Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.

Dismiss

106 views

Skip to first unread message

Apr 18, 2001, 9:57:26â€¯AM4/18/01

to

K>> for issue=current_list disp(issue); disp('---'); end

57

86

86

148

57

86

86

148

---

K>> for issue=current_list(:)' disp(issue); disp('---'); end

57

---

86

---

86

---

148

---

Michael Robbins, CFA

Director, Debt Capital Markets

CIBC World Markets Corp., Canadian Imperial Bank of Commerce

New York, NY USA

michael...@us.cibc.com , rob...@bloomberg.net

Apr 18, 2001, 10:08:58â€¯AM4/18/01

to

"for" expects a row vector not a column vector so if your variable

current_list is a column then the behaviour is as you noticed. This

behaviour can be useful - it allows you to loop over the columns of a matrix

very easily e.g.

current_list is a column then the behaviour is as you noticed. This

behaviour can be useful - it allows you to loop over the columns of a matrix

very easily e.g.

>> for issue=magic(3) disp(issue); disp('---'); end

8

3

4

---

1

5

9

---

6

7

2

---

Chris Hayhurst

The MathWorks

chay...@mathworks.co.uk

Michael Robbins <michael...@us.cibc.com> wrote in message

news:a3hD6.1480$D4.1...@www.newsranger.com...

Apr 18, 2001, 4:38:06â€¯PM4/18/01

to

FOR Loops take columns at a time. For example

K=eye(4);

for i=K

% code

end

During the i-th iteration, i contains the i-th column of K!

When you write for i=1:24

the right hand side of the equal sign is a row vector, so in the i-th

iteration i is the i-th column of the created row vector.

for i=(1:24)' performs the for loop once since the right hand side has one

column!

See pages 163 to 167 in Mastering MATLAB 6.

While Loops and If-Then-Else constructs have vector oriented operation as

well! See the rest of Chapter 10 for more info.

--

Duane Hanselman

Mastering MATLAB 6

ISBN 0-13-019468-9

www.eece.maine.edu/mm

Michael Robbins <michael...@us.cibc.com> wrote in message

news:a3hD6.1480$D4.1...@www.newsranger.com...

Apr 19, 2001, 7:49:39â€¯AM4/19/01

to

"Chris Hayhurst" <chay...@mathworks.co.uk> writes:

> "for" expects a row vector not a column vector so if your

> variable current_list is a column then the behaviour is as you

> noticed. This behaviour can be useful - it allows you to loop

> over the columns of a matrix very easily e.g.

You say it can be useful, and perhaps you are right,

but does someone actually ever use this feature?

(In a "real" program, that is.)

Peter

--

for i=['jamh';'unaa';'sotc';'ttlk';' hae';' ebr';' r '];

i=i'; i=deblank(i); fprintf('%s ',i); end; fprintf('\n');

Apr 19, 2001, 11:33:39â€¯AM4/19/01

to

"Peter J. Acklam" wrote:

> "Chris Hayhurst" <chay...@mathworks.co.uk> writes:

>

> > "for" expects a row vector not a column vector so if your

> > variable current_list is a column then the behaviour is as you

> > noticed. This behaviour can be useful - it allows you to loop

> > over the columns of a matrix very easily e.g.

>

> You say it can be useful, and perhaps you are right,

> but does someone actually ever use this feature?

>

> (In a "real" program, that is.)

In a "real" program I wouldn't use a for loop :~)

On the other hand, in the unlikely occasion that you *had* to (God

forbid) loop over all the elements of a matrix, you can actually gain

from using this trick to first loop over the columns and then loop

within each column:

>> x=rand(1000);

>> tic,for n=1:1000,for m=1:1000,x(n,m);end,end,toc

elapsed_time =

12.4241

>> tic,for n=x,for m=n.',m;end,end,toc

elapsed_time =

3.1942

Notice that changing the order of looping in the first scenario, so that

you run down the columns instead, does not offer much improvement:

>> tic,for m=1:1000,for n=1:1000,x(n,m);end,end,toc

elapsed_time =

12.4097

Using a dummy variable to store each column of x (which at first sight

would seem like an equivalent trick) still offers negligible

improvement:

>> tic,for n=1:1000,y=x(:,n);for m=1:1000,y(m);end,end,toc

elapsed_time =

11.8738

/K (Just another hacklimizer)

Apr 19, 2001, 1:50:20â€¯PM4/19/01

to

Joe Skrap <joes...@hotmail.com> writes:

> In a "real" program I wouldn't use a for loop :~) On the other

> hand, in the unlikely occasion that you *had* to (God forbid)

> loop over all the elements of a matrix, [...]

