MATLAB Central Newsreader - Problem With a Simple For Loop

Problem With a Simple For Loop

Ali Moradi — Sun, 18 Oct 2009 18:58:02 -0400

Hi,

I'm writing a simple for loop for a homework problem and I'm coming across a weird problem that I can't figure out. I have 3 vectors all of size 9x1. I'm doing the following for loop:

for i = 2:9
    dzu(i) = PPZU(i)-PPZU(i-1);
    dzl(i) = PPZL(i)-PPZL(i-1);
    dx(i) = PPX(i)-PPX(i-1);
end

The problem is that when I run it, I get a 9x1 vector as the answer, but it should be a 8x1 vector instead. Does anyone have any idea as to why this is happening? Thank you all in advance for your help.

- Ali

Re: Problem With a Simple For Loop

Bruno Luong — Sun, 18 Oct 2009 19:21:00 -0400

"Ali Moradi" <kimusubi@gmail.com> wrote in message <hbfofq$9mi$1@fred.mathworks.com>...
> Hi,
>
> I'm writing a simple for loop for a homework problem and I'm coming across a weird problem that I can't figure out. I have 3 vectors all of size 9x1. I'm doing the following for loop:
>
> for i = 2:9
> dzu(i) = PPZU(i)-PPZU(i-1);
> dzl(i) = PPZL(i)-PPZL(i-1);
> dx(i) = PPX(i)-PPX(i-1);
> end
>
> The problem is that when I run it, I get a 9x1 vector as the answer, but it should be a 8x1 vector instead.

No, length 9 is correct.

> Does anyone have any idea as to why this is happening?

A simply setting

d(1000)=1

will give 1x1000 vector

Bruno

Re: Problem With a Simple For Loop

Ali Moradi — Sun, 18 Oct 2009 19:53:01 -0400

"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <hbfpqs$4bp$1@fred.mathworks.com>...
> "Ali Moradi" <kimusubi@gmail.com> wrote in message <hbfofq$9mi$1@fred.mathworks.com>...
> > Hi,
> >
> > I'm writing a simple for loop for a homework problem and I'm coming across a weird problem that I can't figure out. I have 3 vectors all of size 9x1. I'm doing the following for loop:
> >
> > for i = 2:9
> > dzu(i) = PPZU(i)-PPZU(i-1);
> > dzl(i) = PPZL(i)-PPZL(i-1);
> > dx(i) = PPX(i)-PPX(i-1);
> > end
> >
> > The problem is that when I run it, I get a 9x1 vector as the answer, but it should be a 8x1 vector instead.
>
> No, length 9 is correct.
>
> > Does anyone have any idea as to why this is happening?
>
> A simply setting
>
> d(1000)=1
>
> will give 1x1000 vector
>
> Bruno

I see what you mean, and I think I fixed it. I just added another loop and made it look like this:

for j = 1:8
    for i = 2:9
        dzu(j) = PPZU(i)-PPZU(i-1);
        dzl(j) = PPZL(i)-PPZL(i-1);
        dx(j) = PPX(i)-PPX(i-1);
    end
end

Does that look about right? Thanks again.

- Ali

Re: Problem With a Simple For Loop

Bruno Luong — Sun, 18 Oct 2009 20:01:04 -0400

"Ali Moradi" <kimusubi@gmail.com> wrote in message
> Does that look about right?

No, sorry it's all wrong.

Bruno

Re: Problem With a Simple For Loop

Steven Lord — Mon, 19 Oct 2009 02:50:22 -0400

"Ali Moradi" <kimusubi@gmail.com> wrote in message
news:hbfrmt$mi$1@fred.mathworks.com...
> "Bruno Luong" <b.luong@fogale.findmycountry> wrote in message
> <hbfpqs$4bp$1@fred.mathworks.com>...
>> "Ali Moradi" <kimusubi@gmail.com> wrote in message
>> <hbfofq$9mi$1@fred.mathworks.com>...

*snip*

> I see what you mean, and I think I fixed it. I just added another loop and
> made it look like this:
>
> for j = 1:8
> for i = 2:9
> dzu(j) = PPZU(i)-PPZU(i-1);
> dzl(j) = PPZL(i)-PPZL(i-1);
> dx(j) = PPX(i)-PPX(i-1);
> end
> end
>
> Does that look about right? Thanks again.

There's no need for a double loop here. Either use the loop from 2 to 9 and
remove the first element after the loop is finished:

dzu(1) = [];

or find some way to 'map' 2:9 to 1:8 and use that 'map' to convert the loop
variable into the appropriate index value. If you understand the following
example, you should be able to adapt it to your needs:

z = zeros(1, 10);
for k = 2:2:20
z(k/2) = k;
end
% Of course, if that was really your goal, you would just use z = 2:2:20;

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ

Re: Problem With a Simple For Loop

Steve Amphlett — Mon, 19 Oct 2009 08:30:05 -0400

"Ali Moradi" <kimusubi@gmail.com> wrote in message <hbfofq$9mi$1@fred.mathworks.com>...
> Hi,
>
> I'm writing a simple for loop for a homework problem and I'm coming across a weird problem that I can't figure out. I have 3 vectors all of size 9x1. I'm doing the following for loop:
>
> for i = 2:9
> dzu(i) = PPZU(i)-PPZU(i-1);
> dzl(i) = PPZL(i)-PPZL(i-1);
> dx(i) = PPX(i)-PPX(i-1);
> end
>
> The problem is that when I run it, I get a 9x1 vector as the answer, but it should be a 8x1 vector instead. Does anyone have any idea as to why this is happening? Thank you all in advance for your help.

Why loop at all? diff() will do your homework for you.