Import Data and plot it with an extra variable as vector

Hello Community,

so I'm quite new to Matlab and I got some beginner struggle:

I imported an Excel sheet with "3" columns (Date, electricity consumption, electricity production) over the function "Import Data". And I want to initialize a variable "quotient" that has the quotient of "production / consumption" for each row. Now i just want to plot (scatter) this quotient with the date. The Errors told me "Vectors must be the same length" so I figured out I need this variable as a vector, but honestly I don't get how to proper initialize a vector and add every quotient for every row to it.

I picture sth like this:

Q = initilize Vector Q
for length of table
    Q = production / consumption
end

scatter(table.Date, Q)

I hope someone can help me with this minor problem (:

Thanks

Answer 1

[Jacob Wood]

Should be an easy fix!

What you are looking for is an element-wise divide: https://www.mathworks.com/help/matlab/ref/rdivide.html

After you have imported your data into a table "T" your code might look like this:

T.quotient = T.production ./ T.consumption;
scatter(T.Date,T.quotient)

[Lynna]

I don't really get how I divide or even initialize a vector/array or similar ^^ It's sth like: vector = 200 : 3 when i have 200 rows and 3 columns right? Or is it not important that its :3 because i just want 1 "new column" with the quotient in it?

And then my problem is I dont know how to get my data into it.

The Quotient is also no column of my table T so is it even possible so say T.quotient = x / y? Or should it be vectorQuotient = x / y?

Answer 2

[Steven Lord]

Since you reference the "Import Data" 'function' I assume you're using the Import Tool opened by clicking the button labeled "Import Data" in the Variable section on the Home tab of the MATLAB Desktop's toolstrip? The one with a green arrow pointing down into a rectangle?

If so, and if you selected to import the data into a table array using the "Output Type" dropdown (in the Imported Data section of the Import tab of the Import Tools toolstrip) then you've read the data into a table array already.

If you have your data in a table array you can reference variables in the table array by typing <name of the table>.<name of the variable>. Let's take the table created by the "Store Related Data Variables in Table" example on the table documentation page. I'm using this code instead of interactively importing a file because it's easier than showing a series of pictures of the steps to import the data using the Import Tool.

LastName = {'Sanchez';'Johnson';'Li';'Diaz';'Brown'};
Age = [38;43;38;40;49];
Smoker = logical([1;0;1;0;1]);
Height = [71;69;64;67;64];
Weight = [176;163;131;133;119];
BloodPressure = [124 93; 109 77; 125 83; 117 75; 122 80];
T = table(LastName,Age,Smoker,Height,Weight,BloodPressure)

The table T has six variables. If I wanted to compute (as an example) each person's weight (stored in the variable Weight in T) divided by their height (the variable Height in T):

y = T.Weight ./ T.Height

If I wanted to store that as a new variable in the table T:

T.WeightOverHeight = T.Weight ./ T.Height

[Lynna]

Yes the Import Data Tool is what i meant. And yes I use the Output Type "Table".

It's good to know that I just can kinda add a new column with a new parameter. I thought I needed to store it in an extra vector. Thanks for the explanation for this.

When I try your suggestion to just create a new T.Quotient "Column" with my wanted information stored I still get the Error Message about the same length.

I added my code (just 3 rows^^) and the error message. The code which is automatically created by Import Data Tool wasn't added 'cause I think Matlab would import it right and I didn't have a Problem here.

I don't know what I'm doing wrong here and I'm sorry it's kinda obvious for someone who uses matlab ^^

% other automatically created import code from the Import Data Tool

% Import the Data
t = readtable("C:\[...]\ImportedFileName.xlsx", opts, "UseExcel", false);


%%%% Code 

%% Clear temporary variables (automatically created)
clear opts


% My Code:
t.quotient = t.production / t.consumption;

scatter(t.Date, t.quotient);
title('Verhältnis Gesamterzeugung zu Gesamtverbrauch über den Tag');


%%%% Error Message I get:
Error using scatter (line 78)
X and Y must be vectors of the same length.

Error in ScatterPlotQuotient (line 34)
scatter(t.Date, t.quotient);


% When I asked for their lenght:
>> length(t.quotient)

ans =

       35040

>> length(t.Date)

ans =

       35040