Well, yes, but the whole point here is that we don't loop over

each element in a matrix, but loop over the columns, and do that

by letting the loop variable contain each column.

> [...] you can actually gain from using this trick to first loop

> over the columns and then loop within each column:

>

> >> x=rand(1000);

>

> >> tic,for n=1:1000,for m=1:1000,x(n,m);end,end,toc

Yes, it's always a good idea to loop over lower dimensions faster

than higher dimensions.

But why are you creating the vector 1:1000 over and over again? I

would create it only once:

tic;

v = 1:1000;

for n=v,

for m=v,

x(m,n);

end

end

toc;

Apr 19, 2001, 3:18:36â€¯PM4/19/01

to

"Peter J. Acklam" wrote:

You are missing the point. I intended to show the difference between

indexed vs. non-indexed access of the contents of x (the part that you

sniped out).

As for for acting columnwise, I guess it is consistent with the way

other vector functions act on matrices. I.e. if I want the largest of

bunch of numbers I use max(x). If however x is a matrix max acts

columnwise. In the same spirit if I want loop through a bunch of numbers

I use for n=x... If x is a matrix for acts columnwise. As for

applications: anywhere the columns of a matrix are (physical) vectors

use for v=x... instead of for n=1:100;v=x(:,n)...

/K

Apr 19, 2001, 5:30:27â€¯PM4/19/01

to

Joe Skrap <joes...@hotmail.com> writes:

> You are missing the point. I intended to show the difference

> between indexed vs. non-indexed access of the contents of x

> (the part that you sniped out).

*suitably humbled* :-)

You are right -- I missed your point. I didn't know about

this, so thanks!

Apr 19, 2001, 8:19:24â€¯PM4/19/01

to

In article <wk8zkxr...@math.uio.no>,

Peter J. Acklam <jac...@math.uio.no> wrote:

>"Chris Hayhurst" <chay...@mathworks.co.uk> writes:

>

>> "for" expects a row vector not a column vector so if your

>> variable current_list is a column then the behaviour is as you

>> noticed. This behaviour can be useful - it allows you to loop

>> over the columns of a matrix very easily e.g.

>

>You say it can be useful, and perhaps you are right,

>but does someone actually ever use this feature?

>

>(In a "real" program, that is.)

>

>Peter

Peter J. Acklam <jac...@math.uio.no> wrote:

>"Chris Hayhurst" <chay...@mathworks.co.uk> writes:

>

>> "for" expects a row vector not a column vector so if your

>> variable current_list is a column then the behaviour is as you

>> noticed. This behaviour can be useful - it allows you to loop

>> over the columns of a matrix very easily e.g.

>

>You say it can be useful, and perhaps you are right,

>but does someone actually ever use this feature?

>

>(In a "real" program, that is.)

>

>Peter

I will admit to:

a) "Inventing" this "clever" feature in the original MATLAB, and

b) Never using it for anything useful.

-- Cleve

mo...@mathworks.com

Apr 20, 2001, 6:04:37â€¯AM4/20/01

to

On 19 Apr 2001 20:19:24 -0400, mo...@mathworks.com (Cleve Moler)

wrote:

wrote:

I have a suspicion that any algorithm which can be implemented using

this 'clever' for-loop can always be vectorized, and probably without

any major programming effort.

Does anybody have a counter-example? Just curious,

Joris

-----

Joris Portegies Zwart

http://www.science.uva.nl/~portegie/

Apr 20, 2001, 7:28:44â€¯AM4/20/01

to

In article <wkr8yob...@math.uio.no>, jac...@math.uio.no says...

> Joe Skrap <joes...@hotmail.com> writes:

>

> > In a "real" program I wouldn't use a for loop :~) On the other

> > hand, in the unlikely occasion that you *had* to (God forbid)

> > loop over all the elements of a matrix, [...]

>

> Well, yes, but the whole point here is that we don't loop over

> each element in a matrix, but loop over the columns, and do that

> by letting the loop variable contain each column.

>

> > [...] you can actually gain from using this trick to first loop

> > over the columns and then loop within each column:

> >

> > >> x=rand(1000);

> >

> > >> tic,for n=1:1000,for m=1:1000,x(n,m);end,end,toc

>

> Yes, it's always a good idea to loop over lower dimensions faster

> than higher dimensions.

