is there a way to make matrices out of a bigger matrix with a changing condition without using a loop

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Kashish TANEJA
Kashish TANEJA 2022-6-1
回答： Rohit Kulkarni 2023-9-8
so i have the points of a field in a 2D matrix and a line is cutting that field , i want to take only points which are below the field . is there a way for me to put some condition when i make another matrix from the field such that i only consider the points below that field without using any loop ?
using a loop will make my code very heavy and hence i want to avoid that .

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回答（1 个）

Rohit Kulkarni
Rohit Kulkarni 2023-9-8
Hi Kashish,
I understand that you have a matrix and a line cut through it, and you would like to obtain the points which are below the line without using a for loop.
The points below the line can be filtered by using logical indexing and matrix operations.
The general approach is:
  • Define line equation.
  • Create logical mask and use it to identify points below the line equation.
The following code snippet shows an example of this:
% Define the parameters of the line equation: y = mx + b.
m = 0.5; % Slope of the line
b = 10; % Y-intercept of the line
% Create a 2D matrix representing your field (random data in this case).
% Replace this with your actual data.
fieldSize = 100;
fieldMatrix = rand(fieldSize, fieldSize) * 100;
% Create an array of x-coordinates corresponding to the columns of the matrix.
xCoordinates = 1:fieldSize;
% Create a matrix of y-coordinates based on the line equation.
yCoordinates = m * xCoordinates + b;
% Create a logical mask to identify points below the line.
mask = (1:fieldSize)' > yCoordinates;
% Extract the points below the field using the logical mask.
pointsBelowField = fieldMatrix;
pointsBelowField(mask) = NaN; % Set points above the line to NaN
% Create a matrix of values for the points above the line.
pointsAboveField = fieldMatrix;
pointsAboveField(~mask) = NaN; % Set points below the line to NaN
% Display both matrices
figure;
% Display the points below the line
subplot(1, 2, 1);
imagesc(pointsBelowField);
colormap('jet');
title('Points Below Line');
colorbar;
% Display the points above the line
subplot(1, 2, 2);
imagesc(pointsAboveField);
colormap('jet');
title('Points Above Line');
colorbar;
% Display both plots side by side
sgtitle('Points Above and Below Line');
Hope it resolves your query!

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