List Manipulation

[Roman]

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:

When I run I get no result

cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))
cnt = cnt + 1

print p

when I change it to the following, I get rows back

cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
print col
cnt = cnt + 1

print p


Thanks in advance

[Mike Kent]

Re: List Manipulation

Roman wrote:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:
>
When I run I get no result
>
cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))
cnt = cnt + 1
>
print p

I'm having trouble deciding what you *intend* this program to do. It
looks like you want to take the first 7 lines of the input file, and
append all the data elements in those lines into one long list. If
that's what you want to do, then you are almost there, although you
could have written it better. If that's NOT what you want to do...
well, there are tutorials.

The problem with this code is in the line 'p[:0].append(str(col )).
Given a list p, p[:0] will give you the part of p *prior* to element 0.
Since there is never anything in a list prior to element 0, you will
always get an empty list back.

I assume this is not what you intended. But just *what* do you intend?
I sure can't say.

[Iain King]

Re: List Manipulation


Roman wrote:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:
>
When I run I get no result
>
cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))

What are you trying to do here? p[:0] returns a new list, of all the
elements in p up to element 0 (which is of course the empty list),
which is then appended to, but is not stored anywhere. If you want to
insert str(col) then use p.insert


Iain

[Laszlo Nagy]

Re: List Manipulation

Roman írta:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:
>
When I run I get no result
>
cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))
>

You are appending to a slice. In that case, p[:0] creates a new list
object. You are not appending to p but to a new object (created from a
slice).
If you need to insert an item at the begining of a list, use the insert
method instead.

>>l = [2,3,4]
>>l.insert(1, 0)
>>l

[2, 0, 3, 4]


Best,

Laszlo

[Steven D'Aprano]

Re: List Manipulation

On Tue, 04 Jul 2006 07:01:55 -0700, Roman wrote:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:
>
When I run I get no result

What do you mean? Does it print None?

cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break

That's a very unPythonic way of doing the job. The usual way of doing
this would be something like this:

for line in reader[:7]:
# no need for the "if cnt 6: break" clause now

for col in line:
p[:0].append(str(col ))

p[:0] creates a new list, which has a string appended to it, and is then
thrown away. What are you trying to do?

If you are trying to insert the new entry at the beginning of the list,
you probably want this:

p.insert(0, str(col))

[Roman]

Re: List Manipulation

Thanks for your help

My intention is to create matrix based on parsed csv file. So, I would
like to have a list of columns (which are also lists).

I have made the following changes and it still doesn't work.


cnt = 0
p=[[], [], [], [], [], [], [], [], [], [], []]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
j = 0
for col in line:
p[j].append(col)
j=j+1
cnt = cnt + 1

print p

Iain King wrote:

Roman wrote:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:

When I run I get no result

cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))

>
What are you trying to do here? p[:0] returns a new list, of all the
elements in p up to element 0 (which is of course the empty list),
which is then appended to, but is not stored anywhere. If you want to
insert str(col) then use p.insert
>
>
Iain

[Sibylle Koczian]

Re: List Manipulation

Roman schrieb:

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:
>
When I run I get no result
>
cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
p[:0].append(str(col ))

This is wrong. I'm not absolutely certain _what_ it does, but it doesn't
append anything to list p. p[:0] is an empty copy of p, you are
appending to this empty copy, not to p. What's wrong with
p.append(str(co l))?

when I change it to the following, I get rows back
>
cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
for col in line:
print col
cnt = cnt + 1
>
print p
>

Here you print every single cell, but p doesn't change.

HTH
Koczian

--
Dr. Sibylle Koczian
Universitaetsbi bliothek, Abt. Naturwiss.
D-86135 Augsburg
e-mail : Sibylle.Koczian @Bibliothek.Uni-Augsburg.DE

[Mike Kent]

Re: List Manipulation

Roman wrote:

Thanks for your help
>
My intention is to create matrix based on parsed csv file. So, I would
like to have a list of columns (which are also lists).
>
I have made the following changes and it still doesn't work.
>
>
cnt = 0
p=[[], [], [], [], [], [], [], [], [], [], []]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break
j = 0
for col in line:
p[j].append(col)
j=j+1
cnt = cnt + 1
>
print p

p[j] does not give you a reference to an element inside p. It gives
you a new sublist containing one element from p. You then append a
column to that sublist. Then, since you do nothing more with that
sublist, YOU THROW IT AWAY.

