MATLAB Central Newsreader - Properly vectorizing code when using "if"

Properly vectorizing code when using "if"

Johan — Sat, 11 Oct 2008 22:46:02 -0400

Is it possible to "vectorize" an if statement?

For example, in the example below, would like to use y = x^2 when x is less than 0 and y = x^3 when x is greater than zero. Both of these functions are positive in their respective domains. However, the implementation below doesn't work as intended.

Any recommendations?
Thanks.

x = -1:0.01:1;
r = ifTest(x);
plot(x,r);

function[ifTest] = ifTest(x)

if (x < 0 )
r = x .* x;
else
r = x .* x .* x;
end
ifTest = r;

Re: Properly vectorizing code when using "if"

Donn Shull — Sat, 11 Oct 2008 23:48:02 -0400

one solution is to use logical operators:

r = ((x>0).*x.*x) + ((x<=0).*x.*x.*x);

Hope this helps,

Donn

"Johan " <robert.mchugh@ips.invensys.com> wrote in message <gcrab9$kns$1@fred.mathworks.com>...
> Is it possible to "vectorize" an if statement?
>
> For example, in the example below, would like to use y = x^2 when x is less than 0 and y = x^3 when x is greater than zero. Both of these functions are positive in their respective domains. However, the implementation below doesn't work as intended.
>
> Any recommendations?
> Thanks.
>
> x = -1:0.01:1;
> r = ifTest(x);
> plot(x,r);
>
> function[ifTest] = ifTest(x)
>
> if (x < 0 )
> r = x .* x;
> else
> r = x .* x .* x;
> end
> ifTest = r;

Re: Properly vectorizing code when using "if"

Lucio Andrade-Cetto — Sun, 12 Oct 2008 00:54:02 -0400

Another a little more elegant

r = x .^ (2+(x<=0));

Lucio

"Donn Shull" <donn.shull.no_spam@aetoolbox.com> wrote in message <gcrdvi$m08$1@fred.mathworks.com>...
> one solution is to use logical operators:
>
> r = ((x>0).*x.*x) + ((x<=0).*x.*x.*x);
>
> Hope this helps,
>
> Donn
>
> "Johan " <robert.mchugh@ips.invensys.com> wrote in message <gcrab9$kns$1@fred.mathworks.com>...
> > Is it possible to "vectorize" an if statement?
> >
> > For example, in the example below, would like to use y = x^2 when x is less than 0 and y = x^3 when x is greater than zero. Both of these functions are positive in their respective domains. However, the implementation below doesn't work as intended.
> >
> > Any recommendations?
> > Thanks.
> >
> > x = -1:0.01:1;
> > r = ifTest(x);
> > plot(x,r);
> >
> > function[ifTest] = ifTest(x)
> >
> > if (x < 0 )
> > r = x .* x;
> > else
> > r = x .* x .* x;
> > end
> > ifTest = r;

Re: Properly vectorizing code when using "if"

Matt Fig — Sun, 12 Oct 2008 00:54:03 -0400

"Johan " <robert.mchugh@ips.invensys.com> wrote in message <gcrab9$kns$1@fred.mathworks.com>...
Both of these functions are positive in their respective domains. However, the implementation below doesn't work as intended.
> x = -1:0.01:1;
> r = ifTest(x);
> plot(x,r);
>

function[x] = ifTest(x)

tmp = x<0;
x(tmp) = x(tmp).^2;
x(~tmp) = x(~tmp).^3

Re: Properly vectorizing code when using "if"

Matt — Sun, 12 Oct 2008 12:59:02 -0400

> if (x < 0 )
> r = x .* x;
> else
> r = x .* x .* x;
> end

Just to explain what went wrong, when you give a vector argument to "if", it tests to see if the real part of all components are non-zero. Otherwise the "else" statements are executed.

So the above code evaluates the "else" statements (on the entire vector x) whenever even one x(i)>0