pred

OpenTuring

predpredecessor function

Syntax   pred ( expn )

Description   The pred function accepts an integer, character, or an enumerated value and returns the integer minus one, the previous character, or the previous value in the enumeration. For example, pred ( 7 ) is 6.

Example   This part of a Turing program fills up array a with the enumerated values red, yellow, green, red, yellow, green, etc.

        type colors : enum ( green, yellow, red )
        var a : array 1 .. 100 of colors
        var c : colors := colors . red
        for i : 1 .. 100
            a ( i ) := c
            if c = colors . green then
                c := colors . red
            else
                c := pred ( c )
            end if
        end for

Details   It is illegal to apply pred to the first value of an enumeration.

See also   succ, lower and upper functions.