I am working on kaprekars constant homework, and everytime I write a while loop I get error.

The idea behind the code is that when you put numbers in an array it will sort in ascending and decending order, and then subtract until I get kaprekars constant 6174. My code works for one iteration, but as soon as I try to write a while loop it put out an error message. I am at my wits end here. The moment the bolded parts go in, the code will not run. If i define n as a number, the code will run, but will keep n as the number.

n=1;
while (n==6174)
    if ~(n==6174)
        a=input("enter number a:");
        b=input("enter number b: ");
        c=input("enter number c: ");
        d=input("enter number d: ");
        if (a==b)&&(b==c)&&(c==d)
            fprintf("error! please enter atleast two different digits ")
        end
        %puts four digits into an array
        arr=[a b c d];
        for i=1:1:4
            %sorts array in ascending order
            B=sort(arr);
            %sorts array in descending order
            C=sort(arr,'descend');
        end
        %converts array into string data for ascending order
        arr2= [B];
        y = 0;
        for j = 1:length(arr2)
            y = y + (10^(j-1))*arr2(length(arr2)+1-j);
        end
        %converts array into string for descending order
        arr3= [C];
        z = 0;
        for k = 1:length(arr3)
            z = z + (10^(k-1))*arr3(length(arr3)+1-k);
        end
        n=z-y;
        fprintf("%.0d-%.0d=%.0d\n",z,y,n)
        arr=(n);
    end
    break ;
end

Answer 1

[Voss]

See below:

% ask for the input number only once at the beginning:
a=input('enter number a: ');
b=input('enter number b: ');
c=input('enter number c: ');
d=input('enter number d: ');
if (a==b)&&(b==c)&&(c==d)
    fprintf('error! please enter atleast two different digits ')
end
%puts four digits into an array
arr=[a b c d];

% n=1;
% while (n==6174)
% if ~(n==6174)
n=1;
while n ~= 6174 % do this as long as n is NOT 6174
    
    % for loop not necessary here
%     for i=1:1:4
        %sorts array in ascending order
        B=sort(arr);
        %sorts array in descending order
        C=sort(arr,'descend');
%     end
    
    %converts array into string data for ascending order
    arr2= [B];
    y = 0;
    for j = 1:length(arr2)
        y = y + (10^(j-1))*arr2(length(arr2)+1-j);
    end
    
    %converts array into string for descending order
    arr3= [C];
    z = 0;
    for k = 1:length(arr3)
        z = z + (10^(k-1))*arr3(length(arr3)+1-k);
    end
    
    n=z-y;
    fprintf('%.0d-%.0d=%.0d\n',z,y,n)
    
%     arr=(n);
    
    % arr must be the 4 digits of n, in a 1-by-4 vector:
    % convert n to a character vector with num2str(),
    % and use '%04d' to add leading zeros if necessary.
    % then subtract the character '0' from each character
    % in num2str(n), giving you numeric digits:
    arr = num2str(n,'%04d')-'0'; 
    
end

[Olivia Langford]

Okay thank you so much! I see where the last part was definitely an issue now! I couldnt figure out how to do it!

[Voss]

You're welcome!

Yes, that last part is a little tricky the first time you have to figure it out. I hope my explanation makes sense!

[Steven Lord]

You can simplify the code a bit by realizing that you can treat the vector of numbers as polynomial coefficients and evaluating it at x = 10.

c = [7 3 9 2]

c = 1×4

7 3 9 2

polyval(c, 10)

ans = 7392

polyval(flip(c), 10)

ans = 2937

polyval(sort(c, 'ascend'), 10)

ans = 2379

polyval(sort(c, 'descend'), 10)

ans = 9732