How can I find x values given a FOR command?

Question

Michael Sabol 2020-4-18

0
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回答： per isakson 2020-4-18

I need to find out what x values are created at certain intervals of t. t is in a matrix of 0:5:55;

clear;
close;
clc;
a = 40*(pi/180);
v = 1200;
g = 32.2;
k=[0,2e-6,10e-6,20e-6];
k=0;
dt=.5;
t = 0:dt:55
vx = v*cos(a)
vy = v*sin(a)
x = zeros(length(t),4);
y = zeros(length(t),4);
x(1,1)=0;
y(1,1)=0;
vx(1,1)=vx;
vy(1,1)=vy;
for j=2:length (t)
    x(j,1) = x(j-1,1)+vx(j-1,1)*dt-.5*k*v(j-1,1).^2*cos(a)*dt.^2
    y(j,1) = y(j-1,1)+vy(j-1,1)*dt-5*k*v(j-1,1).^2*sin(a)*dt^2-0.5*g*dt.^2
    vx(j,1) = vx(j-1,1)-k*v(j-1,1).^2*cos(a)*dt
    vy(j,1) = vy(j-1,1)-k*v(j-1,1).^2*sin(a)*dt-g*dt
    a=atan(vy(j,1)./vx(j,1))
    v(j,1)=vx(j,1).^2+vy(j,1).^2
end
find (x) = 0:5:55

Transferring what I say in my head "find the x values from the equation when t=0:5:55

to the code is what is irking me

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Stijn Haenen 2020-4-18

I'm not sure what you want, the result of your script is an matrix 'x' with values corresponding to t=0:0.5:55.

with the function find you can find the position at which a certain relation holds, for example find(x==0) results in position 1 (the first element of x is equal to 0)

Michael Sabol 2020-4-18

Stijn,

thanks

it seems that my request wasn't as clear as it could be?

What I want is multiple x values that correspond to the t values in the matrix 0:5:55

so perhaps i cant use the find command for doing that?

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Answer 1

per isakson 2020-4-18

1
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在 MATLAB Online 中打开

Replace

find (x) = 0:5:55

by

x(1:round(5/dt):end,:)

This should work since 5/dt is a whole number. round takes care of a possible floating point error. When 5/dt isn't a whole number you can use the function interp1()