Problem with IF statement

why doesn't it give me a result if I enter " if D == l " I wan't to calculate all initial velocities and angles at which the length is 100m.

clear all
close all
clc
%1
v0 = [15:0.001:60]; %initial velocity
kot = [15:60]; %angle
g = 9.81; %gravity
l = 100; %distance

for i = 1:length(v0)
    for j = 1:length(kot)
        D = (v0(i)^2*sind(2*kot(j)))/g; %distance
        if D == l
            fprintf('Vo = %.2f m/s | kot = %.2f° | D = %.2f  \n',v0(i),kot(j), D);
        end
    end
end

Answer 1

[Rik]

Don't use clear all. You don't need it.

I suspect you encountered the wondrous world of floats. Computers don't have infinite memory and may round numbers in ways that you don't expect. This is especially a problem if you are asking for exact equality, while you would be satisfied with equality within machine precision.

Instead of a==b use abs(a-b)<tol. You can use the eps function, or set another value.

[Fe99]

This don’t help but thanks anyway.

[Rik]

Why not? Using the code below you can find how close D gets to 100. So if you pick a tolerance that is above that, your code will print.

v0 = 15:0.001:60; %initial velocity
kot = 15:60; %angle
g = 9.81; %gravity
l = 100; %distance
[i,j] = meshgrid(1:length(v0),1:length(kot));
D = (v0(i).^2.*sind(2*kot(j)))/g; %distance
smallest_diff=min(abs(D(:)-100))

smallest_diff = 4.0932e-05

%repeat your code, but with the edits I described
tol=5e-5;
for i = 1:length(v0)
    for j = 1:length(kot)
        D = (v0(i)^2*sind(2*kot(j)))/g; %distance
        if abs(D-l)<tol
            fprintf('Vo = %.2f m/s | kot = %.2f° | D = %.2f  \n',v0(i),kot(j), D);
        end
    end
end

Vo = 36.93 m/s | kot = 23.00° | D = 100.00  

[Fe99]

sorry my bad. thanks for answer

[Rik]

No problem. If my answer solved your question, please consider marking my answer as accepted answer. If not, feel free to comment with your remaining issues.