For loop not working as desired - matlab

Question

I am calculating the distance and velocity between GPS coordinates (lat,long,height) and it appears to work well when I used single values. However, I have a matrix of 79 GPS coordinates (3x79 matrix) and I want to find the distance and speed between each two consecutive points. When I try to use a for loop the output I get is all zeros apart from the first and last value (which are very high).
I am probably doing something silly but I can spot it...any suggestions are appreciated :)
for k=1:77
R=6378.1e3;
latDistance = copter_llh(1,k+1) - copter_llh(1,k);
lonDistance = copter_llh(2,k+1) - copter_llh(2,k);
a = sin(latDistance / 2) * sin(latDistance / 2) + cos(copter_llh(1,k))...
*cos(copter_llh(1,k+1)) * sin(lonDistance / 2) * sin(lonDistance / 2);
c = 2 *atan2(sqrt(a), sqrt(1 - a));
distance = R * c * 1000; % convert to meters
height = copter_llh(3,k+1) - copter_llh(3,k);
distance = sqrt((distance^ 2) + (height^2));
velocity = distance/0.1*60;
% stepsize =0.1min ___speed in m/s
distance(:,k)=(distance);
velocity(:,k)=(velocity);
end %-----

Answer 1

It looks like you are reusing your distance variable in an unintentional way, as well as your velocity variable at the end of your for loop. You're mutating the distance variable, and then trying to reshape it as a matrix. You'll need to change this variable, and then call where you want to store your matrix as something else. Also, distance and velocity look like they're single arrays, and so the : accessing for the first dimension is superfluous. As such, try doing this instead:
distance = zeros(1,77); %// NEW - Preallocate distance array
velocity = zeros(1,77); %// NEW - Preallocate velocity array
R=6378.1e3; %// NEW - Leave outside for loop. Constant at each iteration
for k=1:77
latDistance = copter_llh(1,k+1) - copter_llh(1,k);
lonDistance = copter_llh(2,k+1) - copter_llh(2,k);
a = sin(latDistance / 2) * sin(latDistance / 2) + cos(copter_llh(1,k))...
*cos(copter_llh(1,k+1)) * sin(lonDistance / 2) * sin(lonDistance / 2);
c = 2 *atan2(sqrt(a), sqrt(1 - a));
dist = R * c * 1000; %// convert to meters - NEW - change variable
height = copter_llh(3,k+1) - copter_llh(3,k);
%// stepsize =0.1min ___speed in m/s
distance(k) = sqrt((dist^ 2) + (height^2)); %// NEW - Assign directly to distance array
velocity(k) = distance/0.1*60; %// NEW - Assign directly to velocity array
end %-----
This code should now store 77 distances and velocities stored in distance and velocity respectively. Try that and let me know if it works!
Sidenote
You can compute this in a completely vectorized way without any for loops. You can use diff to compute those neighbouring distances for you. As such, you can actually do something like this:
R=6378.1e3;
latDistance = diff(copter_llh(1,:)); % // NEW
lonDistance = diff(copter_llh(2,:)); %// NEW
a = sin(latDistance / 2) .* sin(latDistance / 2) + cos(copter_llh(1,1:end-1))...
.*cos(copter_llh(1,2:end)) .* sin(lonDistance / 2) .* sin(lonDistance / 2);
c = 2 *atan2(sqrt(a), sqrt(1 - a));
dist = R * c * 1000; %// convert to meters - NEW - change variable
height = diff(copter_llh(3,:));
distance = sqrt((dist.^2) + (height.^2)); %// NEW
velocity = distance/0.1*60; %// NEW
The above code should be equivalent to your for loop method, but remember to always write code that makes sense to you first and works before you start doing any optimizations. That's the best way to code!