how to take out a specific row out of matrix

Hello I have this matrix :

a = [john 2016;john 2015;mike 2016;leo 2018]

I only need information from 2016 in second column, like:

n = [john 2016;mike 2016]

I am using this code but it does not work. any help ? Thank you !

b = [2016];
n = a(ismember(a(:,2),b),:);

[James Tursa]

What are john, mike, and leo? Please copy & paste the entire error message you are getting.

[farfar]

I have this matrix with some annual information of people. I wanted to go to the second column b=a(:,2) and find all 2016 and give me a new matrix with entire row of 2016 data. I do it with numeric matrix, but this one has name in it.

Index exceeds matrix dimensions.

Error in questhree (line 5) n = a(ismember(a(:,2),b),:);

[James Tursa]

Again I ask you, what are john, mike, and leo? Also, what does size(a) return?

[farfar]

name of people ! I am not sure what you mean .

a is 10125 by 2 matrix in an excel file.

[James Tursa]

Please run the following code:

whos john mike leo a

[farfar]

 Still does not work 
Error:
Name      Size            Bytes  Class     Attributes
  a         4x1                32  double              
Index exceeds matrix dimensions.
Error in questhree (line 5)
n = a(ismember(a(:,2),b),:);

[James Tursa]

The result of the whos function shows that you have no variable named "b" in your workspace, and the 2nd dimension of the variable "a" is 1. This is causing your error since a(:,2) requires the 2nd dimension to be at least two, but "a" is just a column vector.

[farfar]

true. in the excel file, the first column is the name and the second is the numbers. but when i read it with xlsread, the matrix "a" in my workspace has only one column, (the numbers). it is not reading the names.

[Image Analyst]

You do NOT have that matrix. Since your whos statement (assuming you did what James told you to do) didn't show them as variables, they're probably strings you got from Excel, which means that (the badly-named) a and n would have to be cell arrays. But the best, most modern way to do this now is with a table, something like (untested):

t = readtable(filename);
rowsToExtract = t{:, 2} == 2016
t2016 = t(rowsToExtract, :)

Answer 1

[Image Analyst]

Attach your workbook. This can probably be done with tables:

t = readtable(filename);
rowsToExtract = t{:, 2} == 2016
t2016 = t(rowsToExtract, :)