How to extract a specific value from a loop.

2018 8 31
2 个回答
回答已采纳
更新时间：2018 9 3
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Hello
I wrote this code to model a simple projectile motion problem. What I would like to do now is output the value for x after 2 seconds but I don't know how to pull just one value out of a for loop.
%Projectile motion
v0= 10;
a= 45;
h= .04;
a=a*pi/180;
g=9.8; %acceleration due to gravity in m/s^2
xmax=v0^2*sin(2*a)/g;
ymax=v0^2*sin(a)^2/(2*g);
td=2*v0*sin(a)/g; %total time
x1=0;
y1=0;
figure('color','white');
for t=0:h:td+h
plot(x1,y1,'bo')
hold all
xlim([0 1.1*xmax]);
ylim([0 1.1*ymax]);
title('Projectile Motion');
xlabel('Distance in meter');
ylabel('Height in meter');
getframe;
x1=x1+h*v0*cos(a); %Euler's method for x
y1=y1+h*(v0*sin(a)-g*t);%Euler's method for y
% x1(2) % this is where I'm having the problem
end

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 采纳的回答

M
M 2018-8-31

0 个投票

t is your time variable, so you have to use it to test if it's equal to 2seconds.
if t==2 % assumes your time units is seconds
x_2sec=x1;
end
but in your code, t will never reach 2 seconds.

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Westin Messer
Westin Messer 2018-8-31
%Projectile motion
v0= 10;
a= 45;
h= .04;
a=a*pi/180;
g=9.8; %acceleration due to gravity in m/s^2
xmax=v0^2*sin(2*a)/g;
ymax=v0^2*sin(a)^2/(2*g);
td=2*v0*sin(a)/g; %total time
x1=0;
y1=0;
figure('color','white');
for t=0:h:td+h
plot(x1,y1,'bo')
hold all
xlim([0 1.1*xmax]);
ylim([0 1.1*ymax]);
title('Projectile Motion');
xlabel('Distance in meter');
ylabel('Height in meter');
getframe;
x1=x1+h*v0*cos(a); %Euler's method for x
y1=y1+h*(v0*sin(a)-g*t);%Euler's method for y
% x1(2) % this is where I'm having the problem
if t==1.3 % assumes your time units is seconds
x_2sec=x1
end
end
When I do that nothing outputs. Matlab doesn't seem to recognize that if statement. Am I using it wrong?
GK
GK 2018-9-3
Hi, if statement is never true in your code. You can check that by adding the else statement just after that. Shown in below code:
%Projectile motion
v0= 10;
a= 45;
h= .04;
a=a*pi/180;
g=9.8; %acceleration due to gravity in m/s^2
xmax=v0^2*sin(2*a)/g;
ymax=v0^2*sin(a)^2/(2*g);
td=2*v0*sin(a)/g; %total time
x1=0;
y1=0;
figure('color','white');
for t=0:h:td+h
plot(x1,y1,'bo')
hold all
xlim([0 1.1*xmax]);
ylim([0 1.1*ymax]);
title('Projectile Motion');
xlabel('Distance in meter');
ylabel('Height in meter');
getframe;
x1=x1+h*v0*cos(a); %Euler's method for x
y1=y1+h*(v0*sin(a)-g*t);%Euler's method for y
% x1(2) % this is where I'm having the problem
if t==1.3 % assumes your time units is seconds
x_2sec=x1
else % adding else statement tell us that, it is not if is never true
x_2sec=0
end
end
M
M 2018-9-3
When I do that nothing outputs. Matlab doesn't seem to recognize that if statement. Am I using it wrong?
Your time step is equal to 0.4, so your time variable will be succesively equal to t=1.28 and 1.32, and your condition will never be satisfied.
Dennis
Dennis 2018-9-3
To check if t==1.32 would not work either. Due to floating point limitations t will not be exactly 1.32.

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更多回答（1 个）

GK
GK 2018-8-31

0 个投票

Yes you can. 1. If you remove semicolon at the end of expression, MATLAB simply shows it at the command line 2. Alternately, you can also print it in excel file using- xlswrite('x1values.xlsx', x1);

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