Save data from for loop without subscript index
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Mike Mierlo van
am 2 Aug. 2019
Kommentiert: Mike Mierlo van
am 2 Aug. 2019
I am programming a simulation of a falling object and I want to plot its trajectory. When plotting I experience the problem that the data is not stored.
I want to plot h as function of t without its data being overwritten.
I also want the steps in each iteration to be 0.1, so indexing will not work.
The standard solution for these plots is using 'indexing' h(t). This does not work because the subscript index 't' starts with '0' and has steps of 0.1.
My code:
t0 = 0; %start time is 0 sec.
dt = 0.1; % timestep is 0.1 sec.
t_stop = 10; % time stops at 10 sec if h=0 has not been reached.
h0= 30; %start height
g = 9.81; %gravitational constant.
for t = t0:dt:t_stop %starttime:timestep:stoptime
h= h0-0.5*g*(t)^2; %h as function of t
if h=<0 %stop simulation when h=<0
break
end
end
Question: how to plot 'h' as function of 't'?
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Akzeptierte Antwort
Guillaume
am 2 Aug. 2019
Bearbeitet: Guillaume
am 2 Aug. 2019
%... constant definitions
timesteps = t0:dt:t_stop;
h = zeros(size(timesteps)); %preallocate h
for step = 1:numel(timesteps)
t = timesteps(step);
h(step) = h0-0.5*g*t^2;
if h(step)<= 0
break;
end
end
Note that the whole thing can be done without a loop:
%... constant definitions
t = t0:dt:t_stop;
h = h0 - 0.5*g*t.^2; %note the .^ since t is a vector
3 Kommentare
Guillaume
am 2 Aug. 2019
The loop stops as soon as h is negative. You can then shorten the vectors if you so wish (otherwise it's just 0s)
%... constant definitions
timesteps = t0:dt:t_stop;
h = zeros(size(timesteps)); %preallocate h
for step = 1:numel(timesteps)
t = timesteps(step);
h(step) = h0-0.5*g*t^2;
if h(step)<= 0
break;
end
end
tokeep = h > 0;
plot(timesteps(tokeep), h(tokeep));
You can do the same with the vectorised method
%... constant definitions
t = t0:dt:t_stop;
h = h0 - 0.5*g*t.^2; %note the .^ since t is a vector
tokeep = h > 0;
plot(t(tokeep), h(tokeep));
%optional:
t = t(tokeep);
h = h(tokeep);
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