Is it possible to combine "for" and "if" like this?

Hello, Iam new at computing and matlab, I have a problem with the next part of my program.

nodos = [0     0     0     0     1     1; 48     0     0     0     1     1;84     0     0     0     1     1;48    36  -500     0     0     0]

Nnodos=4

for i=1:Nnodos

w=0

   if nodos(i,5)==0    

   nodos(i,7)=w+1

   w=w+1
  
   end
end

When i run the program the matrix "nodos" is not modified. Is the i going from 1 to Nnodos?

thanks for your time.

[Matt J]

When I run your code, nodos does get modified. A 7th column now appears, and it looks like the following

nodos =

       0     0     0     0     1     1     0
      48     0     0     0     1     1     0
      84     0     0     0     1     1     0
      48    36  -500     0     0     0     1

Answer 1

[Geoff Hayes]

Rachuan - when I run the code, the matrix nodos is modified. In fact, a seventh column is appended to the original 4x6 matrix. Look at the condition that when true, updates the matrix

 if nodos(i,5)==0    
   nodos(i,7)=w+1
   w=w+1  
 end

So if the fifth element of the ith row is zero, then you set the seventh element of the ith row to w+1. But since your matrix is 4x6, a seventh column of zeros is automatically added to the matrix with only nodos(i,7) equal to one. The code then increments w and proceeds to the next iteration of the loop. But the first line of the for loop body

 w=0

So even though we have incremented w, we have reset it to zero. I suspect that you only want to initialize w to zero outside of the for loop. So you may want to revise your code as

 nodos = [0 0 0 0 1 1; 48 0 0 0 1 1;84 0 0 0 1 1;48 36 -500 0 0 0]

 Nnodos = 4;
 w      = 0;
 for i=1:Nnodos
    if nodos(i,5)==0    
        nodos(i,7)=w+1;
        w=w+1;
    end
 end

Now decide what should happen when nodos(i,5)==0 0 do you want to add a new column, or update an existing one with nodos(i,??)=w+1;

[Rachuan]

Great, thanks my friend. It was that W inside the loop that was breaking the code.

Answer 2

[dpb]

Yes... :) (the answer to the question asked). You can see this as you run the code as given above since the value of w=0 is printed four times, once for each pass thru the loop.

But...the array nodos is modified; you just added a column instead of, perhaps intending to change one of the existing columns, perhaps.

Count 'em...you now at the end have size(nodos)=[4 7] and the last value of the last column is the '1' you added.

The session at my command line follows...

>> nodos = [0     0     0     0     1     1;
      48     0     0     0     1     1;
      84     0     0     0     1     1;
      48    36  -500     0     0     0]
Nnodos=4
for i=1:Nnodos
w=0
if nodos(i,5)==0    
  nodos(i,7)=w+1
  w=w+1
end
end
nodos =
   0     0     0     0     1     1
  48     0     0     0     1     1
  84     0     0     0     1     1
  48    36  -500     0     0     0
Nnodos =
   4
w =
   0
w =
   0
w =
   0
w =
   0
nodos =
   0     0     0     0     1     1     0
  48     0     0     0     1     1     0
  84     0     0     0     1     1     0
  48    36  -500     0     0     0     1
w =
   1
>>