How to use IF statements to find data greater than or equal to a certain amount

If you have a large amount of data, tens of millions of rows of data, how can you use IF statements in MATLAB to find specific data points that spike above a certain threshold?

Lets say for example you have a load of capacitance data against time. And you wan to find where the data spikes above 1 Farad. An example data table:

Time, Capacitance

1 uS, 0.3

2 uS, 0.5

3 uS, 0.8

4 uS, 1

5 uS, 1.3

6 uS, 1.4

7 uS, 1.2

8 uS, 0.9

9 uS, 0.5

You want to sift through this table and create a new variable that only includes data where the capacitance exceeds 1 Farad

Answer 1

[Karim]

see below for a demonstration

Time = [1 2 3 4 5 6 7 8 9]';
Capacitance = [0.3 0.5 0.8 1 1.3 1.4 1.2 0.9 0.5]';

MyTable = table(Time,Capacitance)

MyTable = 9×2 table
    Time    Capacitance
    ____    ___________

     1          0.3    
     2          0.5    
     3          0.8    
     4            1    
     5          1.3    
     6          1.4    
     7          1.2    
     8          0.9    
     9          0.5    

% delete variables, just to show that we work with the data from the table
clearvars Time Capacitance

% set a treshold value
Treshold = 1;

% use some logice to find the values larger then the treshold (if you want
% to include it use >= instead of >
KeepMe = MyTable.Capacitance > Treshold;

% extract a part of the table using the logical array
NewTable = MyTable( KeepMe , : )

NewTable = 3×2 table
    Time    Capacitance
    ____    ___________

     5          1.3    
     6          1.4    
     7          1.2    

[AluminiumMan]

I have tried yours and the other person aboves suggested methods, that are loosely the same. Sharing my response here also:

This is the error I am getting:

>> A=B.C(C.Capacitance>Threshold);

Subscripting into a table using one subscript (as in t(i)) or three or more subscripts

(as in t(i,j,k)) is not supported. Always specify a row subscript and a variable

subscript, as in t(rows,vars).

Note that A is the new variable I want

B is the struct and C is the table. The table is contained within the struct. It was a csv file imported to MATLAB as a .MAT file and imported as a table within a struct

[AluminiumMan]

Also your code directly called upon Capacitance, since you have a variable named capacitance. But for me, capacitance is a heading in a table, not a variable. So if I type Capacitance into my code directly, MATLAB throws an error. I have to point MATLAB to where the column header is, so its a big differnet to your setup here, where Capacitance is itself a variable that also happens to have been combined into a table

[Karim]

Hello @AluminiumMan, i modified the code to work directly with the data from the table. Have a look at the updated demonsration.

However from your question it is not clear how the data is stored in the struct. Can you add some demo code or the csv file?

[AluminiumMan]

Sure, please find data attached

[Voss]

@AluminiumMan: You left off the second subscript, like the error message suggests:

A=B.C(C.Capacitance>Threshold,:);
%                            ^^ you need this part

Notice that @Karim's answer has it:

NewTable = MyTable( KeepMe , : )

Here's a demo with your mat file and variables, as given:

S = load('CapacitanceData.mat');
B = struct('C',S.CapacitanceData.RecordedCapacitanceData);
C = B.C;
Threshold = 1;
A=B.C(C.Capacitance>Threshold,:)

A = 3×2 table
    Time    Capacitance
    ____    ___________

     3          1.3    
     4          1.6    
     7          1.3    

[AluminiumMan]

Its working. Thank you for your help

Answer 2

[Chunru]

c = randn(20,1)   % random data

c = 20×1

-0.1315 1.3920 0.3866 0.1342 0.7914 -0.5643 2.9497 -0.1842 -0.3139 0.3103

c1 = c(c>1)

c1 = 2×1

1.3920 2.9497

[AluminiumMan]

This hasn't worked for me. Your table only has one column of data. It needs to have a Time and a Capacitance column, and the c1 variable needs to contain both the time and capacitance values of any value over 1 F. I tried your formula and at the moment its not giving me a 2 column table of time and y data

[AluminiumMan]

This is the error I am getting:

>> A=B.C(C.Capacitance>Threshold);

Subscripting into a table using one subscript (as in t(i)) or three or more subscripts

(as in t(i,j,k)) is not supported. Always specify a row subscript and a variable

subscript, as in t(rows,vars).

Note that A is the new variable I want

B is the struct and C is the table. The table is contained within the struct. It was a csv file imported to MATLAB as a .MAT file and imported as a table within a struct

[Chunru]

Can you show what data B and C you have?

[AluminiumMan]

Sure. Please find data attached

[AluminiumMan]

Its working. Thank you for your help