Why do I get Array indices must be positive integers or logical values?

What am I doing wrong? I seem to get the error what ever I try. Can someone help, please :)

alfa = pi/6;

v0 = 300;

[h,dx] = flyrange(v0,alfa);

function [h,dx] = flyrange(v0,alfa)

g = 9.81;

a = v0.*cos(alfa);%a = v0x

b = v0.*sin(alfa);%b = v0y

tn = b./g;

tl = 2.*tn;

t = [0:tl];

h = (-1/2).*g.*tn.^(2) + b.*tn;

dx = a.*tl;

for n = [1:tl];

x(n) = a.*t;

y(n) = (-1/2).*g.*t.^(2) + b.*t;

end

plot(x(t),y(t))

end

Answer 1

[Steven Lord]

t = [0:tl];
%snip a bunch of code, none of which changes t
plot(x(t),y(t))

There's no such thing as element 0 of an array in MATLAB. The first element is element 1.

[Elina Salo]

But what do I do when I need my t to have the values [0 tl]?

[Steven Lord]

You mean something like this?

t = 0:360;
% x and y are functions of t
x = cosd(t);
y = sind(t);
plot(x, y) % no need to specify x(t) or y(t)
axis square % make the axes square rather than rectangular
hold on % let me plot something else on this same axes
plot(x(t == 0), y(t == 0), 'rx') % x marks the starting spot

[Elina Salo]

My code looks like this now:

alfa = pi/6;

v0 = 300;

[h,dx] = flyrange(v0,alfa);

function [h,dx] = flyrange(v0,alfa)

g = 9.81;

% ax = 0;

% ay = g;

a = v0.*cos(alfa);%a = v0x

b = v0.*sin(alfa);%b = v0y

tn = b./g;

tl = 2.*tn;

t = [0:tl];

h = (-1/2).*g.*tn.^(2) + b.*tn;

dx = a.*tl;

for n = [1:t]

t = [0:tl]

x(n) = a.*t;

y(n) = (-1/2).*g.*t.^(2) + b.*t;

end

plot(x,y)

end

And now Matlab has this error:

Unrecognized function or variable 'x'.

Error in Matlab>flyrange (line 26)

plot(x,y)

Error in Matlab (line 8)

[h,dx] = flyrange(v0,alfa);

[Steven Lord]

Here's a piece of your code. I've commented it out so when I run this answer it doesn't try to run this code.

%{
t = [0:tl];
h = (-1/2).*g.*tn.^(2) + b.*tn;
dx = a.*tl;
    for n = [1:t]
        t = [0:tl]
        x(n) = a.*t;
        y(n) = (-1/2).*g.*t.^(2) + b.*t;
    end
plot(x,y)
%}

Why didn't this work? When you use : and one or more of the operands has more than one element MATLAB will only use the first element. So your loop over n where you're iterating over [1:t] will iterate over the vector 1:t(1) which is 1:0. That vector is empty, so x is never assigned a value.

But you don't need to use a loop here since x and y only depend on a, t, g, and b. Use a vectorized calculation.

% Use sample values for a, b, and g
g = 9.8;
a = 3;
b = 4;

% Also use a sample vector of values for t
t = 0:5;

% Compute x and y for ALL the values of t at once
x = a.*t;
y = (-1/2).*g.*t.^(2) + b.*t;
plot(x, y)

Search the documentation for the term "vectorization" for more info.

[Elina Salo]

Thank you for your help! I get it know what I did wrong and I got the code go thru without any problems! :)