MATLAB nested loop

Question

function[output]=tridiag(d,l,r)
A=zeros(3);
for i=1:5 
    A(i,i)=d(i);
    for j=2:5
        A(j,(i-1))=l(j);
        for k=1:4
            A(k,(i+5))=r(k);
        end
    end
end
A
end

on this part where it says,

for j=2:5
    A(j,(i-1))=l(j);

I want the variable "i" to execute from 1 ~ 4. and not 5. That's why I put i-1 there, but obviously it won't work.

How can I make the nested j-loop to repeat i=1:4, instead of i=1:5? Same question goes for k-loop too.

+update: So I want the outside i-loop to perform from 1:5, but for the inner j-loop, i want i=1:4.

Change the line `for i=1:5` to `for i=1:4`. Similarly for the loop over `j` — Drake, Mar 06 '17 at 05:24
@Drake no, because that will make the initial i-loop to repeat from 1 to 4. I want the outside loop to repeat from 1:5. Only the in the inside j-loop, i want i to be 1:4 — Eric Kim, Mar 06 '17 at 05:42
Well, you should have stated that clearly from the beginning. Wrap the inner loop in an if statement: if (i < 5) — Drake, Mar 06 '17 at 05:46
Actually, could you show an example input and output? I can guess you want to create a tri-diagonal matrix but can't decipher what is in `d,l,r` — Yvon, Mar 06 '17 at 05:56

score 0 · Answer 1 · answered Mar 06 '17 at 05:26

0

A(j,(i-1))

i starts from 1. So in the first iteration, this give you:

A(j,(0))

Since the index of an array in MATLAB starts from 1, this is definitely wrong. So change your code to something like this will work:

for j=1:4
    A(j,(i))=l(j);

answered Mar 06 '17 at 05:26

Tianjiao Huang

144
3
14

no, because I want i to loop from 1 to 4, which is 1, 2, 3, 4. The way you suggest me to do will result in i=1:5. – Eric Kim Mar 06 '17 at 05:40

Yvon · Accepted Answer · 2017-03-06T06:05:10.600

To your original question: Use loop control in the inner loop.

for i=1:5 
    A(i,i)=d(i);
    for j=2:5
        if i==5
            continue
        end
        A(j,(i-1))=l(j);
        for k=1:4
            A(k,(i+5))=r(k);
        end
    end
end

I guess you want to create a tridiagonal matrix using d as the major diagonal line, l as the left/lower diagonal, and r as the right/upper diagonal. Your code does not seem to work due to wrong indexing.

d = 1:5;
l = 10:10:50;
r = 100:100:500;

N = length(d);
A=zeros(N);
for ii=1:N
    A(ii,ii)= d (ii);
    if ii~=1
        A(ii,(ii-1)) = l(ii);
    end
    if ii~=N
        A(ii,(ii+1)) = r(ii);
    end
end
A


A =

     1   100     0     0     0
    20     2   200     0     0
     0    30     3   300     0
     0     0    40     4   400
     0     0     0    50     5

can you please explain why you used double ii, instead of just one i? And I'm confused how you can use (ii-1), since ii starts from 1. Shouldn't it make (ii-1) undefined? — Eric Kim, Mar 06 '17 at 09:34

score 0 · Answer 3 · answered Mar 06 '17 at 10:27

function[output]=tridiag(d,l,r)
A=zeros(3);
for i=1:5 
      A(i,i)=d(i);
      j=2;
      while(j<6 & i~=5)
             A(j,i)=l(j);
             j=j+1;
                for k=1:4
                A(k,(i+5))=r(k);
             end
       end
end
A
end

The below three lines make sure that 'j' runs from '2' to '5' for every value of 'i' except i=5 .

j=2;
while(j<6 & i~=5)
j=j+1;

MATLAB nested loop

3 Answers3