Finding the maximal value in a matrix, within a specific column, and show the value in the output.

Hello, I have a matrix that I need to extract the maximal value of column D, which is the 4th column.

I use the following command

D=readtable('tempDataTrollhFlygpl.xlsx')

Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.

D = 6789×7 table
     Tidsperiod_fr_o_m_      Tidsperiod_t_o_m_      H_jd_meter_verHavet_    Latitud_decimalgrader_    Longitud_decimalgrader_    Var6                                                                        Var7                                                                     
    ____________________    ____________________    ____________________    ______________________    _______________________    ____    _____________________________________________________________________________________________________________________________________________

    01-Jan-1961 00:00:00    01-Aug-1979 23:59:59                NaT           {'58.3141'       }           {'12.3292' }          NaN     {0×0 char                                                                                                                                   }
                     NaT                     NaT                NaT           {0×0 char        }           {0×0 char  }          NaN     {0×0 char                                                                                                                                   }
                     NaT                     NaT                NaT           {'Lufttemperatur'}           {'Kvalitet'}          NaN     {'Tidsutsnitt:'                                                                                                                             }
    01-Jan-1961 00:00:01    02-Jan-1961 00:00:00        01-Jan-1961           {'0.1'           }           {'Y'       }          NaN     {'Kvalitetskontrollerade historiska data (utom de senaste 3 mån)'                                                                           }
    02-Jan-1961 00:00:01    03-Jan-1961 00:00:00        02-Jan-1961           {'0.7'           }           {'Y'       }          NaN     {'Tidsperiod (fr.o.m.) = 1961-01-01 00:00:00 (UTC)'                                                                                         }
    03-Jan-1961 00:00:01    04-Jan-1961 00:00:00        03-Jan-1961           {'1.0'           }           {'Y'       }          NaN     {'Tidsperiod (t.o.m.) = 1979-08-01 23:59:59 (UTC)'                                                                                          }
    04-Jan-1961 00:00:01    05-Jan-1961 00:00:00        04-Jan-1961           {'0.4'           }           {'Y'       }          NaN     {'Medelvärdet är beräknat för = 24 timmar'                                                                                                  }
    05-Jan-1961 00:00:01    06-Jan-1961 00:00:00        05-Jan-1961           {'0.0'           }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }
    06-Jan-1961 00:00:01    07-Jan-1961 00:00:00        06-Jan-1961           {'0.2'           }           {'Y'       }          NaN     {'Kvalitetskoderna:'                                                                                                                        }
    07-Jan-1961 00:00:01    08-Jan-1961 00:00:00        07-Jan-1961           {'0.0'           }           {'Y'       }          NaN     {'Grön (G) = Kontrollerade och godkända värden.'                                                                                            }
    08-Jan-1961 00:00:01    09-Jan-1961 00:00:00        08-Jan-1961           {'-1.1'          }           {'Y'       }          NaN     {'Gul (Y) = Misstänkta eller aggregerade värden. Grovt kontrollerade arkivdata och okontrollerade realtidsdata (senaste 2 tim).'            }
    09-Jan-1961 00:00:01    10-Jan-1961 00:00:00        09-Jan-1961           {'-1.1'          }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }
    10-Jan-1961 00:00:01    11-Jan-1961 00:00:00        10-Jan-1961           {'-0.8'          }           {'Y'       }          NaN     {'Orsaker till saknade data:'                                                                                                               }
    11-Jan-1961 00:00:01    12-Jan-1961 00:00:00        11-Jan-1961           {'-5.4'          }           {'Y'       }          NaN     {'stationen eller givaren har varit ur funktion.'                                                                                           }
    12-Jan-1961 00:00:01    13-Jan-1961 00:00:00        12-Jan-1961           {'-6.8'          }           {'Y'       }          NaN     {'kvalitetskontrollerna har felmarkerat data (röd kvalitetskod). Efterföljande manuell granskning godkänner, rättar eller underkänner data.'}
    13-Jan-1961 00:00:01    14-Jan-1961 00:00:00        13-Jan-1961           {'2.6'           }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }

[~,x] = max(D(:,4));

Error using tabular/max
Applying the function 'max' to the variable 'Latitud_decimalgrader_' generated an error.

Caused by:
Error using max
Invalid data type. First argument must be numeric or logical.

B = D(x,:);

However, it does not work, and I get the error

Error using tabular/max

Applying the function 'max' to the variable 'Latitud_decimalgrader_' generated an error.

Error in untitled (line 2)

[~,x] = max(D(:,4));

Caused by:

Error using max

Invalid data type. First argument must be numeric or logical.

I should specify the rows after row 11 I think, but how that done?

