Finding the index of a specific element in a list












4












$begingroup$


I have searched everywhere to find an answer for this and have come up with nothing!



I have a data file containing two columns, one with wavelengths in nanometer (some with decimal digits, others just with a decimal point) and the other with spectral irradiance values.



I am trying to make a new table that takes as the first column all the wavelengths and in the second column I divide the spectral values by photon energy to get photon flux. However, in this new table I want to stop at a certain index i where the wavelength corresponds to a specific photon energy that I calculated. This is the code I use. It works.



Here is a small portion of the data table:



{280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5, 



SSpecUsed = Drop[Table[{SSpecWavelength[[i]],SSpecIntensity[[i]]/Eph[SSpecWavelength[[i]]]*Subscript[En, G, Si]*q /. numval}, {i, 0, 949, 1}], 1];


My issue is that I manually found the index (949) at which the wavelength is 1107. nm. In a later exercise I need to integrate over this data up to different indices (so for example integrating all the way up to wavelength 2000 and in the next step only up to 1999). So I tried replacing the 949 value by:



Position[SSpecWavelength, Subscript[[Lambda], G][Eg_]]/.numval


So basically I try using Position to find the index at which the element in the list is equal to the wavelenght that I calculate based on an energy value. When I run it I get empty curly brackets. I even tried this:



Position[SSpecWavelength, 1107.]


and I still get empty curly brackets. I checked and I know for a fact that the element at index 949 is equal to 1107. so why does it not return me the index?



Is there another (better/simpler) way to accomplish what I want?



Thanks in advance, hope the problem is clear!






list-manipulation numerical-value







share|improve this question











$endgroup$












  • $begingroup$
    Would you please provide a small portion of the data table as an example? That would make live much easier for us.
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
    $endgroup$
    – Henrik Schumacher
    7 hours ago










  • $begingroup$
    FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
    $endgroup$
    – alonoid
    7 hours ago










  • $begingroup$
    How do I only use the returned index number without the curly brackets? How do I remove the brackets?
    $endgroup$
    – alonoid
    7 hours ago
















4












$begingroup$


I have searched everywhere to find an answer for this and have come up with nothing!



I have a data file containing two columns, one with wavelengths in nanometer (some with decimal digits, others just with a decimal point) and the other with spectral irradiance values.



I am trying to make a new table that takes as the first column all the wavelengths and in the second column I divide the spectral values by photon energy to get photon flux. However, in this new table I want to stop at a certain index i where the wavelength corresponds to a specific photon energy that I calculated. This is the code I use. It works.



Here is a small portion of the data table:



{280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5, 



SSpecUsed = Drop[Table[{SSpecWavelength[[i]],SSpecIntensity[[i]]/Eph[SSpecWavelength[[i]]]*Subscript[En, G, Si]*q /. numval}, {i, 0, 949, 1}], 1];


My issue is that I manually found the index (949) at which the wavelength is 1107. nm. In a later exercise I need to integrate over this data up to different indices (so for example integrating all the way up to wavelength 2000 and in the next step only up to 1999). So I tried replacing the 949 value by:



Position[SSpecWavelength, Subscript[[Lambda], G][Eg_]]/.numval


So basically I try using Position to find the index at which the element in the list is equal to the wavelenght that I calculate based on an energy value. When I run it I get empty curly brackets. I even tried this:



Position[SSpecWavelength, 1107.]


and I still get empty curly brackets. I checked and I know for a fact that the element at index 949 is equal to 1107. so why does it not return me the index?



Is there another (better/simpler) way to accomplish what I want?



Thanks in advance, hope the problem is clear!






list-manipulation numerical-value







share|improve this question











$endgroup$












  • $begingroup$
    Would you please provide a small portion of the data table as an example? That would make live much easier for us.
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
    $endgroup$
    – Henrik Schumacher
    7 hours ago










  • $begingroup$
    FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
    $endgroup$
    – alonoid
    7 hours ago










  • $begingroup$
    How do I only use the returned index number without the curly brackets? How do I remove the brackets?
    $endgroup$
    – alonoid
    7 hours ago














4












4








4





$begingroup$


I have searched everywhere to find an answer for this and have come up with nothing!



I have a data file containing two columns, one with wavelengths in nanometer (some with decimal digits, others just with a decimal point) and the other with spectral irradiance values.



I am trying to make a new table that takes as the first column all the wavelengths and in the second column I divide the spectral values by photon energy to get photon flux. However, in this new table I want to stop at a certain index i where the wavelength corresponds to a specific photon energy that I calculated. This is the code I use. It works.



