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

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Amanda Liu el 22 de Jun. de 2021
Comentada: Amanda Liu el 22 de Jun. de 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)..
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KSSV el 22 de Jun. de 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?
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Amanda Liu el 22 de Jun. de 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

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### Más respuestas (1)

Dhruv G el 22 de Jun. de 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
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Amanda Liu el 22 de Jun. de 2021
Okay, thank you.

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