Finding point number for a certain wave value

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Hi,

I'm quite new to igor so sorry if this is an elementary question. I have a wave with a minimum and I want to find the difference between that point and the value of the wave a certain number of points away. My code looks something like:

function findtrapdepth ()

variable trapdepth, minxpoint, waveminimum

waveminimum = wavemin(totalpotential)
print waveminimum

minxpoint = x2pnt(totalpotential,waveminimum)

print minxpoint

End

I would have thought it had something to do with the x2pnt function but that just returns "0" for whatever value of my variable waveminimum.

After that if I want to find the value of a wave at a certain point do I just write:

totalpotential[pointnumber]

Any help appreciated,
Sandeep
smavadia wrote:
I have a wave with a minimum and I want to find the difference between that point and the value of the wave a certain number of points away. My code looks something like: ...

I would have thought it had something to do with the x2pnt function but that just returns "0" for whatever value of my variable waveminimum.

After that if I want to find the value of a wave at a certain point do I just write:

totalpotential[pointnumber]

Any help appreciated,
Sandeep


A few questions first.

1) Is totalpotential the wave that has the minimum?
2) Is totalpotential a scaled wave rather than an unscaled wave?
3) Are you trying to find the value of totalpotential at point N away from its minimum rather than at a distance X (by its scaling) away?

If the answers to all of these are YES, then try this ...

Function/S ValueAtPointNFromMin(ww,N)
    wave ww
    variable N

    variable wmin, minpnt, vatN
    string theValues

    WaveStats/Q/M=1
    wmin = V_min
    minpnt = x2pnt(ww,V_minloc)
    vatN = ww[minpnt+N]
    sprintf theValues, "Minimum %g at point %d and value %g at point %d", wmin, minpnt, vatN, (minpnt+N)

    return theValues
end


At the command line, type ValueAtPointNFromMin(totalpotential,3) to get a printout of what you are asking.

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAH

November 15, 2009 at 05:40 pm - Permalink

Hi

thanks for your help, I still have a few more questions but first answering yours:

1) totalpotential is the wave with the minimum
2) I am using an unscaled wave, it would be nice to use a scaled wave in the future but it's not a problem at the moment.
3) N points away is fine I know what the conversion is.

Having a look at your code I guess the important thing is the V-minloc output of the wavestats function. As for x2pnt it seems to give out the same value the its second argument, no matter what the input wave is e.g.

x2pnt (anywave, 12) gives the value 12.

I had a go at this problem using a loop:

    variable i
    for (i=0; i<197; i+=1)

        if (totalpotential[i] == wavemin(totalpotential))
        minxpoint = i
        endif

    endfor


But this is quite a limited approach, it would be nice to do what I thought x2pnt was meant to do which was find the point number closest the x value that you input.

So the question is does x2pnt do anything?

Thanks
Sandeep

November 17, 2009 at 02:48 am - Permalink

smavadia wrote:
...
1) totalpotential is the wave with the minimum
2) I am using an unscaled wave, it would be nice to use a scaled wave in the future but it's not a problem at the moment.
3) N points away is fine I know what the conversion is.

Having a look at your code I guess the important thing is the V-minloc output of the wavestats function. As for x2pnt it seems to give out the same value the its second argument, no matter what the input wave is e.g.

x2pnt (anywave, 12) gives the value 12.


That is because you are using an unscaled wave. For unscaled waves, the point number and the x-value at that point are the same. Therefore, x2pnt(anywave, ANYVALUE) will always return ANYVALUE as its response.

Based on your new code, all you need to do is the following ...

Function/S LocationofMin(wy)
    wave wy

    variable wmin, minloc
    string theValues

    WaveStats/Q/M=1 wy
    wmin = V_min

    // this only works for UNSCALED waves

     minloc = V_minloc
 
    sprintf theValues, "Minimum %g at point or x value %d", wmin, minloc
 
    return theValues
end


To get a better understand of x2pnt for scaled versus unscaled waves, try the functions below. First run MakeThem(), then try various inputs to Testx2pnt().

Function MakeThem()

    make/O/N=100 wscaled, wunscaled
    SetScale/P x -50,0.5, wscaled
    display/N=Testx2Pnt wscaled, wunscaled
    ModifyGraph rgb(wscaled)=(1,3,39321),rgb(wunscaled)=(52428,1,1)

    return 0
end

Function Testx2pnt(Xv,Np)
    variable Xv,Np
   
    string theValues

    wave wscaled
    wave wunscaled
    wunscaled = x^Np
    wscaled = x^Np
    sprintf theValues, "Unscaled: (%g, %g) at point %g", Xv, wunscaled[x2pnt(wunscaled,Xv)], x2pnt(wunscaled,Xv)
    sprintf theValues, "%s; Scaled: (%g, %g) at point %g", theValues, Xv, wscaled[x2pnt(wunscaled,Xv)], x2pnt(wscaled,Xv)
   
    print theValues
   
    return 0
end

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAH

November 17, 2009 at 08:05 am - Permalink