Matlab: Converting a double vector array to string cell array

Question

map1 = containers.Map({'212','2','12','44'},[4,5,6,7]);
keyset = str2double(keys(map1));

Now I do a set of operations on the keyset which will give back

Keyset= [203,2,12,39];

I tired the following:

num2cell(num2str(keyset));
num2cell(num2str(keyset,1));
num2cell(num2str(keyset,'%11.0g'));
num2cell(num2str(keyset,3));

all of the above gave weird results in the final cell array. I just need the integers to be used as keys for another container map.

Answer 1

I propose 5 additional solutions, three of which are 4-5x faster by than the solutions proposed so far. The lessons learned from this are:

• num2str is slow
• cellfun and arrayfun can add significant overhead
• There are many ways to convert a numeric array to a cell array of strings.

The three highest-performance solutions are very similar in terms of performance:

Looping to assign cell elements

n4 = length(Keyset);
tmp4 = cell(n4,1);
for i4 = 1:n4
    tmp4{i4} = sprintf('%i',Keyset(i4));
end

Converting all to string and calling textscan

tmp6 = textscan(sprintf('%i\n',Keyset'),'%s');
tmp6 = tmp6{1};

Converting all to string and calling regexp.

tmp3 = regexp(sprintf('%i ',Keyset),'(\d+)','match');

---

Here's the full test code with timings:

function t = speedTest

t=zeros(7,1);
for ii=1:100, 
    Keyset=randi(1,10,100); % random keys
    tic; 
    eval( [ 'tmp1 = { ', sprintf(' %d ', Keyset), ' }; '] );
    t(1)=t(1)+toc; 
    tic;
    tmp2=arrayfun(@num2str, Keyset, 'Uniform', false);
    t(2)=t(2)+toc;

    tic;
    tmp3 = regexp(sprintf('%i ',Keyset),'(\d+)','match');
    t(3) = t(3)+toc;

    tic;
    n4 = length(Keyset);
    tmp4 = cell(n4,1);
    for i4 = 1:n4
        tmp4{i4} = sprintf('%i',Keyset(i4));
    end
    t(4) = t(4)+toc;

    tic;
    n5 = length(Keyset);
    tmp5 = cell(n5,1);
    for i5 = 1:n5
        tmp4{i5} = num2str(Keyset(i5));
    end
    t(5) = t(5)+toc;

    tic;
    tmp6 = textscan(sprintf('%i\n',Keyset'),'%s');
    tmp6 = tmp6{1};
    t(6) = t(6)+toc;

    tic;
    tmp7 = num2cell(Keyset);
    tmp7 = cellfun(@(x)sprintf('%i',x),tmp7,'uni',false);
    t(7) = t(7)+toc;


end;
t

t =

    1.7820
   21.7201
    0.4068
    0.3188
    2.2695
    0.3488
    5.9186

Answer 2

How about:

arrayfun(@num2str, Keyset, 'Uniform', false)'

which should yield a 4-by-1 cell array for your example:

ans = 
    '203'
    '2'
    '12'
    '39'