Input with a count

This is what I have so far

while (nargin==0)
    numpts = input('Please enter in the number of points:');
    fprintf('Please enter in points in [x y] form.')
     for count=1:numpts
        xy_points = input('Enter [x y] pair for point %g:', count);
     end
x_points = xy_points(:,1);
y_points = xy_points(:,2);
end

This is the error

Error using input
The second argument to INPUT must be 's'.

Error in Untitled (line 30)
        xy_points = input('Enter [x y] pair for point %g:', count);

and these are the directions for the assignment Write a MATLAB function (not script) that performs a linear regression fit on a set of data. The function should be able to handle two cases:

1. Two input arguments (a vector of x points and a vector of y points), or
2. Zero input arguments, the program should first:
a. Ask the user to input the number of points they will enter
b. Enter (x,y) points one-by-one

The output should return and display the straight-line slope, intercept, and number of points that were used. The program should also graph the data points as discrete (not connected by a line) circles, and on the same axis the straight-fit line ranging across the x values.

[Hikaru]

I think you have to take a look at this part.

    for count=1:numpts
       xy_points = input('Enter [x y] pair for point %g:', count);
    end

You're basically assigning new value to xy_points in each iteration, which is not what you want. Why don't you try making xy_points into a matrix of 2 columns first?

xy_points = zeros(numpts,2); % this is called pre-allocating

Now, can you write the code to tell MATLAB to assign the user input into the variable xy_points? Hint: This is where you could use your count variable.

Answer 1

[Star Strider]

See the documentation for input. (I prefer to use the inputdlg function because it avoids Command Window pollution.)

[Corey K]

I'm still unclear as to how to fix this error. I changed it to this xy_points = input('Enter [x y] pair for point %s:', 'count'); and I'm still getting this error Error using input The second argument to INPUT must be 's'.

Error in linear_regression1 (line 32) xy_points = input('Enter [x y] pair for point %s:', 'count');

I am a pretty basic matlab user, sorry for my limited knowledge.

[Star Strider]

Try this:

xy_points = input('Enter [x y] pair for point %g: ');

No worries! We were all mystified at first, too.

[c k]

I wanted it to say for point 1, 2, 3...numpts, doesn't it need to have counter?

[Star Strider]

I used inputdlg here to test my code snippet (it works), but you would do essentially the same using input:

for count = 1:numpts
    xy_points(count,:) = inputdlg('Enter [x y] pair for point %g: ');
end

[c k]

I just gave up on trying to put the %g part.

This is what I have in my code now and it will show in the command window to enter the number of points, then will ask to enter a point that many times, but from then on it just repeats itself, as if by terms of the while loop it still has nargen==0. Any insight into this?

function[fit_slope, fit_intercept, Npoints] = linear_regression(x_points,y_points)

...

while (nargin==0)
    numpts = input('Please enter in the number of points:');
    fprintf('Please enter in points in [x y] form.')
     for count=1:numpts
        xy_points = input('Enter [x y] pair for point');
        x_points = xy_points(:,1);
        y_points = xy_points(:,2);
     end
end
% % plot x versus y
% plot(x_points,y_points);
% % create matrix
% M=[numpts,sum(x_points);sum(x_points),sum(x_points.^2)];
% %b is y-intercept and n is the slope
% bn=inv(M)*[sum(y_points);sum(x_points.*y_points)]
% %display line equation
% disp('f(x)=nx+b')

[Star Strider]

Forget the while loop. It isn’t necessary. I would simply test for ‘nargin == 0’, and return if true (perhaps first printing an error message to the Command Window).

The problem statement is confusing as I read it. My impression is that your script will ask for the user to enter the ‘x_points’ and ‘y_points’ vectors from the input (or inputdlg) assignments, then passes them to your regression function that then returns the required output. Your script (not the function) would then take the function output and do the plot. You might want to clarify that. That is how I understand the assignment, anyway.

I would also use the backslant (\) operator to do the regression. For example:

x_points = [11:3:39]';          % Column Vector
y_points = [1:10]';             % Column Vector

B = [ones(size(x_points)) x_points]\y_points;
fit_intercept = B(1);
fit_slope     = B(2);

Your ‘x_points’ and ‘y_points’ vectors are already column vectors as you wrote your code, so you can use them as they are in this code snippet.

Also, the way I read the problem statement, it seems to want you to get ‘x_points’ and ‘y_points’ before you call your regression function, not from within it, since you are supposed to pass them to it.