How to shuffle such that two same elements are not together? - matlab

Question

I have a string containing several elements, some identical and some unique. I want my code to check every 2 following elements in my string and if they're equal, it should call a function ShuffleString, where the input variable (randomize) is the string itself, that will re-shuffle the string in a new position. Then, the script should re-check every 2 following elements in the string again until no two identical elements appear next to each other.
I have done the following:
My function file ShuffleString works fine. The input variable randomize, as stated earlier, contains the same elements as MyString but in a different order, as this was needed on an unrelated matter earlier in the script.
function [MyString] = ShuffleString(randomize)
MyString = [];
while length(randomize) > 0
S = randi(length(randomize), 1);
MyString = [MyString, randomize(S)];
randomize(S) = [];
end
The script doesn't work as intended. Right now it looks like this:
MyString = ["Cat" "Dog" "Mouse" "Mouse" "Dog" "Hamster" "Zebra" "Obama"...
"Dog" "Fish" "Salmon" "Turkey"];
randomize = MyString;
while(1)
for Z = 1:length(MyString)
if Z < length(MyString)
Q = Z+1;
end
if isequal(MyString{Z},MyString{Q})
[MyString]=ShuffleString(randomize)
continue;
end
end
end
It just seems to reshuffle the string an infinite amount of times. What's wrong with this and how can I make it work?

Answer 1

You are using an infinite while loop that has no way to break and hence it keeps iterating.
Here is a simpler way:
Use the third output argument of the unique function to get the elements in numeric form for easier processing. Apply diff on it to check if consecutive elements are same. If there is any occurrence of same consecutive elements, the output of diff will give at least one zero which when applied with negated all will return true to continue the loop and vice versa. At the end, use the shuffled indices/numeric representation of the strings obtained after the loop to index the first output argument of unique (which was calculated earlier). So the script will be:
MyString = ["Cat" "Dog" "Mouse" "Mouse" "Dog" "Hamster" "Zebra" "Obama"...
"Dog" "Fish" "Salmon" "Turkey"]; %Given string array
[a,~,c] = unique(MyString);%finding unique elements and their indices
while ~all(diff(c)) %looping until there are no same strings together
c = ShuffleString(c); %shuffling the unique indices
end
MyString = a(c); %using the shuffled indices to get the required string array
For the function ShuffleString, a better way would be to use randperm. Your version of function works but it keeps changing the size of the arrays MyString and randomize and hence adversely affects the performance and memory usage. Here is a simpler way:
function MyString = ShuffleString(MyString)
MyString = MyString(randperm(numel(MyString)));
end