# array: concatenate two columns of integers to one colum

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William Sampson on 13 Sep 2012
(Matlab novice)
I thought this would be so easy...
I have a 2-d array of integers and I want to combine two columns like this:
... 123 456 ...
... 123 456 ...
etc...
I want:
... 123456 ...
... 123456 ...
etc...
as single numbers (that is, I want (123 * 1000) + (456)
The number of rows will be variable from one run to the next.
Any help would be appreciated.

#### 1 Comment

Jan on 13 Sep 2012
The question is not clear. What is the wanted output for: [1,2,3,4; 5,6,7,8; 9,10,11,12] ?

Honglei Chen on 13 Sep 2012
Edited: Honglei Chen on 13 Sep 2012
a = randi([1 100],[5 4]) % a 5x4 integer matrix
b = cellfun(@num2str,{a(:,2:3)},'UniformOutput',false)
c = cellfun(@(x) strrep(x,' ',''), cellstr(b{1}),'UniformOutput',false)
a(:,2) = str2double(c)
a(:,3) = []

Jan on 13 Sep 2012
Faster:
b = sprintf('%d%d*', a(:, 2:3).');
c = sscanf(b, '%d*'); [EDITED, thanks Honglei]
STRREP operates on cell string directly, so there is usually no need to create a time-consuming anonymous function inside CELLFUN.
Honglei Chen on 13 Sep 2012
@Jan, thanks for the tip. That's a good one. I think you missed a b in your second expression.

Tom on 13 Sep 2012
Edited: Tom on 13 Sep 2012
A=[1 2 3;3 5 7];
B=[7 8 9;4 5 6];
%
C=[A B];
w=size(C,2);
D=10.^(w-1:-1:0);
Out=sum(bsxfun(@times,D,[A B]),2)

Jan on 13 Sep 2012
Edited: Jan on 13 Sep 2012
A = [1234, 567; 1, 234];
B = A(:, 1) .* nextpow10(A(:, 2)) + A(:, 2);
function Y = nextpow10(X)
Y = 10 .^ floor(log10(abs(n)) + 1); % [EDITED]
Please test this before using, I cannot run Matlab currently.

#### 1 Comment

Andrei Bobrov on 13 Sep 2012
A=randi(67,6,4)
n = ceil(log10(A(:,2:end)));
out = sum(A(:,1:end-1).*(10.^fliplr(cumsum(fliplr(n),2))),2)+ A(:,end)

William Sampson on 13 Sep 2012
Yeah - so
1) I'm not getting it and
2) I seem to not have been clear enough (sorry...)
what I have is a data array with 59 columns and an indeterminate number of rows and I want to combine 2 (for example) columns as described above. In reality it is 5 columns, 8 - 12, of 16 digits each, but I'll start with getting 2 of them done.
So I have, e.g., rawData(:,9) and rawData(:,10) to combine

Honglei Chen on 13 Sep 2012
It's the same. I updated my answer to show an example of combining 2nd and 3rd column of a matrix
Jan on 13 Sep 2012
See my suggestion:
B = rawData(:, 9) .* nextpow10(rawData(:, 10)) + rawData(:, 10)
result = [rawData(:, 1:8), B, rawData(:, 11:end)]
I do not see, why the suggested method should not work.
Btw. What do you expect as result of two 16 digit numbers? You can store only 16 digits in variables of the type DOUBLE.

Ryan on 13 Sep 2012
Edited: Ryan on 13 Sep 2012
A = [1234, 567; 1, 234]; %Sample Data
% Figure out how many spaces to shift the left column
C = ceil(log10(A(:,2)));
% Find areas where the value is a multiple of 10 or is 1
idx = A(:,2) == 1 | rem(A(:,2),10) == 0;
C(idx) = C(idx) + 1; % Handle the exception
% Determine how much to shift the left column
C = 10.^C;
% Add in the right column
A = A(:,1).*C + A(:,2);