>

> But why are you creating the vector 1:1000 over and over again? I

> would create it only once:

>

> tic;

> v = 1:1000;

> for n=v,

> for m=v,

> x(m,n);

> end

> end

> toc;

> Joe Skrap <joes...@hotmail.com> writes:

>

> > In a "real" program I wouldn't use a for loop :~) On the other

> > hand, in the unlikely occasion that you *had* to (God forbid)

> > loop over all the elements of a matrix, [...]

>

> Well, yes, but the whole point here is that we don't loop over

> each element in a matrix, but loop over the columns, and do that

> by letting the loop variable contain each column.

>

> > [...] you can actually gain from using this trick to first loop

> > over the columns and then loop within each column:

> >

> > >> x=rand(1000);

> >

> > >> tic,for n=1:1000,for m=1:1000,x(n,m);end,end,toc

>

> Yes, it's always a good idea to loop over lower dimensions faster

> than higher dimensions.

>

> But why are you creating the vector 1:1000 over and over again? I

> would create it only once:

>

> tic;

> v = 1:1000;

> for n=v,

> for m=v,

> x(m,n);

> end

> end

> toc;

Though this was not Joe's point, one reason to NOT create the vector v

first is if it's very large. In for loops, the indices are created on

the fly when the : notation is seen, rather than first creating the whole

vector. That way, if you are doing something a squillion times, but

dumping results into a scalar, MATLAB will not unnecessarily run out of

memory on you.

--Loren

Apr 20, 2001, 8:16:16â€¯AM4/20/01

to

lo...@mathworks.com (Loren Shure) writes:

> [...] one reason to NOT create the vector v first is if it's

> very large. In for loops, the indices are created on the fly

> when the : notation is seen, rather than first creating the

> whole vector. That way, if you are doing something a squillion

> times, but dumping results into a scalar, MATLAB will not

> unnecessarily run out of memory on you.

Good point, indeed. I never noticed this feature until Matlab 6.

Is it new or has it been around for a while? (I know it was

introduced to Perl very recently.)

I think it's very useful. Certain loops may run for an unknown

number of time, and it's convenient to be able to say

for i = 1:Inf ... end

so one doesn't need to explicitly increment a loop counter.

Apr 20, 2001, 9:14:57â€¯AM4/20/01

to

Joris Portegies Zwart wrote:

> I have a suspicion that any algorithm which can be implemented using

> this 'clever' for-loop can always be vectorized, and probably without

> any major programming effort.

>

> Does anybody have a counter-example? Just curious,

>

> Joris

>

I would be interested to hear about your definition of vectorization. In

some sense a for loop using this feature would be vectorized since it

acts on vectors.

/K

Apr 20, 2001, 12:15:52â€¯PM4/20/01

to

On Fri, 20 Apr 2001 15:14:57 +0200, Joe Skrap <joes...@hotmail.com>

wrote:

wrote:

Good point. I would say that for me 'vectorized' simply means not

using any for loops. Or, equivalently, that instead of iterating over

the elements of a matrix, the operation is performed on the matrix as

a whole.

Apr 20, 2001, 12:39:37â€¯PM4/20/01

to

>Good point. I would say that for me 'vectorized' simply means not

>using any for loops. Or, equivalently, that instead of iterating over

>the elements of a matrix, the operation is performed on the matrix as

>a whole.

>using any for loops. Or, equivalently, that instead of iterating over

>the elements of a matrix, the operation is performed on the matrix as

>a whole.

I would guess there are degrees of vectorization. This would not be fully

vectorized.

Apr 20, 2001, 1:07:25â€¯PM4/20/01

to

Joris Portegies Zwart wrote:

I would tend to use the term vectorized as a loose synonymous to paralelized

i.e. a piece of code that N CPUs could equally distribute among themselves.

This definition does not cover all the interesting cases of hacklamization

but gives nonetheless the feeling that for loops should be avoided in favor

of functions that act on vectors

Apr 20, 2001, 2:31:37â€¯PM4/20/01

to

In article <wkelunw...@math.uio.no>, jac...@math.uio.no says...

>

> Good point, indeed. I never noticed this feature until Matlab 6.

> Is it new or has it been around for a while? (I know it was

> introduced to Perl very recently.)

>

> Good point, indeed. I never noticed this feature until Matlab 6.

> Is it new or has it been around for a while? (I know it was

> introduced to Perl very recently.)

I don't remember when we introduced this, but no later than some version

of MATLAB 5, I think, certainly before the current version.

--Loren

Apr 20, 2001, 4:20:39â€¯PM4/20/01

to

Joe Skrap <joes...@hotmail.com> writes:

> Joris Portegies Zwart wrote:

>

> > Good point. I would say that for me 'vectorized' simply means

> > not using any for loops. Or, equivalently, that instead of

> > iterating over the elements of a matrix, the operation is

> > performed on the matrix as a whole.

