Save data from for loop without subscript index

Question

Mike Mierlo van 2019-8-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/474588-save-data-from-for-loop-without-subscript-index

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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Answer 1

Guillaume 2019-8-2

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https://ww2.mathworks.cn/matlabcentral/answers/474588-save-data-from-for-loop-without-subscript-index#answer_385940

编辑：Guillaume 2019-8-2

在 MATLAB Online 中打开

%... 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 2019-8-2

在 MATLAB Online 中打开

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);