Using logic to select specific values in a matrix

Hi y'all,

I am trying to create a second table from my original table based on taking all the values in column B, that fulfill the requirement for column A.

Example data:

Edit: attached file

I want to create a second table that only includes row 2 as long as the first row = 1.

This is what I tried:

A = SmallerSample
idnt = A(:,1) == 1 ;
new = A(ind1, 2);

The error I get:

Error using ()

A table row subscript must be a numeric array containing real positive

integers, a logical array, a character vector, a string array, a cell array of

character vectors, or a pattern scalar.

Error in Graph_Bead_Distributions

new = A(ind1, 2);

Answer 1

[Walter Roberson]

A = SmallerSample

assuming that A is a table

idnt = A(:,1) == 1 ;

() subscripting of a table yields a table.

== has, somewhat recently, become defined for tables -- and the result is a table.

So the idnt variable is going to be a table.

new = A(idnt, 2);

You are then trying to index A using a table as the first subscript.

You should have used

idnt = A.(1) == 1;

or

idnt = A{:,1} == 1;

[Stephen23]

See also: https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html

Answer 2

[John D'Errico]

Well, you are doomed to failure, as long as you don't use the right variable names as you created them!

A = [1, 0.1234
    1, 0.1345
    1, 0.1456
    2, 0.1567
    2, 0.1678
    2, 0.1789];
idnt = A(:,1) == 1 ;

You defined the variable idnt. But then you tried to use idn1. Which is a DIFFERENT variable name. Apparently you also had the variable idn1 defined in your workspace, as otherwise, you would have gotten a different error.

new = A(idnt, 2)

new = 3×1

0.1234 0.1345 0.1456

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But using the name you defined, it does indeed work. So you were not really doomed to fail. You just need to be more careful in your code. Look more carefully at the error message next time, as it suggested there was problem with the variable ind1, which is not in your workspace as a valid variable to use as an index.

[Kristine]

I did misswrite it, but in my workspace both variables were equivalent, so I still get an error once correcting the mistake:

>> Graph_Bead_Distributions

Error using () (line 132)

A table row subscript must be a numeric array containing real positive integers,

a logical array, a character vector, a string array, a cell array of character

vectors, or a pattern scalar.

Error in Graph_Bead_Distributions (line 26)

new = A(idnt, 2);

[Kristine]

I think the example table I provided is not as helpful. The actual table I am using is where the issue lies.