# Relationship between (i & j) of a nested for loop and (x & y) coordinates?

4 views (last 30 days)
Amanda Liu on 22 Jun 2021
Commented: Amanda Liu on 22 Jun 2021
If I have a nested for loop:
T=ones(n)
for i = 1 : nx
for j = 1 : ny
T(i,j) = %something
end
end
I understand i goes down and j goes right in a loop but isn't it supposed to be the opposite where i is the horizontal row nd j is the vertical column? for i = 1 : nx
for j = 1 : ny
Does this mean that the T(i,j) I'm plotting is in the direction of: I'm a little confused about (i&j) and (x&y)..

KSSV on 22 Jun 2021
It moves from bottom to top and left to right. Check with below:
nx = 10 ;
ny = 10 ;
figure
hold on
for i = 1:nx
for j = 1:ny
plot(i,j,'*')
drawnow
text(i,j,num2str([i j]))
end
end
Why you are confused with direction? How does it matter?
Amanda Liu on 22 Jun 2021
Hmm..because if I were to assign boundary conditions at the sides of a square (left right top bottom)
should i code like this T(:,1) = 2 % Left
T(:,end) = 3 % Right
T(1,:) = 1 % Top
T(end,:) = 4 % Bottom or
T(:,1) = 4 % Left
T(:,end) = 1 % Right
T(1,:) = 2 % Top
T(end,:) = 3 % Bottom
but i do noticed that the top and bot should be reversed so that it plots in the correct direction
T(1,:) % Should be Bottom value
T(:,end) % Should be Top value

Dhruv G on 22 Jun 2021
As @KSSV has noted, the for loop just means that the indices will increase while the loop runs. In a table, what you've drawn is correct. But in a right-handed coordinate plane ( the normal axes that we're used to), the indices increase when moving right and moving upward. The indices of the for loop will obey this
Amanda Liu on 22 Jun 2021
Okay, thank you.

R2021a

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