memory first. But then, I don't think it makes much of a difference whether you write down a loop or memcpy or std::copy to implement this, I guess. – moooeeeep Jul 02 '17 at 09:19

score 0 · Answer 6 · answered Jun 30 '17 at 12:31

If building the elements (the ints in your question, whatever it is in real life) is free and side-effect-less:

static const int data[] = { 0, 1, 0 };
std::vector<int> orderedSides (data+condition, data+condition+2);

Full program example:

#include <iostream>
#include <vector>

std::vector<int> make(bool cond)
{
    static const int data[] = { 0, 1, 0 };
    return std::vector<int> (data+cond, data+cond+2);
}

std::ostream& operator<<(std::ostream& os, const std::vector<int>& v)
{
    return os << "{ " << v[0] << ", " << v[1] << " }";
}    

int main()
{
    std::cout << "true:  " << make(true) << "\n"
              << "false: " << make(false) << "\n";
}

Prints:

true:  { 1, 0 }
false: { 0, 1 }

Demo

Container initialization in C++98

6 Answers6