modifying a table column

Hey Sasha,

You probably want to look at the Expr class [1] where you set "out" to be
the same as the original array.

Be Well
Anthony

1. http://pytables.github.io/usersguide/libref/expr_class.html

Post by Oleksandr Huziy
I have a huge table imported from other binary files to hdf, and I forgot
to multiply the data by a factor in one case. Is there an easy way to
multiply a column by a constant factor using pytables?
To modify it in place?
Thank you
--
Sasha
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Oleksandr Huziy

2013-08-27 19:43:58 UTC

Thank you Anthony.

Cheers

Post by Anthony Scopatz
Hey Sasha,
You probably want to look at the Expr class [1] where you set "out" to be
the same as the original array.
Be Well
Anthony
1. http://pytables.github.io/usersguide/libref/expr_class.html

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Oleksandr Huziy

2013-08-27 23:50:34 UTC

I just wanted to make sure if it is possible to use an assignment in
expressions? (this gives me a syntax error exception, complains about the
equal sign in the expression)

h = tb.open_file(path, mode="a")
varTable = h.get_node("/", var_name)
coef = 3 * 60 * 60 #output step
expr = tb.Expr("c = c * m", uservars = {"c": varTable.cols.field, "m":
coef })
expr.eval()
varTable.flush()
h.close()

Is this an optimal way of multiplying a column? (this one works, but I
think it loads all the data into memory...right?)

expr = tb.Expr("c * m", uservars = {"c": varTable.cols.field, "m": coef
})
varTable.cols.field[:] = expr.eval()

Thank you

Cheers

Anthony Scopatz

2013-08-27 23:58:12 UTC

I just wanted to make sure if it is possible to use an assignment in
expressions? (this gives me a syntax error exception, complains about the
equal sign in the expression)

Hi Sasha,

Assignment is a statement not an expression, so it is not possible to use
here. This is why you are getting a syntax error.

Post by Oleksandr Huziy
h = tb.open_file(path, mode="a")
varTable = h.get_node("/", var_name)
coef = 3 * 60 * 60 #output step
coef })
expr.eval()
varTable.flush()
h.close()
Is this an optimal way of multiplying a column? (this one works, but I
think it loads all the data into memory...right?)
coef })
varTable.cols.field[:] = expr.eval()

You are right that this loads the entire computed array into memory and is
therefore not optimal. I would do something like the following:

h = tb.open_file(path, mode="a")
varTable = h.get_node("/", var_name)
coef = 3 * 60 * 60 #output step
c = varTable.cols.field
expr = tb.Expr("c = c * m", uservars = {"c": c, "m": coef })
expr.set_output(c)
expr.eval()
varTable.flush()
h.close()

Be Well
Anthony

Post by Oleksandr Huziy
Thank you
Cheers

Post by Oleksandr Huziy
I have a huge table imported from other binary files to hdf, and I
forgot to multiply the data by a factor in one case. Is there an easy way
to multiply a column by a constant factor using pytables?
To modify it in place?
Thank you
--
Sasha
------------------------------------------------------------------------------
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Oleksandr Huziy

2013-08-28 00:44:57 UTC

Post by Anthony Scopatz
You are right that this loads the entire computed array into memory and is
h = tb.open_file(path, mode="a")
varTable = h.get_node("/", var_name)
coef = 3 * 60 * 60 #output step
c = varTable.cols.field
expr = tb.Expr("c * m", uservars = {"c": c, "m": coef })
expr.set_output(c)
expr.eval()
varTable.flush()
h.close()

Aha, this is cool. Thanks Anthony.

Cheers
--
Sasha

On Tue, Aug 27, 2013 at 11:44 AM, Oleksandr Huziy <

Post by Oleksandr Huziy
I have a huge table imported from other binary files to hdf, and I
forgot to multiply the data by a factor in one case. Is there an easy way
to multiply a column by a constant factor using pytables?
To modify it in place?
Thank you
--
Sasha
------------------------------------------------------------------------------
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tutorial videos with LearnDevNow. Subscribe today and save!
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Anthony Scopatz

2013-08-28 00:51:32 UTC

Glad I could help!

Post by Anthony Scopatz
You are right that this loads the entire computed array into memory and
h = tb.open_file(path, mode="a")
varTable = h.get_node("/", var_name)
coef = 3 * 60 * 60 #output step
c = varTable.cols.field
expr = tb.Expr("c * m", uservars = {"c": c, "m": coef })
expr.set_output(c)
expr.eval()
varTable.flush()
h.close()

Aha, this is cool. Thanks Anthony.
Cheers
--
Sasha