Re: fast replace for matrix minor

*To*: mathgroup at smc.vnet.net*Subject*: [mg69527] Re: fast replace for matrix minor*From*: "Ray Koopman" <koopman at sfu.ca>*Date*: Thu, 14 Sep 2006 06:57:24 -0400 (EDT)*References*: <ee8hcp$j7g$1@smc.vnet.net>

Christopher Arthur wrote: > Is there a quick way to replace a small minor block in a large matrix? > Using ReplacePart in a Do loop seems to be very inefficient with > respect to timing > > Chris Arthur In[1]:= a = Table[10i+j,{i,6},{j,9}] Out[1]= {{11, 12, 13, 14, 15, 16, 17, 18, 19}, {21, 22, 23, 24, 25, 26, 27, 28, 29}, {31, 32, 33, 34, 35, 36, 37, 38, 39}, {41, 42, 43, 44, 45, 46, 47, 48, 49}, {51, 52, 53, 54, 55, 56, 57, 58, 59}, {61, 62, 63, 64, 65, 66, 67, 68, 69}} To replace the values in rows 4..5, columns 3..6: In[2]:= b = Table[-10i-j, {i,4,5},{j,3,6}] Out[2]= {{-43,-44,-45,-46}, {-53,-54,-55,-56}} In[3]:= ReplacePart[a, b, Flatten[Table[{i,j},{i,4,5},{j,3,6}],1], Flatten[Table[{i,j},{i,1,2},{j,1,4}],1]] Out[3]= {{11, 12, 13, 14, 15, 16, 17, 18, 19}, {21, 22, 23, 24, 25, 26, 27, 28, 29}, {31, 32, 33, 34, 35, 36, 37, 38, 39}, {41, 42,-43,-44,-45,-46, 47, 48, 49}, {51, 52,-53,-54,-55,-56, 57, 58, 59}, {61, 62, 63, 64, 65, 66, 67, 68, 69}}