The SelectFirst(n, p, L, b1, ..., bn) function selects the first n elements e of L which satisfy p(e, b1, ..., bn).

The SelectLast(n, p, L, b1, ..., bn) function selects the last n elements e of L which satisfy p(e, b1, ..., bn).

If n is not given, the default value of n is 1.

If n is negative, SelectFirst searches backward from the end of L, and SelectLast searches forward from the start of L but reverses the output.

If p is not given, the default value of p is the function which returns true.

If L is an rtable (such as an Array, Matrix, or Vector), the result is equivalent to applying this function to convert(L,list) (with all other arguments the same).  (See convert/list.)

If n is not given, the SelectFirst and SelectLast commands return the found element or NULL. Otherwise, if L is a list or rtable, a list is returned, and if L is a set, a set is returned.

If the option output = form is given, then the elements selected and/or their indices in L are returned, depending on whether form includes the keywords values and/or indices, respectively. If the option output=form is not given, the default value of output is values. The return format is an expression sequence of the lists of values and/or indices (or the value and/or index, or NULL, if n is not given), the order reflecting the order of the keywords in output. The value of output may not include indices when L is a set. Any argument which is interpreted as the output = form option will not be passed as an extra argument to p.

$\mathrm{with}\left(\mathrm{ListTools}\right)\:$

$L≔\left[\mathrm{seq}\left(1..10\right)\right]$

$L≔\left[1\,2\,3\,4\,5\,6\,7\,8\,9\,10\right]$

$\mathrm{SelectFirst}\left(L\right)$

$1$

$\mathrm{SelectFirst}\left(1\,L\right)$

$\left[1\right]$

$\mathrm{SelectFirst}\left(\mathrm{isprime}\,L\right)$

$2$

$\mathrm{SelectLast}\left(\mathrm{isprime}\,L\right)$

$7$

$\mathrm{SelectFirst}\left(3\,\mathrm{type}\,L\,\mathrm{even}\right)$

$\left[2\,4\,6\right]$

$\mathrm{SelectFirst}\left(-3\,\mathrm{type}\,L\,\mathrm{odd}\right)$

$\left[9\,7\,5\right]$

$\mathrm{SelectLast}\left(3\,\mathrm{type}\,L\,\mathrm{odd}\right)$

$\left[5\,7\,9\right]$

$V≔\mathrm{Vector}\left(\left[1\,2\,3\right]\right)$

$V≔\left[\begin{array}{c}1\\ 2\\ 3\end{array}\right]$

$\mathrm{SelectFirst}\left(2\,V\right)$

$\left[1\,2\right]$

$\mathrm{SelectLast}\left(3\,⟨⟨1\left|2\right|3|4⟩\,⟨5\left|6\right|7|8⟩⟩\right)$

$\left[7\,4\,8\right]$

$\mathrm{SelectFirst}\left(2\,\mathrm{type}\,\left[a\,''a''\,a\,''b''\right]\,\mathrm{name}\,\mathrm{output}=\mathrm{indices}\right)$

$\left[1\,3\right]$

$\mathrm{SelectFirst}\left(2\,\mathrm{type}\,\left[a\,''a''\,a\,''b''\right]\,\mathrm{string}\,\mathrm{output}=\left[\mathrm{values}\,\mathrm{indices}\right]\right)$

$\left[''a''\,''b''\right],\left[2\,4\right]$

$S≔\left\{1\,''abc''\,\mathrm{uvwxyz}\,\mathrm{Int}\left(\sin \left(x\right)\,x\right)\right\}$

$S≔\left\{1\,''abc''\,\mathrm{uvwxyz}\,\int \sin \left(x\right)\ⅆx\right\}$

$\mathrm{SelectFirst}\left(3\,S\right)$

$\left\{1\,''abc''\,\mathrm{uvwxyz}\right\}$

$\mathrm{SelectLast}\left(3\,\mathrm{isprime}\,\left[a\,b\,c\right]\right)$

Error, (in ListTools:-SelectFirst) cannot determine if this expression is true or false: isprime(c)