Re: Replace specific element with specific value.

*To*: mathgroup at smc.vnet.net*Subject*: [mg99651] Re: Replace specific element with specific value.*From*: Bill Rowe <readnews at sbcglobal.net>*Date*: Sun, 10 May 2009 05:16:38 -0400 (EDT)

On 5/9/09 at 3:22 AM, nick.maj at mssm.edu wrote: >I tried this code, it gives me right and wrong results spreeded into >nested lists. >data = {{0, 2}, {2, x}, {3, 4}, {4, 6}, {6, y}, {8, 9}, {9, z}} >Table[{#1, #2 + Which[#2 === j, i, #2 =!= j, 0]} & @@@ >data, {i, {xx, yy, zz}}, {j, {x, y, z}}] >needed output: >{0, 2}, {2, x+xx}, {3, 4}, {4, 6}, {6, y+yy}, {8, 9}, {9, z+zz}} >While searching only the second element of each list, if x is found >add xx and if y is found add yy to it etc. Why not use pattern matching? That is: In[1]:= data = {{0, 2}, {2, x}, {3, 4}, {4, 6}, {6, y}, {8, 9}, {9, z}}; In[2]:= data /. {x -> x + xx, y -> y + yy, z -> z + zz} Out[2]= {{0, 2}, {2, x + xx}, {3, 4}, {4, 6}, {6, y + yy}, {8, 9}, {9, z + zz}} Here, I've manually input the replacement rules which seems fine for only three variables. If there where a large number of variables to treat in this manner, I would generate the replacement rules programatically. Perhaps something like In[3]:= Rule @@@ (ToExpression[{#, # <> "+" <> # <> #}] & /@ {"x", "y", "z"}) Out[3]= {x->x+xx,y->y+yy,z->z+zz}