how to multiply only certain columns in a matrix based on a condition in matlab

One way to multiply certain columns in a matrix based on a condition in Matlab is by using logical indexing. Here is an example:

Suppose you have a matrix A and want to multiply only columns whose column sums are greater than a certain threshold threshold:

main.m
A = [1 2 3; 4 5 6; 7 8 9];
threshold = 12;

% Find column sums
colsums = sum(A);

% Find columns that meet condition
columns_to_multiply = colsums > threshold;

% Multiply selected columns by a scalar value
A(:,columns_to_multiply) = A(:,columns_to_multiply) * 2;
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In this example, the matrix A is first defined as a 3x3 matrix. The sum function is used to find the column sums of A, which are stored in the colsums variable. Next, a logical array named columns_to_multiply is created by checking which elements in colsums are greater than the threshold. Finally, the * operator is used to multiply the selected columns of A by a scalar value of 2. Note that the : operator is used to select all rows of A.

This approach is flexible and can be extended to handle more complex conditions or operations.