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

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Sat, 01/22/2022 - 07:38 pm

Hi.

I have a 1D numerical wave MyWave.

I’m trying to write a function that will return the row (Xpoint) at which a given value, XValue, can be found in MyWave for the Nth time.

For example, if MyWave = 0,1,0,1,1,1,1,0,1 and

If XValue = 1, and

     If N = 1, the function should return xPoint = 2.

     If N = 3, the function should return xPoint = 5.

Thanks for your help!

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.

January 23, 2022 at 03:17 am - Permalink

// 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.

January 23, 2022 at 03:40 am - Permalink

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

 

January 23, 2022 at 04:35 am - Permalink