Create 2 new Columns of data in a table in an existing file after each time the script is run

for i = 1:10
    someinfo = readtable("someinfo.csv");
    [q, p] = getqp(someinfo);
    table.column1 = p;
    table.column2 = q*p
end

I have a matlab script that generates a csv file with data but its overwriting data in column1 and column2. I wanted data in newcolumns

Everytime I run my script, I want 2 new columns created beside column1 and 2. The new columns are getting information from a database with changing information however the information is assigned to p and q so the formulas for the first and second new columns I wish to be generated are exactly the same as column 1 and 2 respectively.

table.columnm = p; table.columnn = q*p;

I want these new columns to be added to the existing csv file and have a new name based on each run.

What is confusing me is the logic. I don't understand where in my code I would implement this feature. I know it wouldnt be in this for loop

[Walter Roberson]

"and have a new name based on each run"

What name should they have? "column" followed by the next available column number?

Would "var" followed by the next available column number be acceptable?

[N/A]

Would "var" followed by the next available column number be acceptable? Yes

What name should they have? "column" followed by the next available column number? Yes, this is fine too

I just want new data to append to the existing file instead of overwriting it every time the script is run

Answer 1

[Walter Roberson]

  new_table{:,end+1} = p;
  new_table{:end+1} = p*q;

Question: is p a vector? Is q a scalar or a vector or an array? I am concerned about whether p*q invokes algebraic matrix multiplication and whether the result is a vector of the correct size. Is it possible for some results to be a different size than others?

[N/A]

All very excellent questions.

p is a single value with three significant figures. q is just an integer.

Where in my script would I put the code you gave? within the for loop? outside the for loop? at the end of the main function?

this is what I get when I place it within the for loop

Error

Unable to perform assignment because brace indexing is not supported for variables of this type.

When I run the script using the normal brackets (), it generates a new column for every row of data with the rest of the rows being 0 so thats not quite right.

[Walter Roberson]

In my code, I used new_table as the variable name because using table as the variable name gets confusing between variable name and call to the table() constructor. new_table should be initialized as a table() object for my code to work. It sounds like you might have initialized as a numeric array.

With you indicating that p and q are scalars, then the code I show would add new variables containing only one meaningfully used row; if the table object had multiple rows before then they would be extended with 0's. But if the table object had multiple rows before, I have to wonder what you intended by adding new scalar variables to it ...

[N/A]

function [url, Id, rarity] = getTable(List)
    
    [rows] = size(List);
    
    for i=1:rows
        row = List(i,:);
        
        url = string(row.URL(1));
        Id = string(row.ID(1));
        rarity = string(row.Rarity(1));
        quantity = row.Quantity(1);
                   
        [title, p] = getDetails(url);  
        
        cardList.CurrentPricePer(i,1) = p;
        cardList.CurrentTotalPrice(i,1) = quantity*p;
    end
    
    writetable(List, 'list_currents.csv');
end

Information is being retrieved from the url and the price and name is being retrived. This is used to generate a column with a name and the price associated with that name. There are several rows of names and one column in which the price is placed corresponding to the name.

I've ran the script once. Its generated current price per and current total price. Now I need to do it again but in new columns.

Does this help you understand the problem a bit more?

[Walter Roberson]

Notice the when you use a single output for size() then the result is always a vector with minimum length 2. You then use that vector as the upper bound in for i=1:rows rather than having the bound be a scalar. MATLAB does define the operation: it ignores anything beyond the first element, so the call becomes effectively for i=1:rows(1) ... but along the way you would have managed to confuse people reading the code about whether you were doing this deliberately or accidentally. Perhaps you should use rows = size(List,1)

If List is a table object then you could also use rows = height(List)

Your code can only work if List is either a table object or a nonscalar struct (or it is some kind of object).

You write into the structure cardList but you never save the structure anywhere.

What you writetable() is the original List input, unchanged.

You return url, Id, rarity from the function, but their values are going to be whatever they were left at in the loop -- so they will happen to correspond to the information about the last row of List. Is that useful and meaningful, that you bother to return those for exactly the last row?

[N/A]

The output was supposed to be Name, Id, and rarity, sorry.

Let me preface this by saying there is more code and the script is working.

You are absolutely correct in your observations. Everything is going exactly as planned. It is all deliberate.

The intention for name id and rarity to be outputting from the function is to do exactly what you just stated, to return the information pertinent and then loop back and do the same for the next name.

By having the outputs as those listed, I will be able to add a search feature later on to scour through the data and return the information I request. But thats later.

So now the question comes again

How do I do this such that it appends instead of overwrites?