Applying different equations to specific elements in MATLAB loops

[loukoumas]

Hello again!

Let's say that i am calculating every element of a 100X300 table by using equation A
I use two for loops (i m doing this in matlab)
for i=1:100
for j=1:300
element(i,j)=... (it is the equation A)
end
end
equation A involves the neighboring elements
What if, a few elements of the table must be calculated with a different equation, equation B and this must happen inside the two for loops

for example elements (49,81) (49,82)
(50,81) (50,82)
(51,81) (51,82)
(52,81) (52,82)
i could use if and elseif function for every single element, like
if i==49 && j==81
element(i,j)=...equation B
elseif i==50 && j==81
element(i,j)=...equation B
...

...

end
Is there another, better way to do it ?

Thanks a lot!

 

[NemoReally]

Hello again!

Let's say that i am calculating every element of a 100X300 table by using equation A
I use two for loops (i m doing this in matlab)
for i=1:100
for j=1:300
element(i,j)=... (it is the equation A)
end
end
equation A involves the neighboring elements
What if, a few elements of the table must be calculated with a different equation, equation B and this must happen inside the two for loops

for example elements (49,81) (49,82)
(50,81) (50,82)
(51,81) (51,82)
(52,81) (52,82)
i could use if and elseif function for every single element, like
if i==49 && j==81
element(i,j)=...equation B
elseif i==50 && j==81
element(i,j)=...equation B
...

...

end
Is there another, better way to do it ?

I'm not sure about 'better', but you could put the list of 'alternative' element indices in an array and then use a function to determine whether (i,j) is in that list. I don't use Matlab, but a very quick look at the documentation suggests that ismember may do the trick (it will return a vector of all zeros if (i,j) isn't in The List.

I've attached a picture of what I mean implemented in Mathcad.

I'm presuming that you can't take B outside of the loop.

NR

 

[NemoReally]

I'm not sure about 'better', but you could put the list of 'alternative' element indices in an array and then use a function to determine whether (i,j) is in that list. I don't use Matlab, but a very quick look at the documentation suggests that ismember may do the trick (it will return a vector of all zeros if (i,j) isn't in The List.

I've attached a picture of what I mean implemented in Mathcad.

I'm presuming that you can't take B outside of the loop.

Forgot to add, and I think Matlab should be able to do this, an alternative would be to create a 100x300 array whose elements are the function name of f, and then change the 'special' cases to the function name of g. In your main loop, you then look up the appropriate function for element (i,j) in the function array.

... a quick glance at the Matlab documentation suggests that, although this might be possible, a better route to go might be to create a structure array of f's and g's handles.

NR

 

[loukoumas]

Thanks a lot mate!