Skip to main content
Mathematica

Return to Question

added 82 characters in body
Source Link
rogerl
rogerl
  • 4.3k
  • 3
  • 28
  • 43

I have a list of integers; I would like to replace anything of the form $x,y,z,2$ in such a list with $x+1, y+1, 0, 0$. The problem is that the $2$ may appear at the front, or in the first three elements, of the sequence (in which case I want to just pretend that the missing leading elements are zero), so I'm trying to use optional pattern values to deal with that. Here is what I've tried (with a different pattern output just to figure out what is going on)

{0, 1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 0, 1, 1} (* this is what I'd expect. x matches to the null string *)
{1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 1, 1, 0} (* I would have expected a=0 and b=c=1 *)
{1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 1, 0, 0} (* I would have expected a=b=0, c=1 *)
{2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 0, 0, 0} (* This matched as I would expect *)

Clearly I'm misunderstanding how this pattern works. Can someone enlighten me?

I would really like to know not only what the right way to do this is, but also why what I tried did not work. I've read the section in Shifrin's book on patterns; are there other good resources to understand how patterns work?

I have a list of integers; I would like to replace anything of the form $x,y,z,2$ in such a list with $x+1, y+1, 0, 0$. The problem is that the $2$ may appear at the front, or in the first three elements, of the sequence, so I'm trying to use optional pattern values to deal with that. Here is what I've tried (with a different pattern output just to figure out what is going on)

{0, 1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 0, 1, 1} (* this is what I'd expect. x matches to the null string *)
{1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 1, 1, 0} (* I would have expected a=0 and b=c=1 *)
{1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 1, 0, 0} (* I would have expected a=b=0, c=1 *)
{2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 0, 0, 0} (* This matched as I would expect *)

Clearly I'm misunderstanding how this pattern works. Can someone enlighten me?

I would really like to know not only what the right way to do this is, but also why what I tried did not work. I've read the section in Shifrin's book on patterns; are there other good resources to understand how patterns work?

I have a list of integers; I would like to replace anything of the form $x,y,z,2$ in such a list with $x+1, y+1, 0, 0$. The problem is that the $2$ may appear at the front, or in the first three elements, of the sequence (in which case I want to just pretend that the missing leading elements are zero), so I'm trying to use optional pattern values to deal with that. Here is what I've tried (with a different pattern output just to figure out what is going on)

{0, 1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 0, 1, 1} (* this is what I'd expect. x matches to the null string *)
{1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 1, 1, 0} (* I would have expected a=0 and b=c=1 *)
{1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 1, 0, 0} (* I would have expected a=b=0, c=1 *)
{2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 0, 0, 0} (* This matched as I would expect *)

Clearly I'm misunderstanding how this pattern works. Can someone enlighten me?

I would really like to know not only what the right way to do this is, but also why what I tried did not work. I've read the section in Shifrin's book on patterns; are there other good resources to understand how patterns work?

Source Link
rogerl
rogerl
  • 4.3k
  • 3
  • 28
  • 43

Matching x,y,z,2

I have a list of integers; I would like to replace anything of the form $x,y,z,2$ in such a list with $x+1, y+1, 0, 0$. The problem is that the $2$ may appear at the front, or in the first three elements, of the sequence, so I'm trying to use optional pattern values to deal with that. Here is what I've tried (with a different pattern output just to figure out what is going on)

{0, 1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 0, 1, 1} (* this is what I'd expect. x matches to the null string *)
{1, 1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {x, a, b, c}
    {{}, 1, 1, 0} (* I would have expected a=0 and b=c=1 *)
{1, 2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 1, 0, 0} (* I would have expected a=b=0, c=1 *)
{2} /. {x___, a_: 0, b_: 0, c_: 0, 2, y___} :> {{x}, a, b, c}
    {{}, 0, 0, 0} (* This matched as I would expect *)

Clearly I'm misunderstanding how this pattern works. Can someone enlighten me?

I would really like to know not only what the right way to do this is, but also why what I tried did not work. I've read the section in Shifrin's book on patterns; are there other good resources to understand how patterns work?