Access to a table, why matlabs run with code: T = T(:,[varnames(others) 'SelfAsses​sedHealthS​tatus']); , but later can not run with this one: T(:, ['Systolic' 'Diastolic']) = [];

I have the code below in which 'patients.dat is a sample file in Matlab.

clear all
clc
T = readtable('patients.dat');
T(:, ['Systolic'  'Diastolic']) = [];
varnames = T.Properties.VariableNames;
others = ~strcmp('SelfAssessedHealthStatus', varnames);
T = T(:, [varnames(others)  'SelfAssessedHealthStatus']);
T(1:5,6:7)

When I run this code. Matlab reported an error message:

Error using magic_square (line 5)
Unrecognized variable name 'SystolicDiastolic'.

When I remove line: T(:, ['Systolic' 'Diastolic']) = []; , matlab works well.

My problem is that: the two lines: T = T(:, [varnames(others) 'SelfAssessedHealthStatus']); and T(:, ['Systolic' 'Diastolic']) = []; Have the same syntax, but why matlab reported an error message at the latter (T(:, ['Systolic' 'Diastolic']) = [];)

Can somebody help me explain this phenomenon? many thanks and best regards!

Answer 1

[Stephen23]

The problem is that you misunderstand the meaning of square brackets []. Many beginners think that square brackets construct a list, but in fact they are a concatenation operator. So when you write this:

['Systolic'  'Diastolic']

you concatenate the two strings together into one string:

>> ['Systolic'  'Diastolic']
ans =
SystolicDiastolic

It does not construct a list of those strings! In fact the error message message shows this quite clearly too:

Unrecognized variable name 'SystolicDiastolic'.

You see, actually reading the error message is a good habit, because it contains useful information to understand what is happening. Here it showed you one string that you are using as a variable name, not two strings like you think you are using.

The solution, like it always is, is to read the documentation:

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

"When indexing into a table with parentheses or curly braces, you can specify vars as a colon, numeric indices, logical expressions, a single variable name, or a cell array of row names." There is a bug in the documentation because that should read "a cell array of variable names.", and this is easy to figure out from the context.

A cell array of variable names will resolve your problem:

T(:,{'Systolic','Diastolic'})

[Dung Le]

Thank you for your help!

Could you explain me why: ['Systolic' 'Diastolic'] --> 'SystolicDiastolic' (this syntax can not construct a list and it is so-called concatenation operator)

But: [varnames(others) 'SelfAssessedHealthStatus'] --> this one does not lead to concatenation and it can be used to construct a list although the former and the latter both use square brackets.

[Stephen23]

Because the varnames are in a cell array MATLAB places the trailing string into a cell before concatenating them, like this:

>> [{'A','B'},'C']
ans = 
    'A'    'B'    'C'

MATLAB does an implicit type conversion from string to cell (of a string), thus the output is a cell array of strings and this is correct for using to index tables with.

[Dung Le]

I get it, Thank you so much !