Adding row 2 and row 3 and multiplying it by 3

A= [ 1 1 1 1 1 1 1 1 1 1 6 6 6 6 6 1 1 1 1 1 1 1 1 1 1 ]

[Adam Danz]

'A' only has 1 row.

These lines below add rows 2 and 3 and then multiplies the result by 3 but you haven't specified whether rows 2 and 3 should be added by row or by column.

sum(x([2,3],:)) .* 3
% or
sum(x([2,3],:),2) * 3

[Renesse Cammock]

row 2 and 3 should be added together by rows not columns

[Star Strider]

Please define row 1, row 2, and row 3.

[Renesse Cammock]

How do I get the answer that I got back into the matrix?

[Renesse Cammock]

[ 1 1 1 1 1; 1 1 1 1 1 ; 6 6 6 6 6 ; 1 1 1 1 1; 1 1 1 1 1]

[Adam Danz]

@Renesse Cammock, in response to the PM, let's continue the discussion here. Which solution worked for you? The first one proposed (my comment above) or the second one proposed (Star Strider's in the answer section, half of which is virtually the same as mine)? And how would you like the change the output?

[Renesse Cammock]

Both solutions works but none of them actually put it back in the matrix cause the question wanna a single matlab code which I never stated my apologies but how do I work it so that the answer is in the matrix form as in [ 1 1 1 1 1; 1 1 1 1 1; 6 6 6 6 6; 1 1 1 1 1; 1 1 1 1 1]

[Adam Danz]

You'll have to provide an example of an input and an expected output.

[Renesse Cammock]

Write a single matlab code to add row 2 and 3 and multiply the result by 3 Input A= [1 1 1 1 1; 1 1 1 1 1;1 1 1 1 1 l;1 1 1 1 1;1 1 1 1 1] Output A= [ 1 1 1 1 1;1 1 1 1 1; 6 6 6 6 6; 1 1 1 1 1;1 1 1 1 1]

[Adam Danz]

That's a little better but it's still not clear.

Row 2 + row 3 is merely [1 1 1 1 1] + [1 1 1 1 1] which equals [2 2 2 2 2] if we're adding each element together.

[2 2 2 2 2] * 3 is just [6 6 6 6 6].

The question remains, how would that fit into a matrix? In your output example it's in row 3 but I'm not sure why.

Answer 1

[Star Strider]

The semicolons make all the difference.

I’m not at all certain what you want to do. Try this:

A = [1 1 1 1 1; 1 1 1 1 1 ; 6 6 6 6 6 ; 1 1 1 1 1; 1 1 1 1 1]; 

R23x3 = 3*(A(2,:)+A(3,:));

Anew = [A(1,:); R23x3; A(4:end,:)];                                 % Replace A([2 3],:) wisth ‘R23x3’

Experiment to get the result you want.

[Renesse Cammock]

Thanks

[Star Strider]

My pleasure.

If my Answer helped you solve your problem, please Accept it!

Answer 2

[Adam Danz]

A= [
    1 1 1 1 1; 
    1 1 1 1 1;
    1 1 1 1 1;
    1 1 1 1 1;
    1 1 1 1 1];

A(3,:) = sum(A([2,3],:)) * 3; 
% ^ Not sure why this should go in row 3 but that matches your output example

Result

A =
     1     1     1     1     1
     1     1     1     1     1
     6     6     6     6     6
     1     1     1     1     1
     1     1     1     1     1

[Adam Danz]

"If u were multiplying first is there a specific function I should put at the front like how to add sum it at the front?"

A(3,:) = prod(A([2,3],:)) + 3; 
%        ^^^^

But 1 * 1 = 1 so I doubt that's what you want to do.

"How to write a single matlab code to multiply rows 1 and 2 by 2 then square the result" [from PM]

prod(A([1,2],:)).^2

But again, if you're dealing with all 1s, then the result will still be all 1s because 1*1 =1 and 1^2=1.

[Renesse Cammock]

It wouldn’t be 1 if u multiplying by 2 then squared

[Adam Danz]

Sorry, I missread. My previous comment multiplied row 1 by row 2. Instead, we're supposed to multiply row 1 & 2 by 2 (and then square it).

(A([1,2],:) *2).^2

while will give you [2 x 5] matrix of 4s since 1*2 = 2 and 2^2 = 4.

[Adam Danz]

BTW, I think this will all be clear to you if you spend a few minutes reading about indexing in Matlab.

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

Answer 3

[Renesse Cammock]

That’s how I saw it on the question paper but it works thanks ??

Answer 4

[Renesse Cammock]

If u were multiplying first is there a specific function I should put at the front like how to add sum it at the front?

[Adam Danz]

Please use the comment sections for discussion and the answer sections for answers. Otherwise the thread will become difficult to follow. The order of appearence of the answers is not fixed.