how to get variable of function in for loop (2024)

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Amron Rosyada about 21 hours ago

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Answered: Steven Lord about 11 hours ago

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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)

fmax = 354

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VBBV about 21 hours ago

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Do you mean display x and y values only or f values ?

x = 1:20 % delete the semi colon

x = 1x20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

<mw-icon class=""></mw-icon>

y = 1:13 %

y = 1x13

1 2 3 4 5 6 7 8 9 10 11 12 13

<mw-icon class=""></mw-icon>

disp(x)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

disp(y)

1 2 3 4 5 6 7 8 9 10 11 12 13

Amron Rosyada about 8 hours ago

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Edited: Amron Rosyada about 8 hours ago

you didnt get my question. i got the fmax. but at which variable of x and y that excactly subtituted in that f function. i mean, when (the value of x and y subtituted) fmax happens?

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Steven Lord about 2 hours ago

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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.')

f = 13x20

-26 -24 -20 -14 -6 4 16 30 46 64 84 106 130 156 184 214 246 280 316 354 -31 -29 -25 -19 -11 -1 11 25 41 59 79 101 125 151 179 209 241 275 311 349 -40 -38 -34 -28 -20 -10 2 16 32 50 70 92 116 142 170 200 232 266 302 340 -53 -51 -47 -41 -33 -23 -11 3 19 37 57 79 103 129 157 187 219 253 289 327 -70 -68 -64 -58 -50 -40 -28 -14 2 20 40 62 86 112 140 170 202 236 272 310 -91 -89 -85 -79 -71 -61 -49 -35 -19 -1 19 41 65 91 119 149 181 215 251 289 -116 -114 -110 -104 -96 -86 -74 -60 -44 -26 -6 16 40 66 94 124 156 190 226 264 -145 -143 -139 -133 -125 -115 -103 -89 -73 -55 -35 -13 11 37 65 95 127 161 197 235 -178 -176 -172 -166 -158 -148 -136 -122 -106 -88 -68 -46 -22 4 32 62 94 128 164 202 -215 -213 -209 -203 -195 -185 -173 -159 -143 -125 -105 -83 -59 -33 -5 25 57 91 127 165

<mw-icon class=""></mw-icon>

[fmaxval, fmaxloc] = max(f, [], 'all')

fmaxval = 354

fmaxloc = 248

What does that location of 248 correspond to in terms of x and y?

[row, col] = ind2sub(size(f), fmaxloc)

row = 1

col = 20

fprintf("Maximum of f is %d at x = %d, y = %d.\n", fmaxval, col, row)

Maximum of f is 354 at x = 20, y = 1.

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))

ans = 354

That matches the maximum value found by max (and by inspection of the displayed f array.)

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VBBV about 9 hours ago

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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

f = 20x13

<mw-icon class=""></mw-icon>

fmax = max(max(f));

display(fmax)

fmax = 354

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VBBV 1 minute ago

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You can follow the for loop syntax given below

https://in.mathworks.com/help/matlab/ref/for.html

Amron Rosyada about 21 hours ago

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thanks, but my question is how to get the variables ( x and y) that build the fmax?

VBBV about 21 hours ago

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Edited: VBBV about 8 hours ago

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Ran in:

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

disp(f)

-26 -31 -40 -53 -70 -91 -116 -145 -178 -215 -256 -301 -350 -24 -29 -38 -51 -68 -89 -114 -143 -176 -213 -254 -299 -348 -20 -25 -34 -47 -64 -85 -110 -139 -172 -209 -250 -295 -344 -14 -19 -28 -41 -58 -79 -104 -133 -166 -203 -244 -289 -338 -6 -11 -20 -33 -50 -71 -96 -125 -158 -195 -236 -281 -330 4 -1 -10 -23 -40 -61 -86 -115 -148 -185 -226 -271 -320 16 11 2 -11 -28 -49 -74 -103 -136 -173 -214 -259 -308 30 25 16 3 -14 -35 -60 -89 -122 -159 -200 -245 -294 46 41 32 19 2 -19 -44 -73 -106 -143 -184 -229 -278 64 59 50 37 20 -1 -26 -55 -88 -125 -166 -211 -260 84 79 70 57 40 19 -6 -35 -68 -105 -146 -191 -240 106 101 92 79 62 41 16 -13 -46 -83 -124 -169 -218 130 125 116 103 86 65 40 11 -22 -59 -100 -145 -194 156 151 142 129 112 91 66 37 4 -33 -74 -119 -168 184 179 170 157 140 119 94 65 32 -5 -46 -91 -140 214 209 200 187 170 149 124 95 62 25 -16 -61 -110 246 241 232 219 202 181 156 127 94 57 16 -29 -78 280 275 266 253 236 215 190 161 128 91 50 5 -44 316 311 302 289 272 251 226 197 164 127 86 41 -8 354 349 340 327 310 289 264 235 202 165 124 79 30

fmax = max(max(f))

fmax = 354

[r,c] = find(f==fmax)

r = 20

c = 1

display(x(r))

disp(y(c))

VBBV 1 minute ago

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The x and y values are 20 and 1 that produce max value in f

Akira Agata about 7 hours ago

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Ran in:

By using meshgrid, you can avoid for-loop, like:

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)

ans = 'max(f) = 354'

sprintf('(x,y) = (%d, %d)', xGrid(idx), yGrid(idx))

ans = '(x,y) = (20, 1)'

% Visualize f

figure

surf(xGrid, yGrid, f)

VBBV about 13 hours ago

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@akira Agata what does "+1' mean ?

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