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Mathematics Physics Mathematica Algebra Numerical Computation

Constructing matrix elements using functions and looping

hbar omega

hbar omega, TIFR, India

Posted 10 years ago

Hi I'm trying to construct a tridiagonal matrix which contains a function f(x,y)=2(cos x+cosy). My code is: Nmax = 2; For[iw = 1, iw <= 5, iw++, A[x_, y_] := Normal[SparseArray[{{i_, j_} /; j == i :> iw + 2 (Cos[x] + Cos[y]), {i_, j_} /; j - i == 1 :> 1, {i_, j_} /; i - j == 1 :> 1}, {Nmax, Nmax}]]; Print["Testing A-matrix at (x,y)=(\[Pi]/2,\[Pi]/2)"]; MatrixForm[A[Pi/2, Pi/2]] // N ] MatrixForm[A[Pi/2, Pi/2]] // N Why does it fail to show the matrix inside the For-loop? Thanks

Hi I'm trying to construct a tridiagonal matrix which contains a function f(x,y)=2(cos x+cosy). My code is:

Nmax = 2;
For[iw = 1, iw <= 5, iw++, 
 A[x_, y_] := 
  Normal[SparseArray[{{i_, j_} /; j == i :> 
      iw + 2 (Cos[x] + Cos[y]), {i_, j_} /; j - i == 1 :> 
      1, {i_, j_} /; i - j == 1 :> 1}, {Nmax, Nmax}]];
 Print["Testing A-matrix at (x,y)=(\[Pi]/2,\[Pi]/2)"];
 MatrixForm[A[Pi/2, Pi/2]] // N
 ]
MatrixForm[A[Pi/2, Pi/2]] // N

Why does it fail to show the matrix inside the For-loop?

Thanks

POSTED BY: hbar omega

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David Reiss

David Reiss, Scientific Arts

Posted 10 years ago

A For loop, like a Do loop, does not produce an output--it simply executes the code within. So you could get what you are wishing for by adding a Print statement to the MatrixForm expression that you want to see as in: Nmax = 2; For[iw = 1, iw <= 5, iw++, A[x_, y_] := Normal[SparseArray[{{i_, j_} /; j == i :> iw + 2 (Cos[x] + Cos[y]), {i_, j_} /; j - i == 1 :> 1, {i_, j_} /; i - j == 1 :> 1}, {Nmax, Nmax}]]; Print["Testing A-matrix at (x,y)=(\[Pi]/2,\[Pi]/2)"]; Print@MatrixForm[A[Pi/2, Pi/2]] // N] MatrixForm[A[Pi/2, Pi/2]] // N

A For loop, like a Do loop, does not produce an output--it simply executes the code within. So you could get what you are wishing for by adding a Print statement to the MatrixForm expression that you want to see as in:

Nmax = 2;
For[iw = 1, iw <= 5, iw++, 
 A[x_, y_] := 
  Normal[SparseArray[{{i_, j_} /; j == i :> 
      iw + 2 (Cos[x] + Cos[y]), {i_, j_} /; j - i == 1 :> 
      1, {i_, j_} /; i - j == 1 :> 1}, {Nmax, Nmax}]];
 Print["Testing A-matrix at (x,y)=(\[Pi]/2,\[Pi]/2)"];
 Print@MatrixForm[A[Pi/2, Pi/2]] // N]
MatrixForm[A[Pi/2, Pi/2]] // N

POSTED BY: David Reiss

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