Two different ways of using iterations

I have decided to do some repetition in Matlab during this summer so I'm doing a lot of assignments again that I have already finished a couple of months ago. Though, I stumble upon a certain assignment and my solution would be:

load tal
v_stor=[];
v_liten=[];
v_mellan=[];

for i=1:length(V)
    if V(i)>=10
        v_stor = v_stor + V(i);
    elseif V(i)<=-10
        v_liten = v_liten + V(i);
    else
        v_mellan = v_mellan + V(i);
    end
end
v_stor
v_mellan
v_liten

I think it's pretty straightfoward what I want my code to do. This gave me empty vectors and I was confused why, so I checked my earlier solution:

load tal

t = 0;

v_stor = [];
v_mellan = [];
v_liten = [];

for i= 1:length(V)
    if V(t+1)>= 10
        v_stor=[v_stor V(t+1)];
    elseif V(t+1)<=-10
        v_liten = [v_liten V(t+1)];
    else
        v_mellan=[v_mellan V(t+1)];
    end
    t=t+1;
end
v_stor
v_mellan
v_liten

So my question is, why is my second solution wrong and how come, in my earlier solution, that I used V(t+1)? It looks the same to me.

Edit: Vector V is of size 1000x1

Answer 1

[dipak nigam]

load tal
v_stor=[];
v_liten=[];
v_mellan=[];
for i=1:length(V)
    if V(i)>=10
        v_stor = [v_stor V(i)];
    elseif V(i)<=-10
        v_liten = [v_liten V(i)];
    else
        v_mellan = [v_mellan V(i)];
    end
end
v_stor
v_mellan
v_liten

You were trying to add values to a vector by using '+' operator. You need to change your code like above to get the desired result.

[Michael Peyron]

Yes, I was able to find it too.

Thanks anyway!

Answer 2

[KSSV]

In the first case, you are not saving the result. Check the below.

load tal
v_stor=zeros(1,length(V));
v_liten=zeros(1,length(V));
v_mellan=zeros(1,length(V));

for i=1:length(V)
    if V(i)>=10
        v_stor(i) =  V(i);
    elseif V(i)<=-10
        v_liten(i) =  V(i);
    else
        v_mellan(i) =  V(i);
    end
end
v_stor
v_mellan
v_liten

[Michael Peyron]

I fixed my error:

for i=1:length(V)
    if V(i)>=10
        v_stor = [v_stor  V(i)];
    elseif V(i)<=-10
        v_liten = [v_liten  V(i)];
    else
        v_mellan = [v_mellan  V(i)];
    end
end

And achieved the correct answer.

How silly of me to make that mistake. Good that I'm doing repeat of the assignments.

[KSSV]

The above method takes time..the right way is what I have shown. Also you need not to use loop for your case.

v_stor = V(V>=10) ;
v_liten = V(V<=-10) ;
v_mellan = V(V>=10 & V<10) ; 

[Michael Peyron]

I did not know this way of extracting the values existed. Thanks for telling. They only taught us how to do it with for loops.

[KSSV]

The basic building block is a loop. But in MATLAB you can avoid loop most of the times....remember the full form of MATLAB.

[Michael Peyron]

Thanks! I added your code to my notes!