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Re: Dynamic iterators in Do loop


On 5/6/06 at 11:50 PM, iwillfail at gmail.com (sai) wrote:

>I have an (n,n) matrix. I want to define an (m,m) matrix [m<n] whose
>elements are defined from the (n,n) matrix. The value of n and m
>changes from problem to problem. How can i do this in mathematica?

There are a number of ways to accomplish what you want. Which is best would depend on precisely what you want to do. For example, the upper m X m sub-matrix can be extracted from the n X n matrix (a) as follows:

Take[Take[#,m]&/@a,m]

Once the sub-matrix is obtained Map can be used to Map any arbitrary function to each element of the matrix.

For example,

In[30]:=
n=4;
m=3;
a=Table[10 i+j,{i,n},{j,n}];
b=Take[Take[#,m]&/@a,m];
Map[Total[IntegerDigits@#]&,b,{2}]

Out[34]=
{{2, 3, 4}, {3, 4, 5}, {4, 5, 6}}
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