I tried islocalmax instead, with plot command, but also this case didn't work.

A=readtable('tempDataTrollhFlygpl.xlsx')
plot(A(:,3:end), A(:,4:end))
xlabel('Temperature')
ylabel('Time')
TF1 = islocalmax(A,'FlatSelection','first');

Thanks

[Torsten]

What do you want to define

max('Lufttemperatur',0x0 char ,'-6.8')

?

Answer 1

[Voss]

D(:,4) is a table (consisting of one column - the 4th column of D), not a numeric array; that's what the error is trying to tell you.

To get the data out of the table, you can use the syntax D{:,4}. However, in this case, column 4 of your table is a cell array of character vectors, not a numeric array, so you'll get a similar error using that syntax. (Note the single quotes around the numbers in column 4 below, indicating character vectors.)

D=readtable('tempDataTrollhFlygpl.xlsx')

Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.

D = 6789×7 table
     Tidsperiod_fr_o_m_      Tidsperiod_t_o_m_      H_jd_meter_verHavet_    Latitud_decimalgrader_    Longitud_decimalgrader_    Var6                                                                        Var7                                                                     
    ____________________    ____________________    ____________________    ______________________    _______________________    ____    _____________________________________________________________________________________________________________________________________________

    01-Jan-1961 00:00:00    01-Aug-1979 23:59:59                NaT           {'58.3141'       }           {'12.3292' }          NaN     {0×0 char                                                                                                                                   }
                     NaT                     NaT                NaT           {0×0 char        }           {0×0 char  }          NaN     {0×0 char                                                                                                                                   }
                     NaT                     NaT                NaT           {'Lufttemperatur'}           {'Kvalitet'}          NaN     {'Tidsutsnitt:'                                                                                                                             }
    01-Jan-1961 00:00:01    02-Jan-1961 00:00:00        01-Jan-1961           {'0.1'           }           {'Y'       }          NaN     {'Kvalitetskontrollerade historiska data (utom de senaste 3 mån)'                                                                           }
    02-Jan-1961 00:00:01    03-Jan-1961 00:00:00        02-Jan-1961           {'0.7'           }           {'Y'       }          NaN     {'Tidsperiod (fr.o.m.) = 1961-01-01 00:00:00 (UTC)'                                                                                         }
    03-Jan-1961 00:00:01    04-Jan-1961 00:00:00        03-Jan-1961           {'1.0'           }           {'Y'       }          NaN     {'Tidsperiod (t.o.m.) = 1979-08-01 23:59:59 (UTC)'                                                                                          }
    04-Jan-1961 00:00:01    05-Jan-1961 00:00:00        04-Jan-1961           {'0.4'           }           {'Y'       }          NaN     {'Medelvärdet är beräknat för = 24 timmar'                                                                                                  }
    05-Jan-1961 00:00:01    06-Jan-1961 00:00:00        05-Jan-1961           {'0.0'           }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }
    06-Jan-1961 00:00:01    07-Jan-1961 00:00:00        06-Jan-1961           {'0.2'           }           {'Y'       }          NaN     {'Kvalitetskoderna:'                                                                                                                        }
    07-Jan-1961 00:00:01    08-Jan-1961 00:00:00        07-Jan-1961           {'0.0'           }           {'Y'       }          NaN     {'Grön (G) = Kontrollerade och godkända värden.'                                                                                            }
    08-Jan-1961 00:00:01    09-Jan-1961 00:00:00        08-Jan-1961           {'-1.1'          }           {'Y'       }          NaN     {'Gul (Y) = Misstänkta eller aggregerade värden. Grovt kontrollerade arkivdata och okontrollerade realtidsdata (senaste 2 tim).'            }
    09-Jan-1961 00:00:01    10-Jan-1961 00:00:00        09-Jan-1961           {'-1.1'          }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }
    10-Jan-1961 00:00:01    11-Jan-1961 00:00:00        10-Jan-1961           {'-0.8'          }           {'Y'       }          NaN     {'Orsaker till saknade data:'                                                                                                               }
    11-Jan-1961 00:00:01    12-Jan-1961 00:00:00        11-Jan-1961           {'-5.4'          }           {'Y'       }          NaN     {'stationen eller givaren har varit ur funktion.'                                                                                           }
    12-Jan-1961 00:00:01    13-Jan-1961 00:00:00        12-Jan-1961           {'-6.8'          }           {'Y'       }          NaN     {'kvalitetskontrollerna har felmarkerat data (röd kvalitetskod). Efterföljande manuell granskning godkänner, rättar eller underkänner data.'}
    13-Jan-1961 00:00:01    14-Jan-1961 00:00:00        13-Jan-1961           {'2.6'           }           {'Y'       }          NaN     {0×0 char                                                                                                                                   }

"I should specify the rows after row 11 I think, but how that done?"

