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Wolfram Language

Should the indexes also be defined as local variables in a custom function?

Hongyi Zhao

Posted 2 years ago

See my following example: In[627]:= getTrMat//ClearAll; getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x}, If[ Length[ns]==1, Partition[ns//First,dim], Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim] ] ] In[639]:= ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 0, 0, 1}}; getTrMat[ns] Out[640]= {{3/4, 5/8, 5/8, 0}, {5/8, 3/4, 5/8, -(1/8)}, {5/8, 5/8, 3/ 4, -(1/8)}, {0, 0, 0, 1}} In the above example, should the index variable i also be defined as local variables or not? More specifically, should I use which of the following forms at the beginning of the Block command? Block[{dim=Sqrt[Length[ns[[1]]]], x}, Block[{dim=Sqrt[Length[ns[[1]]]], x, i}, Regards, Zhao

See my following example:

In[627]:= getTrMat//ClearAll;
getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x},
If[ Length[ns]==1,
Partition[ns//First,dim],
Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim]
]
]

In[639]:= ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 
  0, 0, 1}};
getTrMat[ns]

Out[640]= {{3/4, 5/8, 5/8, 0}, {5/8, 3/4, 5/8, -(1/8)}, {5/8, 5/8, 3/
  4, -(1/8)}, {0, 0, 0, 1}}

In the above example, should the index variable i also be defined as local variables or not? More specifically, should I use which of the following forms at the beginning of the Block command?

Block[{dim=Sqrt[Length[ns[[1]]]], x},
Block[{dim=Sqrt[Length[ns[[1]]]], x, i},

Regards, Zhao

POSTED BY: Hongyi Zhao

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Hans Milton

Posted 2 years ago

You are right, there is no problem with your use. My mistake, sorry.

POSTED BY: Hans Milton

Hongyi Zhao

Posted 2 years ago

Thank you for pointing this out. Then the following confirms my previous conclusion: In[26]:= i=7 getTrMat//ClearAll; getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x}, If[ Length[ns]==1, (Partition[ns//First,dim],) Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim] ] ] ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 0, 0, 1}}; getTrMat[ns] Out[26]= 7 Out[30]= {{(3 x[1])/4, (5 x[1])/8, (5 x[1])/8, 0}, {(5 x[1])/8, ( 3 x[1])/4, (5 x[1])/8, -(x[1]/8)}, {(5 x[1])/8, (5 x[1])/8, ( 3 x[1])/4, -(x[1]/8)}, {0, 0, 0, x[1]}}

Thank you for pointing this out. Then the following confirms my previous conclusion:

In[26]:= i=7
getTrMat//ClearAll;
getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x},
If[ Length[ns]==1,
(*Partition[ns//First,dim],*)
Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim]
]
]

ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 
  0, 0, 1}};
getTrMat[ns]

Out[26]= 7

Out[30]= {{(3 x[1])/4, (5 x[1])/8, (5 x[1])/8, 0}, {(5 x[1])/8, (
  3 x[1])/4, (5 x[1])/8, -(x[1]/8)}, {(5 x[1])/8, (5 x[1])/8, (
  3 x[1])/4, -(x[1]/8)}, {0, 0, 0, x[1]}}

POSTED BY: Hongyi Zhao

Hongyi Zhao

Posted 2 years ago

Hi Milton, I read the referred discussion above, which suggested the method *ClearAll["Global`"]**. But this will erase all user defined variable and function names, but I just want to clear the current definition of the function name if it has been defined.

POSTED BY: Hongyi Zhao

Hans Milton

Posted 2 years ago

Zhao, somewhat off topic: Your use of `ClearAll` in getTrMat // ClearAll; does not really do anything. See this recent discussion.

POSTED BY: Hans Milton

Eric Rimbey

Posted 2 years ago

Also, if you must have a localized codeblock, `Module` is the more expected construct here rather than `Block`.

POSTED BY: Eric Rimbey

Eric Rimbey

Posted 2 years ago

You haven't tested the branch that actually uses `i`, so you can't conclude that yet.

POSTED BY: Eric Rimbey

Hongyi Zhao

Posted 2 years ago

So i is already a local variable in itself and no additional declaration is required: In[740]:= i=7 getTrMat//ClearAll; getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x}, If[ Length[ns]==1, Partition[ns//First,dim], Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim] ] ] ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 0, 0, 1}}; getTrMat[ns] Out[740]= 7 Out[744]= {{3/4, 5/8, 5/8, 0}, {5/8, 3/4, 5/8, -(1/8)}, {5/8, 5/8, 3/ 4, -(1/8)}, {0, 0, 0, 1}}

So i is already a local variable in itself and no additional declaration is required:

In[740]:= i=7
getTrMat//ClearAll;
getTrMat[ns_]:=Block[{dim=Sqrt[Length[ns[[1]]]],x},
If[ Length[ns]==1,
Partition[ns//First,dim],
Partition[Sum[x[i] ns[[i]], {i, Length[ns]}],dim]
]
]

ns ={{3/4, 5/8, 5/8, 0, 5/8, 3/4, 5/8, -(1/8), 5/8, 5/8, 3/4, -(1/8), 0, 
  0, 0, 1}};
getTrMat[ns]

Out[740]= 7

Out[744]= {{3/4, 5/8, 5/8, 0}, {5/8, 3/4, 5/8, -(1/8)}, {5/8, 5/8, 3/
  4, -(1/8)}, {0, 0, 0, 1}}

POSTED BY: Hongyi Zhao

Eric Rimbey

Posted 2 years ago

You could just test. In the form where `i` is not declared locally inside the Block, just add a line before the Block like this: `i=7`. Now, if that breaks your function, you know that `i` needs to be declared locally.

POSTED BY: Eric Rimbey

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