converting lists to strings to lists

[robin]

hi,

i'm doing some udp stuff and receive strings of the form '0.870000
0.250000 0.790000;\n'
what i'd need though is a list of the form [0.870000 0.250000 0.790000]
i got to the [0:-3] part to obtain a string '0.870000 0.250000
0.790000' but i can't find a way to convert this into a list. i tried
eval() but this gives me the following error:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
.870000 0.250000 0.79000

and i have the same problem the other way round. e.g. i have a list
which i need to convert to a string in order to send it via udp.
btw: i cannot use pickle, since i'm sending stuff to a LISP programme.

thank you in advance for your help!
best,

robin

 

[Eric Deveaud]

hi,

i'm doing some udp stuff and receive strings of the form '0.870000
0.250000 0.790000;\n'
what i'd need though is a list of the form [0.870000 0.250000 0.790000]
i got to the [0:-3] part to obtain a string '0.870000 0.250000
0.790000' but i can't find a way to convert this into a list. i tried
eval() but this gives me the following error:

check pydoc string about split

my_string = '0.870000 0.250000 0.790000;\n'
my_list = my_string.split()
print my_list

and i have the same problem the other way round. e.g. i have a list
which i need to convert to a string in order to send it via udp.

my_new_string = ' '.join(my_list)
print my_new_string

Eric

 

[gry]

Read about string split and join. E.g.:
l = '0.870000 0.250000 0.790000'
floatlist = [float(s) for s in l.split()]

In the other direction:
floatlist = [0.87, 0.25, 0.79000000000000004]
outstring = ' '.join(floatlist)

If you need to control the precision(i.e. suppress the 00004), read
about
the string formatting operator "%".

-- George Young

 

[robin]

thanks for your answer. split gives me a list of strings, but i found a
way to do what i want:

input='0.1, 0.2, 0.3;\n'
input = list(eval(input[0:-2]))
print input

[0.10000000000000001, 0.20000000000000001, 0.29999999999999999]

this does fine... but now, how do i convert this list to a string?

my_new_string = ' '.join(input)

gives me:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: sequence item 0: expected string, float found

thanks,

robin

 

[Steven D'Aprano]

hi,

i'm doing some udp stuff and receive strings of the form '0.870000
0.250000 0.790000;\n'
what i'd need though is a list of the form [0.870000 0.250000 0.790000]
i got to the [0:-3] part to obtain a string '0.870000 0.250000
0.790000' but i can't find a way to convert this into a list. i tried
eval() but this gives me the following error:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
.870000 0.250000 0.79000

# untested!
def str2list(s):
"""Expects a string of the form '0.87 0.25 0.79;\n' and
returns a list like [0.87, 0.25, 0.79]

WARNING: this function has only limited error checking.
"""
s = s.strip() # ignore leading and trailing whitespace
if s.endswith(';'):
s = s[:-1]
L = s.split()
assert len(L) == 3, "too many or too few items in list."
return [float(f) for f in L]

and i have the same problem the other way round. e.g. i have a list
which i need to convert to a string in order to send it via udp.

That's even easier.

def list2str(L):
"""Expects a list like [0.87, 0.25, 0.79] and returns a string
of the form '0.870000 0.250000 0.790000;\n'.
"""
return "%.6f %.6f %.6f;\n" % tuple(L)

 

[Peter Hansen]

i'm doing some udp stuff and receive strings of the form '0.870000
0.250000 0.790000;\n'
what i'd need though is a list of the form [0.870000 0.250000 0.790000]
i got to the [0:-3] part to obtain a string '0.870000 0.250000

Actually, that's already a bug. You want [0:-2] if you're going to do
it that way. Unless you meant that your string actually has a backslash
and an "n" character, which is doubtful...

You should probably use something more like Steven's solution, although
I'd personally use s.strip(';') or at least s.rstrip(';') if I had
semicolons to remove, rather than a combined endswith(';') and slicing
the last character off.

0.790000' but i can't find a way to convert this into a list. i tried
eval() but this gives me the following error:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
.870000 0.250000 0.79000

(If you'd posted the full error (that's an incomplete traceback),
someone would probably have pointed out how to interpret what Python
told you so that you'd already have figured out what was wrong with that...)

-Peter

 

[robin]

yo!

thank you everone! here's how i finally did it:
converting the string into a list:

input = net.receiveUDPData()
input = list(eval(input[0:-2]))

converting the list into a string:

sendout = "%.6f %.6f %.6f;\n" % tuple(winningvector)

maybe i'll even find a way to generalize the list2string bit, so it
accepts arbitrary sized lists...
thanks,

robin

 

[bruno at modulix]

thanks for your answer. split gives me a list of strings,

Of course, why should it be otherwise ?-)

More seriously : Python doesn't do much automagical conversions. Once
you've got your list of strings, you have to convert'em to floats.

but i found a
way to do what i want:

input='0.1, 0.2, 0.3;\n'
input = list(eval(input[0:-2]))

- eval() is potentially harmful. Using it on untrusted inputs is a bad
idea. In fact, using eval() (or exec) is a bad idea in most cases.

- removing trailing or leading chars is best done with str.strip()

Also, your code is not as readable as the canonical solution (split() +
float)

print input

[0.10000000000000001, 0.20000000000000001, 0.29999999999999999]

input = map(float, input.strip('\n;').split(","))
[0.10000000000000001, 0.20000000000000001, 0.29999999999999999]

this does fine... but now, how do i convert this list to a string?

my_new_string = ' '.join(input)

gives me:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: sequence item 0: expected string, float found

Same as above - you need to do the conversion:
' '.join(map(str, input))

 

[bruno at modulix]

yo!

thank you everone! here's how i finally did it:
converting the string into a list:

input = net.receiveUDPData()
input = list(eval(input[0:-2]))

You'll run into trouble with this.

converting the list into a string:

sendout = "%.6f %.6f %.6f;\n" % tuple(winningvector)

maybe i'll even find a way to generalize the list2string bit, so it
accepts arbitrary sized lists...

sendout = "%s;\n" % " ".join(["%.6f" % float(s) for s in winningvector])

 

[Eric Deveaud]

yo!

thank you everone! here's how i finally did it:
converting the string into a list:

input = net.receiveUDPData()
input = list(eval(input[0:-2]))

no pun intented but as you did not know how to use split and join,
please please DON'T USE eval

Eric

 

[Dave Hansen]

hi,

i'm doing some udp stuff and receive strings of the form '0.870000
0.250000 0.790000;\n'
what i'd need though is a list of the form [0.870000 0.250000 0.790000]
i got to the [0:-3] part to obtain a string '0.870000 0.250000
0.790000' but i can't find a way to convert this into a list. i tried
eval() but this gives me the following error:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 1
.870000 0.250000 0.79000

and i have the same problem the other way round. e.g. i have a list
which i need to convert to a string in order to send it via udp.
btw: i cannot use pickle, since i'm sending stuff to a LISP programme.

Here's a little playing around I did the the interactive interpreter.
Not guaranteed to be the most efficient or Pythonic, but it seems to
work...

instr = '0.870000 0.250000 0.790000;\n'
instr '0.870000 0.250000 0.790000;\n'
ell = [float(x) for x in instr[:-2].split()]
print ell [0.87, 0.25, 0.79000000000000004]
outstr = " ".join(["%8.6f"%x for x in ell]) + ";\n"
outstr '0.870000 0.250000 0.790000;\n'
instr == outstr

True

Regards,
-=Dave