Specify 'NumHeaderLines', 10 in your readtable call:

D=readtable('tempDataTrollhFlygpl.xlsx', 'NumHeaderLines', 10) % specifying 10 header lines

D = 6786×7 table
            Var1               Var2           Var3          Var4      Var5     Var6                                                                        Var7                                                                     
    ____________________    ___________    ___________    ________    _____    ____    _____________________________________________________________________________________________________________________________________________

    01-Jan-1961 00:00:01    02-Jan-1961    01-Jan-1961    {'0.1' }    {'Y'}    NaN     {'Kvalitetskontrollerade historiska data (utom de senaste 3 mån)'                                                                           }
    02-Jan-1961 00:00:01    03-Jan-1961    02-Jan-1961    {'0.7' }    {'Y'}    NaN     {'Tidsperiod (fr.o.m.) = 1961-01-01 00:00:00 (UTC)'                                                                                         }
    03-Jan-1961 00:00:01    04-Jan-1961    03-Jan-1961    {'1.0' }    {'Y'}    NaN     {'Tidsperiod (t.o.m.) = 1979-08-01 23:59:59 (UTC)'                                                                                          }
    04-Jan-1961 00:00:01    05-Jan-1961    04-Jan-1961    {'0.4' }    {'Y'}    NaN     {'Medelvärdet är beräknat för = 24 timmar'                                                                                                  }
    05-Jan-1961 00:00:01    06-Jan-1961    05-Jan-1961    {'0.0' }    {'Y'}    NaN     {0×0 char                                                                                                                                   }
    06-Jan-1961 00:00:01    07-Jan-1961    06-Jan-1961    {'0.2' }    {'Y'}    NaN     {'Kvalitetskoderna:'                                                                                                                        }
    07-Jan-1961 00:00:01    08-Jan-1961    07-Jan-1961    {'0.0' }    {'Y'}    NaN     {'Grön (G) = Kontrollerade och godkända värden.'                                                                                            }
    08-Jan-1961 00:00:01    09-Jan-1961    08-Jan-1961    {'-1.1'}    {'Y'}    NaN     {'Gul (Y) = Misstänkta eller aggregerade värden. Grovt kontrollerade arkivdata och okontrollerade realtidsdata (senaste 2 tim).'            }
    09-Jan-1961 00:00:01    10-Jan-1961    09-Jan-1961    {'-1.1'}    {'Y'}    NaN     {0×0 char                                                                                                                                   }
    10-Jan-1961 00:00:01    11-Jan-1961    10-Jan-1961    {'-0.8'}    {'Y'}    NaN     {'Orsaker till saknade data:'                                                                                                               }
    11-Jan-1961 00:00:01    12-Jan-1961    11-Jan-1961    {'-5.4'}    {'Y'}    NaN     {'stationen eller givaren har varit ur funktion.'                                                                                           }
    12-Jan-1961 00:00:01    13-Jan-1961    12-Jan-1961    {'-6.8'}    {'Y'}    NaN     {'kvalitetskontrollerna har felmarkerat data (röd kvalitetskod). Efterföljande manuell granskning godkänner, rättar eller underkänner data.'}
    13-Jan-1961 00:00:01    14-Jan-1961    13-Jan-1961    {'2.6' }    {'Y'}    NaN     {0×0 char                                                                                                                                   }
    14-Jan-1961 00:00:01    15-Jan-1961    14-Jan-1961    {'0.3' }    {'Y'}    NaN     {0×0 char                                                                                                                                   }
    15-Jan-1961 00:00:01    16-Jan-1961    15-Jan-1961    {'2.5' }    {'Y'}    NaN     {0×0 char                                                                                                                                   }
    16-Jan-1961 00:00:01    17-Jan-1961    16-Jan-1961    {'-2.7'}    {'Y'}    NaN     {0×0 char                                                                                                                                   }

Now, you need to convert those character vectors to numbers, either by setting the appropriate import options before readtable is called, or after the fact by converting e.g. using str2double

val = str2double(D{:,4})

val = 6786×1

0.1000 0.7000 1.0000 0.4000 0 0.2000 0 -1.1000 -1.1000 -0.8000

[~,x] = max(val)

x = 5333

B = D(x,:)

B = 1×7 table
            Var1               Var2           Var3          Var4      Var5     Var6       Var7   
    ____________________    ___________    ___________    ________    _____    ____    __________

    08-Aug-1975 00:00:01    09-Aug-1975    08-Aug-1975    {'25.3'}    {'Y'}    NaN     {0×0 char}