how to get variable of function in for loop
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Hello, someone please help me.
I created a polynomial function f(x,y) using two variables. I varied the values of both variables to get the maximum f(x,y) using for loop and i got it. But, i dont know how to get or display the values of the x and y that made it. can you tell me the syntax to get them? thanks!!
x = 1:20;
y = 1:13;
f = [];
for i = 1:length(x), j = 1:length(j);
f(i,j) = x(i).^2 - x(i) -2*y(j).^2 + y(j) -25;
end
fmax = max(max(f));
display(fmax)
2 个评论
Do you mean display x and y values only or f values ?
x = 1:20 % delete the semi colon
y = 1:13 %
disp(x)
disp(y)
Amron Rosyada
2024-5-29
编辑：Amron Rosyada
2024-5-29
回答（2 个）
Don't call max twice. Call max once with two output arguments and specify 'all' as the dimension over which to operate. That second output will be the linear index of the location where the maximum value returned as the first output was located. Then use ind2sub to get the row and column indices.
x = 1:20;
y = 1:13;
I've written your function as a function handle so I can call it from multiple places without retyping it.
fh = @(x, y) x.^2-x-2*y.^2+y-25;
Note that as originally written, this for loop doesn't do what you think it does. You can't have one for loop iterate over two arrays like this.
% f = [];
% for i = 1:length(x), j = 1:length(j);
% f(i,j) = fh(x(i), y(j));
% end
You need two for loops. Or you could use the vectorized approach others have suggested. But there's no need to grid the data explicitly as others have done; you can take advantage of the implicit expansion behavior of the basic operators in MATLAB like plus.
f = fh(x, y.')
[fmaxval, fmaxloc] = max(f, [], 'all')
What does that location of 248 correspond to in terms of x and y?
[row, col] = ind2sub(size(f), fmaxloc)
fprintf("Maximum of f is %d at x = %d, y = %d.\n", fmaxval, col, row)
Note that the index into x is the col index and the index into y is the row index. Let's check by evaluating the function once more at those specific values of x and y.
fh(x(col), y(row))
That matches the maximum value found by max (and by inspection of the displayed f array.)
x = 1:20;
y = 1:13;
f = [];
for i = 1:length(x),
for j = 1:length(y);
f(i,j) = x(i).^2 - x(i) -2*y(j).^2 + y(j) -25;
end
end
f
fmax = max(max(f));
display(fmax)
6 个评论
VBBV
2024-5-29
You can follow the for loop syntax given below
https://in.mathworks.com/help/matlab/ref/for.html
Amron Rosyada
2024-5-29
x = 1:20;
y = 1:13;
f = [];
for i = 1:length(x),
for j = 1:length(y);
f(i,j) = x(i).^2 - x(i) -2*y(j).^2 + y(j) -25;
end
end
disp(f)
fmax = max(max(f))
[r,c] = find(f==fmax)
display(x(r))
disp(y(c))
VBBV
2024-5-29
The x and y values are 20 and 1 that produce max value in f
+1
x = 1:20;
y = 1:13;
% Create x- and y-grid
[xGrid, yGrid] = meshgrid(x, y);
% Calculate f
f = xGrid.^2 - xGrid - 2*(yGrid.^2) + yGrid -25;
% Find max(f) and it's linear index
[fmax, idx] = max(f(:));
% Show the result
sprintf('max(f) = %d', fmax)
sprintf('(x,y) = (%d, %d)', xGrid(idx), yGrid(idx))
% Visualize f
figure
surf(xGrid, yGrid, f)
VBBV
2024-5-29
@akira Agata what does "+1' mean ?
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2024-5-29
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