Here is a small portion of the data table:



{280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5, 



SSpecUsed = Drop[Table[{SSpecWavelength[[i]],SSpecIntensity[[i]]/Eph[SSpecWavelength[[i]]]*Subscript[En, G, Si]*q /. numval}, {i, 0, 949, 1}], 1];


My issue is that I manually found the index (949) at which the wavelength is 1107. nm. In a later exercise I need to integrate over this data up to different indices (so for example integrating all the way up to wavelength 2000 and in the next step only up to 1999). So I tried replacing the 949 value by:



Position[SSpecWavelength, Subscript[[Lambda], G][Eg_]]/.numval


So basically I try using Position to find the index at which the element in the list is equal to the wavelenght that I calculate based on an energy value. When I run it I get empty curly brackets. I even tried this:



Position[SSpecWavelength, 1107.]


and I still get empty curly brackets. I checked and I know for a fact that the element at index 949 is equal to 1107. so why does it not return me the index?



Is there another (better/simpler) way to accomplish what I want?



Thanks in advance, hope the problem is clear!






list-manipulation numerical-value







share|improve this question











$endgroup$




I have searched everywhere to find an answer for this and have come up with nothing!



I have a data file containing two columns, one with wavelengths in nanometer (some with decimal digits, others just with a decimal point) and the other with spectral irradiance values.



I am trying to make a new table that takes as the first column all the wavelengths and in the second column I divide the spectral values by photon energy to get photon flux. However, in this new table I want to stop at a certain index i where the wavelength corresponds to a specific photon energy that I calculated. This is the code I use. It works.



Here is a small portion of the data table:



{280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5, 



SSpecUsed = Drop[Table[{SSpecWavelength[[i]],SSpecIntensity[[i]]/Eph[SSpecWavelength[[i]]]*Subscript[En, G, Si]*q /. numval}, {i, 0, 949, 1}], 1];


My issue is that I manually found the index (949) at which the wavelength is 1107. nm. In a later exercise I need to integrate over this data up to different indices (so for example integrating all the way up to wavelength 2000 and in the next step only up to 1999). So I tried replacing the 949 value by:



Position[SSpecWavelength, Subscript[[Lambda], G][Eg_]]/.numval


So basically I try using Position to find the index at which the element in the list is equal to the wavelenght that I calculate based on an energy value. When I run it I get empty curly brackets. I even tried this:



Position[SSpecWavelength, 1107.]


and I still get empty curly brackets. I checked and I know for a fact that the element at index 949 is equal to 1107. so why does it not return me the index?



Is there another (better/simpler) way to accomplish what I want?



Thanks in advance, hope the problem is clear!






list-manipulation numerical-value




list-manipulation numerical-value






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 6 hours ago









m_goldberg

86.7k872196




86.7k872196










asked 7 hours ago









alonoidalonoid

262




262












  • $begingroup$
    Would you please provide a small portion of the data table as an example? That would make live much easier for us.
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
    $endgroup$
    – Henrik Schumacher
    7 hours ago










  • $begingroup$
    FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
    $endgroup$
    – alonoid
    7 hours ago










  • $begingroup$
    How do I only use the returned index number without the curly brackets? How do I remove the brackets?
    $endgroup$
    – alonoid
    7 hours ago


















  • $begingroup$
    Would you please provide a small portion of the data table as an example? That would make live much easier for us.
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
    $endgroup$
    – Henrik Schumacher
    7 hours ago






  • 1




    $begingroup$
    In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
    $endgroup$
    – Henrik Schumacher
    7 hours ago










  • $begingroup$
    FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
    $endgroup$
    – alonoid
    7 hours ago










  • $begingroup$
    How do I only use the returned index number without the curly brackets? How do I remove the brackets?
    $endgroup$
    – alonoid
    7 hours ago
















$begingroup$
Would you please provide a small portion of the data table as an example? That would make live much easier for us.
$endgroup$
– Henrik Schumacher
7 hours ago




$begingroup$
Would you please provide a small portion of the data table as an example? That would make live much easier for us.
$endgroup$
– Henrik Schumacher
7 hours ago




1




1




$begingroup$
You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
$endgroup$
– Henrik Schumacher
7 hours ago




$begingroup$
You may try something like FirstPosition[SSpecWavelength, _?(# > 1107. &)] to find the first position with a value larger than 1107..
$endgroup$
– Henrik Schumacher
7 hours ago




1




1




$begingroup$
In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
$endgroup$
– Henrik Schumacher
7 hours ago




$begingroup$
In general Position may take floating point numbers more literate than you. The value from the list could be 1107.0000001 or so but only the leading digits are shown. This difference may be small, but Position won't match 1107. with 1107.0000001.
$endgroup$
– Henrik Schumacher
7 hours ago












$begingroup$
FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
$endgroup$
– alonoid
7 hours ago




$begingroup$
FirstPosition works actually. But why does Position not work? The element in the list is exactly 1107.
$endgroup$
– alonoid
7 hours ago












