position

(position item sequence &key test test-not key start end from-end)

Returns the 0-based index of the first element of sequence that matches item, or nil if no element matches. The comparison is eql by default; :test takes a function designator to use a different comparison and :test-not its negation, while :key takes a selector function applied to each element before the comparison. :start/:end bound the scanned region (the returned index still counts from the start of the whole sequence), and with :from-end true the LAST match wins. The sequence may be a list or a string; the elements of a string are characters. Unlike find, which returns the element, position returns its integer position. The full keyword set also works through the first-class value, e.g. (apply #'position item seq other-keys).