Try doing:

p[j] = p[j].append(col)

However, this will still result in inefficient code. Since every line
you read in via the csv reader is already a list, try this (untested)
instead:

reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
p = [ line for line in reader[:7] ]

[Diez B. Roggisch]

Re: List Manipulation

p[j] does not give you a reference to an element inside p. It gives

you a new sublist containing one element from p. You then append a
column to that sublist. Then, since you do nothing more with that
sublist, YOU THROW IT AWAY.

Not correct.

p = [[]]
p[0].append(1)
print p

yields

[[1]]

p[0] _gives_ you a reference to an object. If it is mutable (list are) and
append mutates it (it does), the code is perfectly alright.

I don't know what is "not working" for the OP, but actually his code works
if one replaces the csv-reading with a generated list:

cnt = 0
p=[[], [], [], [], [], [], [], [], [], [], []]
reader = [["column_%i" % c for c in xrange(5)] for l in xrange(7)]
for line in reader:
if cnt 6:
break
j = 0
for col in line:
p[j].append(col)
j=j+1
cnt = cnt + 1
print p


You are right of course that it is the most unpythonic way imaginabe to do
it. But it works.

Diez

[Roman]

Re: List Manipulation

Nothing got printed.

Could you tell me what would be pythonic version of what I am trying to
do?


Diez B. Roggisch wrote:

p[j] does not give you a reference to an element inside p. It gives
you a new sublist containing one element from p. You then append a
column to that sublist. Then, since you do nothing more with that
sublist, YOU THROW IT AWAY.

>
Not correct.
>
p = [[]]
p[0].append(1)
print p
>
yields
>
[[1]]
>
p[0] _gives_ you a reference to an object. If it is mutable (list are) and
append mutates it (it does), the code is perfectly alright.
>
I don't know what is "not working" for the OP, but actually his code works
if one replaces the csv-reading with a generated list:
>
cnt = 0
p=[[], [], [], [], [], [], [], [], [], [], []]
reader = [["column_%i" % c for c in xrange(5)] for l in xrange(7)]
for line in reader:
if cnt 6:
break
j = 0
for col in line:
p[j].append(col)
j=j+1
cnt = cnt + 1
print p
>
>
You are right of course that it is the most unpythonic way imaginabe to do
it. But it works.
>
Diez

[Bruno Desthuilliers]

Re: List Manipulation

Roman wrote:
(please dont top-post - corrected)

>
Iain King wrote:
>

>>Roman wrote:
>>

>>>I would appreciate it if somebody could tell me where I went wrong in
>>>the following snipet:
>>>

(snip)

>>What are you trying to do here? p[:0] returns a new list, of all the
>>elements in p up to element 0 (which is of course the empty list),
>>which is then appended to, but is not stored anywhere. If you want to
>>insert str(col) then use p.insert
>>
>>
>>Iain

>
>
Thanks for your help
>
My intention is to create matrix based on parsed csv file. So, I
would like to have a list of columns (which are also lists).

csv = [
['L0C0', 'L0C1', 'L0C2'],
['L1C0', 'L1C1', 'L1C2'],
['L2C0', 'L2C1', 'L2C2'],
]

matrix = [[[] for l in range(len(csv))] for c in range(len(csv[0]))]

for numline, line in enumerate(csv):
for numcol, col in enumerate(line) :
matrix[numcol][numline] = col

assert matrix == [
['L0C0', 'L1C0', 'L2C0'],
['L0C1', 'L1C1', 'L2C1'],
['L0C2', 'L1C2', 'L2C2']
]

NB : There are probably more elegant solutions.

I have made the following changes and it still doesn't work.

"doesn't work" is the worst possible description of a problem...

(snip)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Roman]

Re: List Manipulation

I am getting

TypeError: unsubscriptable object

when specifying

for line in reader[:7]:


Steven D'Aprano wrote:

On Tue, 04 Jul 2006 07:01:55 -0700, Roman wrote:
>

I would appreciate it if somebody could tell me where I went wrong in
the following snipet:

When I run I get no result

>
What do you mean? Does it print None?
>

cnt = 0
p=[]
reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
quotechar="'", delimiter='\t')
for line in reader:
if cnt 6:
break

>
That's a very unPythonic way of doing the job. The usual way of doing
this would be something like this:
>
for line in reader[:7]:
# no need for the "if cnt 6: break" clause now
>
>

for col in line:
p[:0].append(str(col ))

>
p[:0] creates a new list, which has a string appended to it, and is then
thrown away. What are you trying to do?
>
If you are trying to insert the new entry at the beginning of the list,
you probably want this:
>
p.insert(0, str(col))

