Finding the x position of the Nth iteration of a given value in a wave

execute

DisplayHelpTopic "FindValue"

DisplayHelpTopic "FindLevels"

DisplayHelpTopic "FindDuplicates"

to learn about some operations that may be useful here.

FindDuplicates with the /INDX flag looks like a good solution.

// returns point value of nth occurrence of value
function FindNthValue(variable value, variable instance, wave w)
    duplicate /free w w_x
    w_x = x
    extract /free/O w_x, w_x, w == value
    if (numpnts(w_x) >= instance)
        return w_x[instance-1]
    endif
    return NaN
end

You may want to check for values that match within some tolerance, rather than exact matches. Depending on how the data are created, you could run into trouble comparing, say, a single precision wave value with a double precision variable value.

Note that tony's example code returns the point value in Igor's zero based counting system, so you need to add 1 to get the result your want (N = 1 gives result 2 etc.). Here is a slightly altered code which uses Extract in a different way and also returns 1-based results:

function FindNthValue(variable value, variable instance, wave w)
    extract/free/indx w, occurence, w == value
    if (instance > 0 && numpnts(occurence) >= instance)
        return occurence[instance-1]+1
    endif
    return NaN
end