27.17 PositionSet

PositionSet( list, elm )
PositionSet( list, elm, func )

In the first form PositionSet returns the position of the element elm, which may be an object of any type, with respect to the sorted list list.

In the second form PositionSet returns the position of the element elm, which may be an object of any type with respect to the list list, which must be sorted with respect to func. func must be a function of two arguments that returns true if the first argument is less than the second argument and false otherwise.

PositionSet returns pos such that list[pos-1] < elm and elm = list[pos]. That means, if elm appears once in list, its position is returned. If elm appears several times in list, the position of the first occurrence is returned. If elm is not an element of list, then false is returned.

    gap> PositionSet( [1,4,5,5,6,7], 0 );
    false
    gap> PositionSet( [1,4,5,5,6,7], 2 );
    false
    gap> PositionSet( [1,4,5,5,6,7], 4 );
    2
    gap> PositionSet( [1,4,5,5,6,7], 5 );
    3
    gap> PositionSet( [1,4,5,5,6,7], 8 );
    false 

PositionSet is very similar to PositionSorted (see PositionSorted) but returns false when elm is not an element of list.

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