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Function std.algorithm.sorting.makeIndex

Computes an index for r based on the comparison less.

SortedRange!(RangeIndex,(a,b)=>binaryFun!less(*a,*b)) makeIndex(alias less, SwapStrategy ss = SwapStrategy.unstable, Range, RangeIndex) (
  Range r,
  RangeIndex index
if (isForwardRange!Range && isRandomAccessRange!RangeIndex && is(ElementType!RangeIndex : ElementType!Range*) && hasAssignableElements!RangeIndex);

void makeIndex(alias less, SwapStrategy ss = SwapStrategy.unstable, Range, RangeIndex) (
  Range r,
  RangeIndex index
if (isRandomAccessRange!Range && !isInfinite!Range && isRandomAccessRange!RangeIndex && !isInfinite!RangeIndex && isIntegral!(ElementType!RangeIndex) && hasAssignableElements!RangeIndex);

The index is a sorted array of pointers or indices into the original range. This technique is similar to sorting, but it is more flexible because (1) it allows "sorting" of immutable collections, (2) allows binary search even if the original collection does not offer random access, (3) allows multiple indexes, each on a different predicate, and (4) may be faster when dealing with large objects. However, using an index may also be slower under certain circumstances due to the extra indirection, and is always larger than a sorting-based solution because it needs space for the index in addition to the original collection. The complexity is the same as sort's.

The first overload of makeIndex writes to a range containing pointers, and the second writes to a range containing offsets. The first overload requires Range to be a forward range, and the latter requires it to be a random-access range.

makeIndex overwrites its second argument with the result, but never reallocates it.


less The comparison to use.
ss The swapping strategy.
r The range to index.
index The resulting index.


The pointer-based version returns a SortedRange wrapper over index, of type SortedRange!(RangeIndex, (a, b) => binaryFun!less(*a, *b)) thus reflecting the ordering of the index. The index-based version returns void because the ordering relation involves not only index but also r.


If the second argument's length is less than that of the range indexed, an exception is thrown.


immutable(int[]) arr = [ 2, 3, 1, 5, 0 ];
// index using pointers
auto index1 = new immutable(int)*[arr.length];
makeIndex!("a < b")(arr, index1);
assert(isSorted!("*a < *b")(index1));
// index using offsets
auto index2 = new size_t[arr.length];
makeIndex!("a < b")(arr, index2);
    ((size_t a, size_t b){ return arr[a] < arr[b];})


Andrei Alexandrescu


Boost License 1.0.