Save data from for loop without subscript index

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Mike Mierlo van
Mike Mierlo van 2019-8-2
评论： Mike Mierlo van 2019-8-2
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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采纳的回答

Guillaume
Guillaume 2019-8-2
编辑：Guillaume 2019-8-2
%... 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
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Guillaume
Guillaume 2019-8-2
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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