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POHow to insert a column into a matrix, the correct Mathematica way
 

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  1. POHow to insert a column into a matrix, the correct Mathematica way
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    <p>I think Mathematica is biased towards rows not columns.</p> <p>Given a matrix, to insert a row seems to be easy, just use <code>Insert[]</code></p> <pre><code>(a = {{1, 2, 3}, {4, 0, 8}, {7 , 8, 0}}) // MatrixForm 1 2 3 4 0 8 7 8 0 row = {97, 98, 99}; (newa = Insert[a, row, 2]) // MatrixForm 1 2 3 97 98 99 4 0 8 7 8 0 </code></pre> <p>But to insert a column, after some struggle, I found 2 ways, I show below, and would like to ask the experts here if they see a shorter and more direct way (Mathematica has so many commands, and I could have overlooked one that does this sort of thing in much direct way), as I think the methods I have now are still too complex for such a basic operation.</p> <h2>First method</h2> <p>Have to do double transpose:</p> <pre><code>a = {{1, 2, 3}, {4, 0, 8}, {7 , 8, 0}} column = {97, 98, 99} newa = Transpose[Insert[Transpose[a], column, 2]] 1 97 2 3 4 98 0 8 7 99 8 0 </code></pre> <h2>Second method</h2> <p>Use SparseArray, but need to watch out for index locations. Kinda awkward for doing this:</p> <pre><code>(SparseArray[{{i_, j_} :&gt; column[[i]] /; j == 2, {i_, j_} :&gt; a[[i, j]] /; j == 1, {i_, j_} :&gt; a[[i, j - 1]] /; j &gt; 1}, {3, 4}]) // Normal 1 97 2 3 4 98 0 8 7 99 8 0 </code></pre> <p>The question is: Is there a more functional way, that is little shorter than the above? I could ofcourse use one of the above, and wrap the whole thing with a function, say <code>insertColumn[...]</code> to make it easy to use. But wanted to see if there is an easier way to do this than what I have.</p> <p>For reference, this is how I do this in Matlab:</p> <pre><code>EDU&gt;&gt; A=[1 2 3;4 0 8;7 8 0] A = 1 2 3 4 0 8 7 8 0 EDU&gt;&gt; column=[97 98 99]'; EDU&gt;&gt; B=[A(:,1) column A(:,2:end)] B = 1 97 2 3 4 98 0 8 7 99 8 0 </code></pre>
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    1. COduplicate question: [Inserting into a 2d list](http://stackoverflow.com/questions/4270802/inserting-into-a-2d-list)
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    2. CO@WReach The question you linked is a bit more narrowly formulated, the request there was to add the column at the end (although some solutions there were more general and suited also for this one).
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    3. CO[This](http://stackoverflow.com/q/1244782/499167) question is also similar. [There](http://stackoverflow.com/questions/1244782/how-to-prepend-a-column-to-a-matrix/2274679#2274679) [Janus](http://stackoverflow.com/users/212538/janus) draws attention to the 'trick' of inserting a column of zeros (say) by the `ArrayFlatten` method as follows: `ArrayFlatten@{{a[[All, ;; 1]], 0, a[[All, 2 ;; 3]]}}` (It seems to me that a [recent question](http://stackoverflow.com/q/7534005/499167) has been treated a lot more harshly than this one as a 'possible duplicate')
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