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From: matlab_learner on 11 Apr 2010 23:36
i am tryin to capture the value of a variable at dift points in time
(using an index to count) and matlab completely misses that argument
(doesn't return a value).

here is what i am trying to do:

-defined/initialized my variables
-start computations
-limit for which i am computing the value of velocity is from time 0 =
time 20 so i do while time <=20, code runs

-now, i need to make velocity plots of velocity vector u,v at dift
time levels. the change in time (delta_t) changes for each iteration
level so i have code that finds what delta_t should be...so
effectively, i know that i am running my code from time 0 to
20...follow so far?

so i was doing something like this to capture my velocity u,v at a
single time (t =0.0122, although i need to pick velocity at 8 dift
times i want to get 1 to work first):
-i first defined my counter W outside so W = 0;
then during the computations i did:
t = t+delta_t %increase time t
W = W+1; %increase the counter along with changes in delta_t
velocity_plots(W) = t;
if velocity_plots == ceil(16.7783) %16.7783 is the last time
point i need to store/grab velocity values
maxU = u
MaxV = v
end
--end of code....
then when iterations stop i do whos

but matlab never returns maxU or MaxV. there is no error but it is
missing it completely.

any suggestions? tnx.
From: TideMan on 12 Apr 2010 00:10
On Apr 12, 3:36 pm, matlab_learner <cib...(a)gmail.com> wrote:
> i am tryin to capture the value of a variable at dift points in time
> (using an index to count) and matlab completely misses that argument
> (doesn't return a value).
>
> here is what i am trying to do:
>
> -defined/initialized my variables
> -start computations
> -limit for which i am computing the value of velocity is from time 0 =
> time 20 so i do while time <=20, code runs
>
> -now, i need to make velocity plots of velocity vector u,v at dift
> time levels. the change in time (delta_t) changes for each iteration
> level so i have code that finds what delta_t should be...so
> effectively, i know that i am running my code from time 0 to
> 20...follow so far?
>
> so i was doing something like this to capture my velocity u,v at a
> single time (t =0.0122, although i need to pick velocity at 8 dift
> times i want to get 1 to work first):
> -i first defined my counter W outside so W = 0;
> then during the computations i did:
>    t = t+delta_t  %increase time t
>  W = W+1;  %increase the counter along with changes in delta_t
>         velocity_plots(W) = t;
>         if velocity_plots == ceil(16.7783)  %16.7783 is the last time
> point i need to store/grab velocity values
>             maxU = u
>             MaxV = v
>         end
> --end of code....
> then when iterations stop i do whos
>
> but matlab never returns maxU or MaxV. there is no error but it is
> missing it completely.
>
> any suggestions? tnx.

This:
> velocity_plots(W) = t;
tells me that velocity_plots is a vector.
But here:
> if velocity_plots == ceil(16.7783) %16.7783 is the last time
will only be true all the values in velocity_plots are identically
equal to ceil(16.7783), i.e., 17.

So, you've got it wrong on two counts in the if statement:
1. You need to use velocity_plots(W), not velocity_plots
2. You need to express the argument within a tolerance, something
like this:
if abs(velocity_plots - 16.7783) < 1e-5
From: TideMan on 12 Apr 2010 00:11
On Apr 12, 4:10 pm, TideMan <mul...(a)gmail.com> wrote:
> On Apr 12, 3:36 pm, matlab_learner <cib...(a)gmail.com> wrote:
>
>
>
> > i am tryin to capture the value of a variable at dift points in time
> > (using an index to count) and matlab completely misses that argument
> > (doesn't return a value).
>
> > here is what i am trying to do:
>
> > -defined/initialized my variables
> > -start computations
> > -limit for which i am computing the value of velocity is from time 0 =
> > time 20 so i do while time <=20, code runs
>
> > -now, i need to make velocity plots of velocity vector u,v at dift
> > time levels. the change in time (delta_t) changes for each iteration
> > level so i have code that finds what delta_t should be...so
> > effectively, i know that i am running my code from time 0 to
> > 20...follow so far?
>
> > so i was doing something like this to capture my velocity u,v at a
> > single time (t =0.0122, although i need to pick velocity at 8 dift
> > times i want to get 1 to work first):
> > -i first defined my counter W outside so W = 0;
> > then during the computations i did:
> >    t = t+delta_t  %increase time t
> >  W = W+1;  %increase the counter along with changes in delta_t
> >         velocity_plots(W) = t;
> >         if velocity_plots == ceil(16.7783)  %16.7783 is the last time
> > point i need to store/grab velocity values
> >             maxU = u
> >             MaxV = v
> >         end
> > --end of code....
> > then when iterations stop i do whos
>
> > but matlab never returns maxU or MaxV. there is no error but it is
> > missing it completely.
>
> > any suggestions? tnx.
>
> This:>         velocity_plots(W) = t;
>
> tells me that velocity_plots is a vector.
> But here:>         if velocity_plots == ceil(16.7783)  %16.7783 is the last time
>
> will only be true all the values in velocity_plots are identically
> equal to ceil(16.7783), i.e., 17.
>
> So, you've got it wrong on two counts in the if statement:
> 1.  You need to use velocity_plots(W), not velocity_plots
> 2.  You need to express the argument within a tolerance, something
> like this:
>           if abs(velocity_plots - 16.7783) < 1e-5

OOps, that should be:
> if abs(velocity_plots(W) - 16.7783) < 1e-5
From: Walter Roberson on 12 Apr 2010 00:16
matlab_learner wrote:

> velocity_plots(W) = t;

So velocity_plots is an array

> if velocity_plots == ceil(16.7783) %16.7783 is the last time

And you are comparing the entire array to a particular value. That will
evaluate to a vector of logical values. When 'if' is presented with a
vector of logical values, it considers the condition to be true only if
*all* of the logical values are true. Which will never be the case for
your code.

Try

if velocity_plots(W) == ceil(16.7783)

From: matlab_learner on 12 Apr 2010 12:34
thank you all.

i will attempt this tonight.

tnx...

i just want to grab the values at dift time values.
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