Tally Instances of Negative Numbers in an Array (for loop)

Hi there.

I'm an absolute beginner. I'm trying to create a script that, given an array, will count (tally) the number of negative values, and print how many there are. This is my code

A=load('ArrayWithNegatives.mat');

Count = 0; %defines variable Count = 0
for n=(1:length(A)) 
    if A(n) <= 0
        Count = Count+1;
    else Count = Count;
    end
    
    end
Count

The error I get is "Error using <= // Invalid data type. Argument must be numeric, char, or logical."

However, the code works when I manually input A=[list of numbers] instead of loading it.

Any tips or pointers to move in the right direction? I don't have to use a for loop but so far it's the only way I can think of with what I've learned.

[Voss]

FYI the line

else Count = Count;

has no effect and can be removed.

[DGM]

FWIW

% load a specific variable from a mat file
u = load('wind.mat').u;

% count negative values using nnz and a logical test
numnegative = nnz(u<0)

numnegative = 1973

See also: nnz

[Elena]

TIL! Thank you

Answer 1

[Matt J]

We don't know the names of the variables in the .mat file. If it contains a variable named B that you are trying to load into A, then you can do,

A=load('ArrayWithNegatives.mat').B;

[Elena]

This fixed it, thank you!

[Matt J]

You're welcome, but please Accept-click the answer to indicate so.

Answer 2

[Fangjun Jiang]

A=load() returns A as a structure, not a typical matrix. If you make A an matrix, this code would work if you make the following change.

numel(A) should be used instead of length(A). Check the document to see the difference.

[Elena]

Is there a command to import data as an array rather than a structure, or to create an array from the structure? In general, what could I look up to learn how to write scripts / functions that incorporate loading elements?

Ty very much

[Walter Roberson]

A=load() returns A as a structure, not a typical matrix.

Small modification to that:

If you load() a text file, then the result is a numeric array.

If you load() a mat file, then the result is a struct containing one field for each variable that was loaded.

[Walter Roberson]

@Elena

Is there a command to import data as an array rather than a structure, or to create an array from the structure?

There are two possibilities here: either you know the name of the variable being loaded, or you do not know the name of the variable being loaded.

If you know the name of the variable being loaded then use structure field referencing on the result of the load command, like

A=load('ArrayWithNegatives.mat').B;

to put the content of variable B from the file into variable A

If you do not know the name of the variable being loaded, then you need to do something like

structinfo = load('ArrayWithNegatives.mat');
varnames = fieldnames(structinfo);
A = structinfo.(varnames{1});

which would load the "first" variable from the file into A.

There is also the possibility of using an undocumented function, struct2array() https://www.mathworks.com/matlabcentral/answers/support/493612-any-fixes-for-undefined-function-or-variable-struct2array#answer_403725

A = struct2array(load('ArrayWithNegatives.mat'));

Given a scalar struct containing one or more variables, struct2array() attempts to create an array by vertcat()'ing all of the variables together.