>

> I would tend to use the term vectorized as a loose synonymous to

> paralelized i.e. a piece of code that N CPUs could equally

> distribute among themselves.

That's the definition I use too. I know some people let

"vectorize" mean "remove any for-loop", but I think that

definition is too loose.

Apr 20, 2001, 4:20:46â€¯PM4/20/01

to

port...@science.uva.nl (Joris Portegies Zwart) writes:

> I have a suspicion that any algorithm which can be implemented

> using this 'clever' for-loop can always be vectorized, and

> probably without any major programming effort.

>

> Does anybody have a counter-example? Just curious,

Well, then you can start with vectorizing my signature. :-)

It is possible, and it doesn't require "major programming effort",

but it is ceratinly going to be more messy than it is now. What

makes it tricky is the fact that the words have different lengths.

Apr 23, 2001, 6:10:44â€¯AM4/23/01

to

Well,

I think I found a rather nice solution:

i =['jamh';'unaa';'sotc';'ttlk';' hae';' ebr';' r '];

fprintf([sscanf([i ; repmat(' ', 1, size(i, 2))], '%s%c') '\n']);

But it wasn't exactly as trivial as I thought it would be... :-)

Apart from that, this wasn't exactly the application I had in mind,

though I'm sure somebody could conjure up a similar but numerical

problem.

Regards,

Joris

Apr 23, 2001, 9:42:09â€¯AM4/23/01

to

jac...@math.uio.no (Peter J. Acklam) writes:

> "Chris Hayhurst" <chay...@mathworks.co.uk> writes:

>

> > "for" expects a row vector not a column vector so if your

> > variable current_list is a column then the behaviour is as you

> > noticed. This behaviour can be useful - it allows you to loop

> > over the columns of a matrix very easily e.g.

>

> You say it can be useful, and perhaps you are right,

> but does someone actually ever use this feature?

>

> (In a "real" program, that is.)

I think most of us would use the more transparent:

for i = 1:size(Y, 2)

x = Y(:, i);

% do something with x

end

instead of

for x = Y

% do stuff

end

Although I must admit, the second option looks quite appealing to the

vectorizer's eye.

-PB

Apr 23, 2001, 9:46:26â€¯AM4/23/01

to

jac...@math.uio.no (Peter J. Acklam) writes:

> lo...@mathworks.com (Loren Shure) writes:

>

> > [...] one reason to NOT create the vector v first is if it's

> > very large. In for loops, the indices are created on the fly

> > when the : notation is seen, rather than first creating the

> > whole vector. That way, if you are doing something a squillion

> > times, but dumping results into a scalar, MATLAB will not

> > unnecessarily run out of memory on you.

>

> Good point, indeed. I never noticed this feature until Matlab 6.

> Is it new or has it been around for a while? (I know it was

> introduced to Perl very recently.)

I never realized this... good features. Just another data point:

Octave has a "range" data type, which ALWAYS stores : notation vectors

in unexpanded form, until a function needs it to be expanded. Good

design IMO.

-PB

Apr 23, 2001, 10:38:58â€¯AM4/23/01

to

J Portegies Zwart <portegi...@fel.tno.nl> writes:

> "Peter J. Acklam" wrote:

> >

> > port...@science.uva.nl (Joris Portegies Zwart) writes:

> >

> > > I have a suspicion that any algorithm which can be

> > > implemented using this 'clever' for-loop can always be

> > > vectorized, and probably without any major programming

> > > effort.

> > >

> > > Does anybody have a counter-example? Just curious,

> >

> > Well, then you can start with vectorizing my signature. :-)

>

> [...] this wasn't exactly the application I had in mind, though

> I'm sure somebody could conjure up a similar but numerical

> problem.

Well, you said "any algorithm", and that pretty much

covers everything. :-)

Apr 23, 2001, 10:39:21â€¯AM4/23/01

to

Pete Boettcher <boet...@ll.mit.edu> writes:

> I think most of us would use the more transparent:

>

> for i = 1:size(Y, 2)

> x = Y(:, i);

> % do something with x

> end

>

> instead of

>

> for x = Y

> % do stuff

> end

Yep, but the last loop is actually equivalent to

for i = 1 : prod(size(Y))/size(Y,1)

x = Y(:,i);

...

end

which, in the 2D case, reduces the first loop above.

0 new messages

Search

Clear search

Close search

Google apps

Main menu