[Bruno Desthuilliers]

Re: List Manipulation

Roman wrote:
(please dont top-post - corrected)

>
Steven D'Aprano wrote:
>

>>On Tue, 04 Jul 2006 07:01:55 -0700, Roman wrote:
>>
>>

(snip)

>>

>>>cnt = 0
>>>p=[]
>>>reader = csv.reader(file ("f:\webserver\ inp.txt"), dialect="excel" ,
>> quotechar="'", delimiter='\t')
>>>for line in reader:
>> if cnt 6:
>> break

>>
>>That's a very unPythonic way of doing the job. The usual way of doing
>>this would be something like this:
>>
>>for line in reader[:7]:
> # no need for the "if cnt 6: break" clause now
>>

I am getting
>
TypeError: unsubscriptable object
>
when specifying
>
for line in reader[:7]:
>

Should have been :

for line in list(reader)[:7]:
# code here


--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Roman]

Re: List Manipulation

Thanks for spending so much time with me. I had since made the
following change.

matrix = [[[] for l in range(len(list( reader)[:10]))] for c in
range(len(list( reader)[7]))]
for numline, line in enumerate(reade r):
for numcol, col in enumerate(line) :
matrix[numcol][numline] = col

print matrix

I don't get mistakes anymore. However, still nothing gets printed
Bruno Desthuilliers wrote:

Roman wrote:
(please dont top-post - corrected)

Iain King wrote:

>Roman wrote:
>
>>I would appreciate it if somebody could tell me where I went wrong in
>>the following snipet:
>>

(snip)
>

>What are you trying to do here? p[:0] returns a new list, of all the
>elements in p up to element 0 (which is of course the empty list),
>which is then appended to, but is not stored anywhere. If you want to
>insert str(col) then use p.insert
>
>
>Iain

Thanks for your help

My intention is to create matrix based on parsed csv file. So, I
would like to have a list of columns (which are also lists).

>
csv = [
['L0C0', 'L0C1', 'L0C2'],
['L1C0', 'L1C1', 'L1C2'],
['L2C0', 'L2C1', 'L2C2'],
]
>
matrix = [[[] for l in range(len(csv))] for c in range(len(csv[0]))]
>
for numline, line in enumerate(csv):
for numcol, col in enumerate(line) :
matrix[numcol][numline] = col
>
assert matrix == [
['L0C0', 'L1C0', 'L2C0'],
['L0C1', 'L1C1', 'L2C1'],
['L0C2', 'L1C2', 'L2C2']
]
>
NB : There are probably more elegant solutions.
>

I have made the following changes and it still doesn't work.

>
"doesn't work" is the worst possible description of a problem...
>
(snip)
>
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Bruno Desthuilliers]

Re: List Manipulation

Roman a écrit :

<ot>
Roman, please stop top-posting and learn to quote.
</ot>

Bruno Desthuilliers wrote:
>

>>Roman wrote:
>>(please dont top-post - corrected)
>>

>>>
>>>My intention is to create matrix based on parsed csv file. So, I
>>>would like to have a list of columns (which are also lists).

>>
>>csv = [
> ['L0C0', 'L0C1', 'L0C2'],
> ['L1C0', 'L1C1', 'L1C2'],
> ['L2C0', 'L2C1', 'L2C2'],
>]
>>
>>matrix = [[[] for l in range(len(csv))] for c in range(len(csv[0]))]
>>
>>for numline, line in enumerate(csv):
> for numcol, col in enumerate(line) :
> matrix[numcol][numline] = col
>>
>>assert matrix == [
> ['L0C0', 'L1C0', 'L2C0'],
> ['L0C1', 'L1C1', 'L2C1'],
> ['L0C2', 'L1C2', 'L2C2']
>]
>>

Thanks for spending so much time with me. I had since made the
following change.
>
matrix = [[[] for l in range(len(list( reader)[:10]))] for c in
range(len(list( reader)[7]))]

Technically; this won't work. The first call to list(reader) will
consume reader.

It's also somewhat dumb (len(list(reade r)[:10]) is always 10) and
inefficient (you're calling list(reader) twice, when you could call it
just once).

Instead of trying anything at random, read the fine manual and try to
understand the example I gave you.

for numline, line in enumerate(reade r):
for numcol, col in enumerate(line) :
matrix[numcol][numline] = col
>
print matrix
>
I don't get mistakes anymore. However, still nothing gets printed

If you don't print anything, nothing will be printed.