Parse expression with indexing

[Loïc Gouarin]

Hi,

I would like to parse a string expression like that

from sympy.parsing.sympy_parser import parse_expr

parse_expr("m[0][0]")

But I have an error with 'Symbol' object does not support indexing.

If I do

sympy.Symbol("m[0][0]") 

everything is ok.

I try to write my own transformation funtion that takes the "m[x][y]" and creates a Symbol and add it to the standard_transformations in parse_expr but the problem is still the same because the problem is in auto_symbol and it parses again my string 'm[x][y]'.

Could you tell me how I can do that ?

Thanks,

Loic

[Aaron Meurer]

Does it work if you transform it literally to sympy.Symbol("m[0][0]")?

Aaron Meurer

>
> Could you tell me how I can do that ?
>
> Thanks,
> Loic
>

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[Loïc Gouarin]

Le dimanche 26 avril 2015 21:03:02 UTC+2, Aaron Meurer a écrit :

On Sun, Apr 26, 2015 at 12:27 AM, Loïc Gouarin <loic.g...@gmail.com> wrote:
> Hi,
>
> I would like to parse a string expression like that
>
> from sympy.parsing.sympy_parser import parse_expr
> parse_expr("m[0][0]")
>
> But I have an error with 'Symbol' object does not support indexing.
>
> If I do
>
> sympy.Symbol("m[0][0]")
>
> everything is ok.
>
> I try to write my own transformation funtion that takes the "m[x][y]" and
> creates a Symbol and add it to the standard_transformations in parse_expr
> but the problem is still the same because the problem is in auto_symbol and
> it parses again my string 'm[x][y]'.

Does it work if you transform it literally to sympy.Symbol("m[0][0]")?

Hi Aaron,

 

your solution is almost what I want. There is no error in the parse_exp but I have in the result Symbol("m[x][y]") and not m[x][y]. I know thatit  is the same but I parse again the result and I need m[x][y]. 

Is there an easy way to have on the ouput m[x][y] and not Symbol("m[x][y]") ?

Thanks,

Loic 

[Loïc Gouarin]

Le lundi 27 avril 2015 08:59:38 UTC+2, Loïc Gouarin a écrit :

Le dimanche 26 avril 2015 21:03:02 UTC+2, Aaron Meurer a écrit :

On Sun, Apr 26, 2015 at 12:27 AM, Loïc Gouarin <loic.g...@gmail.com> wrote:
> Hi,
>
> I would like to parse a string expression like that
>
> from sympy.parsing.sympy_parser import parse_expr
> parse_expr("m[0][0]")
>
> But I have an error with 'Symbol' object does not support indexing.
>
> If I do
>
> sympy.Symbol("m[0][0]")
>
> everything is ok.
>
> I try to write my own transformation funtion that takes the "m[x][y]" and
> creates a Symbol and add it to the standard_transformations in parse_expr
> but the problem is still the same because the problem is in auto_symbol and
> it parses again my string 'm[x][y]'.

Does it work if you transform it literally to sympy.Symbol("m[0][0]")?

Hi Aaron,

 

your solution is almost what I want. There is no error in the parse_exp but I have in the result Symbol("m[x][y]") and not m[x][y]. I know thatit  is the same but I parse again the result and I need m[x][y]. 

Is there an easy way to have on the ouput m[x][y] and not Symbol("m[x][y]") ?

 

Ok I found my error. My transformation was incorrect. I wrote

from sympy.parsing.sympy_parser import parse_expr, standard_transformations

def auto_moments(tokens, local_dict, global_dict):
    result = []
    i = 0
    while(i < len(tokens)):
        tokNum, tokVal = tokens[i]
        if tokVal == 'm':
            name = ''.join([val for n, val in tokens[i:i+7]])
            result.append((tokNum, 'Symbol("{0}")'.format(name)))

            i += 7
        else:
            result.append(tokens[i])
            i += 1
    return result

s = parse_expr("m[0][0]", transformations=(auto_moments,) + standard_transformations)

The result was 

Symbol("m[0][0]")

But if I write 

from sympy.parsing.sympy_parser import parse_expr, standard_transformations

def auto_moments(tokens, local_dict, global_dict):
    result = []
    i = 0
    while(i < len(tokens)):
        tokNum, tokVal = tokens[i]
        if tokVal == 'm':
            name = ''.join([val for n, val in tokens[i:i+7]])
            result.extend([(1, 'Symbol'), (51, '('), (3, "'{0}'".format(name)), (51, ')')])
            i += 7

        else:
            result.append(tokens[i])
            i += 1
    return result

s = parse_expr("m[0][0]", transformations=(auto_moments,) + standard_transformations)
print s

Now it is ok. I have

m[0][0]

Thanks,

Loic