For Loop and if statement

Can someone explain how can use for loop for a data where i have 4 columns as shown below and for each of 3 categories i want to sum the the corresponding x y and z values?

x y z category

9 6 7 1

7 8 6 2

9 1 2 1

5 7 8 3

8 5 1 2

Answer 1

[Kevin Holly]

m = [9    6    7        1
7    8    6        2
9    1    2        1
5    7    8        3
8    5    1        2];


a=[];
for i=1:3
a = [a; [sum(m(m(:,4)==i,1:3),1) i]];
end
a

a = 3×4

18 7 9 1 15 13 7 2 5 7 8 3

Breakdown:

m(:,4)==1

ans = 5×1 logical array
   1
   0
   1
   0
   0

The above creates a logical array of all row in the matrix (m) where the 4 column is equal to 1.

The specific elements within the matrix were selected as m(rows,columns). A colon (:) was used for rows, indicating to select all rows, which in this case was 5. This could also be describe as an array from 1 to 5 or 1:5. For the columns, only the 4th was selected, thus you end up with m(:,4).

m(:,4)

ans = 5×1

1 2 1 3 2

Now, let's look at the first three columns 1 through 3.

m(:,1:3)

ans = 5×3

9 6 7 7 8 6 9 1 2 5 7 8 8 5 1

Let's use the logical array to select only the one where the 4th column in matrix m is equal to one.

m(m(:,4)==1,1:3)

ans = 2×3

9 6 7 9 1 2

Now, let's sum the columns.

sum(m(m(:,4)==1,1:3),1)

ans = 1×3

18 7 9

Now, let's add the 4th column back by concatenating with a bracket.

[sum(m(m(:,4)==1,1:3),1) 1]

ans = 1×4

18 7 9 1

Finally, replace the 1 with an i and create a for loop to calculate for catergories 1 through 3. The variable a is created to hold the results in a single matrix. Each result is appended as a row.

a=[];
for i=1:3
a = [a; [sum(m(m(:,4)==i,1:3),1) i]];
end
a

a = 3×4

18 7 9 1 15 13 7 2 5 7 8 3