For loop not working - How to properly loop inconsistent iterations with matrices?

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cancel077
cancel077 2018-11-17
评论： cancel077 2018-11-18
Good day,
I am trying to loop and plot the results, but it is not working. The first iteration I made it manual because it is more of an 'initialization', and the code is not consistent as the 2,3rd, etc. iteration. When I run everything manually (no loop), it works, but when I start playing with the for loop, it gives me the following first error, which I cannot get rid of or do not know how:
In an assignment A(:) = B, the number of elements in A and B must be the same.
Error in PartA_loop (line 48)
a(i+1) = (P(i+1)*area)./m;
Below is my full code, and it appears to fail after at the 2nd iteration iteration (i=2): Please note the commented if statement area, is failing for me as well. I am not too worried about that, I would like to get the code working, and if the "if" statement works as well it's a bonus!
I know it has to do with matrices, but I think I labeled everything correctly, so it should be pulling the correct numbers.
clear all
close all
clc
% constants
d=0.015; % value is in m, or 15mm
area = (1/4)*pi*(0.015)^2;
gamma = 1.66;
m = 0.002; %in kg
time_step = 0.00003; %intervals of 0.0003
% initial conditions
Po = 3.4474E+7; % Pa
c(1) = 1400; % m/s,
x(1) = 0.508; %in meters]
V0 = pi()*((d/2)^2).*x(1);
Pv_gamma0 = Po*V0^gamma;
t(1) = x(1)./c(1);
theta(1) = atand(t(1)./x(1));
a(1) = (Po*area)/m;
u0_0 = 0; %velocity before piston started to move is 0
u(1) = u0_0 + a(1)*time_step;
hyp(1) = -sqrt((t(1)^2)+(x(1)^2));
m(1) = -(t(1)./x(1));
p1 = [x(1),0];
p2 = [0,t(1)];
figure
plot([p1(1) p2(1)],[p1(2) p2(2)])
hold on
for i=1:5
disp(i) = u(i).*time_step;
x(i+1) = x(i) + disp(i);
V(i+1) = pi().*((d/2)^2).*(x(i+1));
P(i+1) = Pv_gamma0./(V(i)^gamma);
c(i+1) = u(i) - c(i);
% if c(i+1) < 0
% c(i+1) = abs(c(i+1));
% disp('CHECK OK: NEGATIVE VALUE')
% elseif c(i+1) > 0
% disp('ERROR CHECK: POSITIVE VALUE')
% end
t(i+1) = x(i+1)./c(i+1);
theta(i+1) = atand(t(i+1)./x(i+1));
a(i+1) = (P(i+1)*area)./m;
u(i+1) = u(i) + a(i+1).*time_step;
hyp(i+1) = -sqrt(((t(i+1))^2)+((x(i+1))^2));
m(i+1) = -(t(i+1))./(x(i+1));
p1 = [x(i),time_step.*i];
p2 = [0,t(i)];
plot([p1(1) p2(1)],[p1(2) p2(2)])
hold on
array1((i+1),1:2) = p1';
end
x
u
m
c
theta
hyp
I also attached the figure on how it should look like when I run the manual code instead.
Thank you in advance, I hope someone can assist!

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采纳的回答

Image Analyst
Image Analyst 2018-11-17
In the line
a(i+1) = (P(i+1)*area)./m;
m is an array after the first iteration. Do you mean
a(i+1) = (P(i+1)*area)./m(i);
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Image Analyst
Image Analyst 2018-11-18
It works fine for me. See:
>> disp('CHECK OK: NEGATIVE VALUE')
CHECK OK: NEGATIVE VALUE
>>
I don't even see what index it's talking about on that line. There is no index at all. You don't have any variable called "disp" do you? What does this say?
which -all disp
cancel077
cancel077 2018-11-18
Yeah I do have other variables called "disp", that was the issue. Just another simple mistake I did not see!
Thank you appreciate it! Just gotta figure it out now why it's failing on the 3rd iteration now.
Cheers!

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