Newbie help for matlab

how do i write a function that has two 3x3 input matrices and in output it gives new matrix that contains the rows of the input that has the least sum .I.e. a=[1 2 3;2 3 4; 3 4 5] and b=[1 2 3;2 3 4; 3 4 5] then print c=[1 2 3;1 2 3] . I have the logic in my head but i can't code it in matlab since i have only 1 lesson on it. Thanks

Answer 1

[Star Strider]

See if this does what you want:

a=[1 2 3;2 3 4; 3 4 5];
b=[1 2 3;2 3 4; 3 4 5];

sa = sum(a,2);
sb = sum(b,2);

mab = [a; b];
sab = [sa; sb];
[srtab,ix] = sort(sab,'ascend');
c = mab(ix(1:2),:)

producing:

c =

       1     2     3
       1     2     3

[Image Analyst]

He added a requirement that it must have the min row in a, and the min row in b. So your code would not work in this case:

a=[1, 2, 3; 1, 2, 3; 3, 4, 5];
b=[2, 3, 4; 2, 3, 4; 3, 4, 5];

since it will take both of the 1,2,3 rows from a and no row(s) at all from b. I have a question pending with him about what if the min occurs with two rows that have the same sum but different numbers. Wonder how he'll reply.

[Nikola]

That is exactly what i was looking for Star Strider, THANKS A LOT !

[Star Strider]

My pleasure!

Answer 2

[Image Analyst]

Try this:

% Create sample data
a=[1 2 3;2 3 4; 3 4 5] 
b=[3 2 1;2 3 4; 2, 2, 2] 
% Find the sums over columns for both arrays.
columnSumsA = sum(a, 2)
columnSumsB = sum(b, 2)
% Find the overall minimum sum.
MinSumOverall = min([columnSumsA; columnSumsA])
% Find out where (what row) that sum occurs in matrix a & b.
rowLocationInA = find(columnSumsA == MinSumOverall)
rowLocationInB = find(columnSumsB == MinSumOverall)
% Construct C from those rows extracted out from a and b
c = [a(rowLocationInA,:);, b(rowLocationInB, :)]

In the command window, you can see the individual steps results:

a =
     1     2     3
     2     3     4
     3     4     5
b =
     3     2     1
     2     3     4
     2     2     2
columnSumsA =
     6
     9
    12
columnSumsB =
     6
     9
     6
MinSumOverall =
     6
rowLocationInA =
     1
rowLocationInB =
     1
     3
c =
     1     2     3
     3     2     1
     2     2     2

[Nikola]

that's not i am looking for ,the input is 3x3 but the output is 3x2, one vector from both matrices that has the least sum should be printed in new variable, so c should be c=[vector with least sum from a;vector with least sum with b] ;. I know it is hard for me explain. Also thanks.

[Image Analyst]

That's what it would do if the sum occurred only once. But look at my example. You can have a sum of 6 with 1,2,3 or with 2,2,2. Well, what if both of those occur in "b"? Which row(s) do you extract? I extracted both/all. If you wanted to take just first one, or the last one, you could do that. What do you want to do if b looks like this:

b =
     3     2     1
     2     3     4
     2     2     2

such that the min sum occurs on more than 1 row?

[EDIT] To take the first one, do this:

% Create sample data
a=[1 2 3;2 3 4; 3 4 5] 
b=[3 2 1;2 3 4; 2, 2, 2] 
% Find the sums over columns for both arrays.
columnSumsA = sum(a, 2)
columnSumsB = sum(b, 2)
% Find out where (what row) that sum occurs in matrix a & b.
[minSumA, rowLocationInA] = min(columnSumsA)
[minSumB, rowLocationInB] = min(columnSumsB)
% Construct C from those rows extracted out from a and b
c = [a(rowLocationInA,:);, b(rowLocationInB, :)]