multiply specific column of a matrix by specific element

Say I have a matrix [1 2 3; 4 5 6; 7 8 9]. I want to multiply only the 2nd column by 2 & get the result as [1 4 3; 4 10 6; 7 16 9]

Answer 1

[per isakson]

>> A = [1 2 3; 4 5 6; 7 8 9];
>> A(:,2) = A(:,2)*2
A =
     1     4     3
     4    10     6
     7    16     9
>>

And see Array Indexing

In response to comment

>> A .* [1,2,3]
ans =
     1     4     9
     4    10    18
     7    16    27     

And see Compatible Array Sizes for Basic Operations

[Debanjan Maity]

if i want to multiply the third column with another number say 3 alongwith the 2nd column multiplied by 2 and want to get result as

[1 4 9; 4 10 18; 7 16 27]

[per isakson]

See my answer.

[Rajat Maheshwari]

A = [1 2 3 4; 5 6 7 8; 9 10 11 12];
c = A.*[rats(1./sum(A,1))]

I want to Write one line expression that will multiply each column of A by a scalar so that, in the resulting matrix, every column sums to 1. this [rats(1./sum(A,1))] is giving the reciprocal 1*4 vector but when I am multiplying with A.* it is giving error

[per isakson]

One-liners are more difficult to debug. And more difficult to read and understand.

You need to read the documentation on rats() again, especially: "S = rats(X) returns a character vector ". Why do you try to use rats() in the first place?

%%
C = A.*(1./sum(A,1));
sum(C,1)

outputs

ans =
            1            1            1            1
>>

C = A./sum(A,1); is shorter and introduces less floating point errors