$begingroup$
How do I only use the returned index number without the curly brackets? How do I remove the brackets?
$endgroup$
– alonoid
7 hours ago




$begingroup$
How do I only use the returned index number without the curly brackets? How do I remove the brackets?
$endgroup$
– alonoid
7 hours ago










1 Answer
1






active

oldest

votes


















5












$begingroup$

Given 



data = {280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5};


you could use



data[[;; Position[data, 282.][[1, 1]]]]


or 



data[[;; FirstPosition[data, 282.][[1]]]]


both return




{280., 280.5, 281., 281.5, 282.}




I recommend that you use the 2nd form; i.e., the one using FirstPosition



Now suppose data was



data = {280., 280.5, 281., 281.5, 282.0000001, 282.5, 283., 283.5, 284., 284.5};


Then, although 



data[[5]]


prints out as




282.




Position[data, 282.][[1, 1]]


generates the error message



msg



To fix this you can write



Position[data, _?(Round[#, .1] == 282. &)]



{{5}}




The same change in its 2nd argument will fix the problem should FirstPostion be used in place of Position.






share|improve this answer









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    1 Answer
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    5












    $begingroup$

    Given 



    data = {280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5};


    you could use



    data[[;; Position[data, 282.][[1, 1]]]]


    or 



    data[[;; FirstPosition[data, 282.][[1]]]]


    both return




    {280., 280.5, 281., 281.5, 282.}




    I recommend that you use the 2nd form; i.e., the one using FirstPosition



    Now suppose data was



    data = {280., 280.5, 281., 281.5, 282.0000001, 282.5, 283., 283.5, 284., 284.5};


    Then, although 



    data[[5]]


    prints out as




    282.




    Position[data, 282.][[1, 1]]


    generates the error message



    msg



    To fix this you can write



    Position[data, _?(Round[#, .1] == 282. &)]



    {{5}}




    The same change in its 2nd argument will fix the problem should FirstPostion be used in place of Position.






    share|improve this answer









    $endgroup$


















      5












      $begingroup$

      Given 



      data = {280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5};


      you could use



      data[[;; Position[data, 282.][[1, 1]]]]


      or 



      data[[;; FirstPosition[data, 282.][[1]]]]


      both return




      {280., 280.5, 281., 281.5, 282.}




      I recommend that you use the 2nd form; i.e., the one using FirstPosition



      Now suppose data was



      data = {280., 280.5, 281., 281.5, 282.0000001, 282.5, 283., 283.5, 284., 284.5};


      Then, although 



      data[[5]]


      prints out as




      282.




      Position[data, 282.][[1, 1]]


      generates the error message



      msg



      To fix this you can write



      Position[data, _?(Round[#, .1] == 282. &)]



      {{5}}




      The same change in its 2nd argument will fix the problem should FirstPostion be used in place of Position.






      share|improve this answer









      $endgroup$
















        5












        5








        5





        $begingroup$

        Given 



        data = {280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5};


        you could use



        data[[;; Position[data, 282.][[1, 1]]]]


        or 



        data[[;; FirstPosition[data, 282.][[1]]]]


        both return




        {280., 280.5, 281., 281.5, 282.}




        I recommend that you use the 2nd form; i.e., the one using FirstPosition



        Now suppose data was



        data = {280., 280.5, 281., 281.5, 282.0000001, 282.5, 283., 283.5, 284., 284.5};


        Then, although 



        data[[5]]


        prints out as




        282.




        Position[data, 282.][[1, 1]]


        generates the error message



        msg



        To fix this you can write



        Position[data, _?(Round[#, .1] == 282. &)]



        {{5}}




        The same change in its 2nd argument will fix the problem should FirstPostion be used in place of Position.






        share|improve this answer









        $endgroup$



        Given 



        data = {280., 280.5, 281., 281.5, 282., 282.5, 283., 283.5, 284., 284.5};


        you could use



        data[[;; Position[data, 282.][[1, 1]]]]


        or 



        data[[;; FirstPosition[data, 282.][[1]]]]


        both return




        {280., 280.5, 281., 281.5, 282.}




        I recommend that you use the 2nd form; i.e., the one using FirstPosition



        Now suppose data was



        data = {280., 280.5, 281., 281.5, 282.0000001, 282.5, 283., 283.5, 284., 284.5};


        Then, although 



        data[[5]]


        prints out as




        282.




        Position[data, 282.][[1, 1]]


        generates the error message



        msg



        To fix this you can write



        Position[data, _?(Round[#, .1] == 282. &)]



        {{5}}




        The same change in its 2nd argument will fix the problem should FirstPostion be used in place of Position.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 6 hours ago









        m_goldbergm_goldberg

        86.7k872196




        86.7k872196






























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