Hi, I am new to MATLAB. I was trying to subtract the consecutive number in the series with the previous number and then storing this number to a new matrix. I implemented the following code but I need some help.

I have a series num: 1

2

3

4

5

And I want the following series num2: 0

1

1

1

1

Below is my code:

len= length(num);
v=2;
j=1;
for i=i:len
    num2{v,1} = minus (num{v,1},num{j,1});
    v= v+1;
    j=j+1;
end

[Guillaume]

There are several issues with your loop:

• matrix indexing uses () not {}, so it's num(v, 1), etc. Also since you're dealing with vectors you shouldn't be using 2D indexing. num(v) would do the same.
• The start bound of your loop is wrong.
• The v and j variables are completely pointless. j is always going to be the same as i and v is always going to be i+1. So your expression can be simplified to

    num2(i+1) = num(i+1) - num(i)

Also note that I've replaced minus(..) by its more standard -, there's no point in making the code more obscure.

But.. see Thiago's answer for the proper way of doing this calculation in matlab.

Answer 1

[Thiago Henrique Gomes Lobato]

In Matlab what you want to do is a one line code, you should always try to vectorize your operations or use build in function, since loops are generally not so efficient:

num=[1:5]';
num2 = [0;diff(num)];

[Guillaume]

Yes, despite what you may have been taught (unfortunately) loops are rarely an efficient solution in matlab. It usually forces you to write more code for no benefit. diff, as per Thiago's answer does exactly what you need. Another option is write the expression as the difference of two vectors:

num2 = [0; num(2:end) - num(1:end-1)];