# How to sort one array based on another of a different size

5 views (last 30 days)
rbrbrb on 23 Jun 2020
Edited: Stephen Cobeldick on 24 Jun 2020
Hello,
I would like to sort one cell array A according to the order define by a separate cell array B of a smaller size. For example:
A = [a a b c c d d d d e e]
B = [d a c b e]
After sorting, array A would be:
A = [d d d d a a c c b e e]
Thank you for the help.

#### 1 Comment

Rik on 23 Jun 2020
I would write code that replaced every value in your top array by the index in your second array. Then you can sort and use indexing to get the original values back.

Stephen Cobeldick on 24 Jun 2020
Edited: Stephen Cobeldick on 24 Jun 2020
As Rik wrote, the MATLAB solution is to use ismember, e.g.:
>> A = {'a','a','b','c','c','d','d','d','d','e','e'};
>> B = {'d','a','c','b','e'};
>> [X,Y] = ismember(A,B);
>> [~,Z] = sort(Y(X));
>> C = A(Z)
C =
'd' 'd' 'd' 'd' 'a' 'a' 'c' 'c' 'b' 'e' 'e'

Aakash Mehta on 23 Jun 2020
Edited: Aakash Mehta on 23 Jun 2020
A = ['a' 'a' 'b' 'c' 'c' 'd' 'd' 'd' 'd' 'e' 'e']
B = ['d' 'a' 'c' 'b' 'e']
sizeA = length(A)
sizeB = length(B)
You can use the container.Map to track the frequency of the characters. This approach will also work when A is not sorted.
M = containers.Map
for i=1:sizeA
if isKey(M,A(i))==1
M(A(i)) = M(A(i)) + 1;
else
M(A(i)) = 1;
end
end
You will get the map M, which contains the characters as a key and frequency as their value.
Now using following code you can get the desired output.
str = "";
for i=1:sizeB
if isKey(M,B(i))==1
for j=1:M(B(i))
str = strcat(str,B(i));
end
end
end

#### 1 Comment

Rik on 23 Jun 2020
I suspect this can be vectorized with ismember. I suspect that the mapping you are proposing has a lot of unnecessary overhead.