MaplePrimes - answers and comments on Question, Loop variable problems cat - Error, invalid parameters for inline function

wrong structure

pagan — Tue, 15 Jun 2010 18:37:13 Z

I suspect that you are likely using structures not best suited for your overall tasks, ie. lists instead of mutable Matrix & Vector.

a1:=Matrix([[1,2,3,4,5,6,7],[9,8,7,6,5,4,3],[4,5,4,5,4,5,4]]);

# one way
b1:=LinearAlgebra:-ColumnOperation(a1,[4,6]);

# alternatively
b1:=copy(a1):
b1[1..-1,4],b1[1..-1,6] := b1[1..-1,6],b1[1..-1,4]:
b1;

Why use lists of lists? And why use || to form the names (in other posts as well) instead of just a[i] and b[i]?

Anyway, if you really must...

a1:=[[1,2,3,4,5,6,7],[9,8,7,6,5,4,3],[4,5,4,5,4,5,4]];
G:=(L,x,y)->[op(L[1..x-1]),L[y],op(L[x+1..y-1]),L[x],op(L[y+1..-1])]:
map(G,a1,4,6);

possible solution

Scott03 — Tue, 15 Jun 2010 23:47:19 Z

I believe that the suggestion that pagan suggested is likely the most efficient, but if you just want to use your list of lists and use map then the following should work.

for i to 3 do
b || i := map((x) -> [x[4], x[6]], a || i)
end do;

This should allow you to go through each of the 'a' lists and take the 4th and 6th position.

If you wanted to have the 'a' values as Matrices you could simplify this as the following

a1 := Matrix([[1, 2, 3, 4, 5, 6, 7], [9, 8, 7, 6, 5, 4, 3], [4, 5, 4, 5, 4, 5, 4]]):
a2 := Matrix([[2, 3, 4, 5, 6, 7, 8], [8, 7, 6, 5, 4, 3, 2], [3, 4, 3, 4, 3, 4, 3]]):
a3 := Matrix([[3, 4, 5, 6, 7, 8, 9], [7, 6, 5, 4, 3, 2, 1], [2, 3, 2, 3, 2, 3, 2]]):

seq(assign(b || i, a || i( .. , [4, 6])), i = 1 .. 3):

I hope this helps.

Scott

with index

pagan — Tue, 15 Jun 2010 23:58:10 Z

I don't see why it wouldn't work for you with a[i] and b[i] as indexed names.

a[1]:=[[1,2,3,4,5,6,7],[9,8,7,6,5,4,3],[4,5,4,5,4,5,4]]:
a[2]:=[[2,3,4,5,6,7,8],[8,7,6,5,4,3,2],[3,4,3,4,3,4,3]]:
a[3]:=[[3,4,5,6,7,8,9],[7,6,5,4,3,2,1],[2,3,2,3,2,3,2]]:

G:=(L,x,y)->[op(L[1..x-1]),L[y],op(L[x+1..y-1]),L[x],op(L[y+1..-1])]:

for i from 1 to 3 do
  b[i]:=map(G,a[i],4,6);
end do;

ps. Scott, he wanted to create new objects, where b[i] is a[i] with swapped 4th and 6th entries. He didn't just want to extract just those two entries (which would be simpler to do, 'natch.)

structure

Christopher2222 — Tue, 15 Jun 2010 20:26:51 Z

For some reason I was thinking b[i] might wipe out the position of b[i].

Okay, so if I change my name structure so it's

a[1]:= [[ ...

a[2]:=[[ ...

a[3]:=[[ ...

and use b[i] it still doesn't work like I thought it would.

It appears map doesn't like variable b[i][4] and b[i][6]

objects

Christopher2222 — Wed, 16 Jun 2010 00:14:52 Z

I like to add new objects, heh heh. No, I just wanted to know why I couldn't get map to work with indicies in my loop.

Thanks for the inputs.