Size mismatch error when trying to find a row in an array that is the same as a variable vector.

idx = find(all(waypoints == target, 2), 1);

So this is a line of code in a function of a simulink function block.

The array waypoints is just an array with two columns and any amount of rows.

I'm trying to find the index at which the [2 x 1] vector target is equal to a row within the waypoints array.

But when the simulation is run, it tells me there is a size mismatch where I first try to define idx. Surely they don't need to match as the purpose is just to find where one of waypoints rows are equal to target?

function targetnew = setwaypoint(d2w,target,accuracy)

    waypoints = [100 150; 200 400; 175 275; 600 300]; % pre-defined waypoints (in the future imported as a large array from an external source.
    
    if isempty(target)
        target = waypoints(1,:); % initializes the first waypoint.
    end
    
    idx = find(all(waypoints == target, 2), 1);  % find the first row that match
    
    if isempty(idx) % no row found!
        error('could not find target in the waypoints');
    end
    
    if d2w > accuracy
        idx = idx + 1; % if target waypoint is reached, add one to index.
    end
    
    if idx > size(waypoints, 1) % attempt to avoid any issues if simulation time is greater than time taken to reach final waypoint.
    targetnew = waypoints(end, :);
    else
    targetnew = waypoints(idx, :);
    end 
    
end

[Walter Roberson]

Have you considered ismembertol() with the 'rows' option ?

[Sam Elliott]

I never knew about this, but using ismember with 'rows' worked, thank you. I'm still having issues but they're unrelated to this specific issue now so I'll post it in a new question.

Answer 1

[Fangjun Jiang]

The problem is here.

If target is 1x2 and waypoints is nx2, you can't do waypoints==target

waypoints = [100 150; 200 400; 175 275; 600 300];
target=[]
if isempty(target)
   target = waypoints(1,:); % initializes the first waypoint.
end
waypoints==target

[Sam Elliott]

How can I make matlab pick out the index of the array waypoints where the row is equal to the vector target then?

[Fangjun Jiang]

It looks like you want to match a 1x2 target to a row of a nx2 waypoints. Assume the floating point equality comparison is not an issue, you can try this with the example above.

idx = find(~all(waypoints-target, 2), 1)

[Sam Elliott]

I get the same error.

[Fangjun Jiang]

If you are using R2018b as indicated, you should have no error running this:

waypoints = [100 150; 200 400; 175 275; 600 300];
target=[200 400];
idx = find(~all(waypoints-target, 2), 1)
idx =
     2