having problem to solve somthing in matlab

i have this question it says that i have a matrix A [m,n]

and i need to get the amount of the first row and the last row for example if i have a matrix like this :

7 5 3
9 1 8 
10 1 3 

i wanna get the first row ==> 7+5+3 =15

and the last row ==>10+1+3=14

and after that i need to do 15+14=29 ( as first row + the last row)

so the problem i have that i cant use sum function but i can use for

can anyone help me with this ?

Answer 1

[Jon]

If you think about how basic matrix, vector multiplication works, you can see that you can sum rows of a matrix by multiplying by a column vector of ones. You can sum selected elements of a column vector by premultplying by a row vector consisting of zeros and ones (with zeros for the elements that you don't want to include in the sum). Putting these ideas together in your case gives

y = [1 0 1]*A*[1;1;1]

You could also accomplish what you want using MATLAB's indexing abilities along with the sum function. Note that A(1,:) means row 1, every column of A. Similarly A(3,:) means row 3 every column of A

y = sum(A(1,:)) + sum(A(3,:))

[ays ays]

thank you for that but i gave an exmaple the matrix i work on i dont what number in each row

so i need help with that and i cant sum at all

i think there is no other way than loop if u have idea

thank you

[Jon]

I'm sorry, but I do not understand what you are trying to say in your comment. Please clarify what is the problem that you are trying to solve that you think you can only do using a loop.

[ays ays]

i am sorry i am kinda weak in english trying my best and i am solving exams to get better at matlab so the question is saying i have a matrix i dont know how many rows in it or columns and in the question it says also i cant use sum

so thats why i need other way if i could use sum i will have done long time ago

and again i am sorry too weak in english >_<

[Jon]

So if the problem is that you want the sum of the elements of the first and last rows of an arbitrary matrix A without using the sum function, you could generalize the approach I gave earlier using:

[m,n] = size(A)
y = [1 1]*[A(1,:);A(m,:)]*ones(n,1)

[ays ays]

thx

Answer 2

[Jan]

S = sum(x)

is equivalent to

S = 0;
for k = 1:numel(x)
    S = S + x(k);
end

If you want to, you can call this as a subfunction. Or you can collect the 2 sums in one loop:

Sfirst = 0;
Slast  = 0;
[s1, s2] = size(matrix);

Then run a loop over the columns of the matrix and accumulate the 2 sums.

[Jon]

I was thinking the OP was specifically looking for a solution that did not utilize either the sum function or looping, which is why I suggested the matrix multiply approach.