matlab: Subscripted assignment dimension mismatch...i can understand why it is showing but cannot get a solution

i am having problem with this code. when i am preallocating Pw then it shows this error

Subscripted assignment dimension mismatch.

i can understand that Pw is a matrix of 1x10000000 and cnt is a matrix of 1x1 but i cannot understand how to solve this

cnt = 1;

%%plane : Xw = 1
Pw = zeros(1, 1000000);%preallocating Pw

for i=0.2:0.2:0.8,%it means it starts from 0.2 gives an interval 0.2 and goes till 0.8
 for j=0.2:0.2:0.8,
   Pw(cnt,:) = [1 i j];%is the cnt-th row of pw , pw is a matrix whos cnt-th row is a row matrix, cnt is a variable that starts from 1 and after forming 
   %the row matrix, it goes on till i and j conditions are fulfilled. 
   cnt = cnt + 1;
 end
end

please let me know how to solve this

Answer 1

[Youssef Khmou]

hi,

 you have :

 i=0.2:0.2:0.8; % as your loop

The length is 4, then initialize the matrix Pw as :

 Pw=zeros(4,3); % if you expect 4 values 
 %or
 Pw=zeros(16,3); % if you expect (1:4,1:4) values

[Walter Roberson]

zeros((4*4), 3) -- remember the "j" loop.

[Youssef Khmou]

correct,

[Sat m]

thank you... it is working. but i applied the same thing for another loop. there is some problem showing.

%%surface Xw = 1
cnt = 1;
Pc = zeros(16,3);
p = zeros(16,3);

for cnt = 1:1:16,
   Pc(cnt,:) = (R*Pw(cnt,:)' + T)';
   p(cnt,:)  = [(Ox - Fx*Pc(cnt,1)/Pc(cnt,3)) (Oy - Fy*Pc(cnt,2)/Pc(cnt,3))];
end
plot(p(:,1), p(:,2), 'r+');
axis([0 512 0 512]);
grid;
hold;

this time, it is showing

Subscripted assignment dimension mismatch.

Error in calib (line 82)
   p(cnt,:)  = [(Ox - Fx*Pc(cnt,1)/Pc(cnt,3)) (Oy - Fy*Pc(cnt,2)/Pc(cnt,3))];

please let me know

[Youssef Khmou]

the vector " [(Ox - Fx*Pc(cnt,1)/Pc(cnt,3)) (Oy - Fy*Pc(cnt,2)/Pc(cnt,3))]" has more than 3 as length, try :

 cnt=1;
 n=length([(Ox - Fx*Pc(cnt,1)/Pc(cnt,3)) (Oy - Fy*Pc(cnt,2)/Pc(cnt,3))]);
 p=zeros(16,n);

and try again ....

[Walter Roberson]

We do not have evidence that that vector would be longer than length 3. If Ox and Oy and Fx and Fy are all scalars, then the vector would have length 2, which would not match the dimension of 3 that is needed for ":" to work.

[Sat m]

thank you. it is working.. thank you

Answer 2

[Walter Roberson]

When you use a destination dimension of ":" for an array, then the number of elements you are writing in must match the size of the array in that dimension. You set the second dimension of Pw to be 1000000, but your assignment statement is only writing 3 values instead of that 1000000.

Notice too that your second assignment to Pw would be writing to Pw(2,:) but you only allocated one row for Pw. The assignment will be allowed to go through, but you will not be taking advantage of preallocation.

[I, J] = ndgrid(0.2:0.2:0.8, 0.2:0.2:0.8);
Pw = [ ones(numel(I), 1), I(:), J(:) ];