Simple for loop efficiency

Hello, this is my first post here. Currently I am working on a script that should recalculate the column numbers as following:

firstvalue = table{1,1};
i = 1;
height = height(table);
for i = 1:height
    x = table{i,1};
    table{i,1} = (x-firstvalue)/fs*1000;
    i = i+1;
end

This loop does what I want, but is terribly slow. Is there a way to make this loop execute faster? The loop works in the table with around 50.000 rows. I think the loop could be more efficient but don't know how.

Thank you!

[Adam]

What is 'table'? Is it a Table object? I'm not familiar with tables, but if it is a table then I would imagine it is quicker to calculate all your results into a numeric array first (extract all the first column of the table at once) and then assign the whole thing to the table in one go rather than updating each row of the table directly. Not sure if this is possible with tables though since I don't use them. It is a usual Matlab strategy for objects such as uitable though.

[Jan]

Omit the "i=i+1". It is only confusing inside a loop. A corresponding warning should appear in the editor.

[Daniel van Dijk]

Sorry i was trying to simplify but made it confusing. I updated the first post.

Sounds logical. I dont know what kind of table it is, to me they are all the same. I'll take a look!

Answer 1

[Akira Agata]

How about doing this.

table{:,1} = (table{:,1}-table{1,1})/fs*1000;

And let's check the calculation time.

 fs = 100;
 table1 = array2table(rand(1000,2));
 table2 = table1;

 % Slightly modified original code
 tic
 firstvalue = table1{1,1};
 i = 1;
 n = height(table1);
 for i = 1:n
    x = table1{i,1};
    table1{i,1} = (x-firstvalue)/fs*1000;
 end
 toc

 % Proposed code
 tic
 table2{:,1} = (table2{:,1}-table2{1,1})/fs*1000;
 toc

 % Check the answer
 isequal(table1, table2)

In my PC, the result is 0.131066 sec -> 0.003073 sec and table2 is the same as table1.