add elements to indexed list locations

[leventyilmaz]

Hi,

I have a very simple problem, but do not know an elegant way to
accomplish this.
###
# I have a list of names:
names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']

# and another set of names that I want to insert into
# the names list at some indexed locations:
surnames = { 1: 'kent', 3:'parker', 5:'wayne' }

# The thing I couldn't figure out is, after I insert a
# surname the rest of the indices are not valid.
# That is, the following won't work:
for i, x in surnames.iteritems():
names.insert(i,surnames)
###

I am searching a nice way to do this. For instance, is there a more
robust way to store indices (as some sort of pointers maybe?)

- Levent

 

[Alexis Roda]

En/na (e-mail address removed) ha escrit:

Hi,

I have a very simple problem, but do not know an elegant way to
accomplish this.
###
# I have a list of names:
names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']

# and another set of names that I want to insert into
# the names list at some indexed locations:
surnames = { 1: 'kent', 3:'parker', 5:'wayne' }

# The thing I couldn't figure out is, after I insert a
# surname the rest of the indices are not valid.
# That is, the following won't work:
for i, x in surnames.iteritems():
names.insert(i,surnames)
###

In my previous post I've misunderstood the problem. Here is a valid
solution:

keys = surnames.keys()
keys.sort()
count = 0
for i in keys :
names.insert(i + count, surnames)
count = count + 1


HTH

 

[Diez B. Roggisch]

Hi,

I have a very simple problem, but do not know an elegant way to
accomplish this.
###
# I have a list of names:
names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']

# and another set of names that I want to insert into
# the names list at some indexed locations:
surnames = { 1: 'kent', 3:'parker', 5:'wayne' }

# The thing I couldn't figure out is, after I insert a
# surname the rest of the indices are not valid.
# That is, the following won't work:
for i, x in surnames.iteritems():
names.insert(i,surnames)
###

I am searching a nice way to do this. For instance, is there a more
robust way to store indices (as some sort of pointers maybe?)

Use a dictionary for both of them?

The concept of indices is that they IMPLY a position. So either you work
in a way that you e.g. add the surnames in a defined order and adjust
the subsequent indices, or you discard the approach entirely and e.g use
a map of first to surnames.

Diez

 

[Steven Bethard]

# I have a list of names:
names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']

# and another set of names that I want to insert into
# the names list at some indexed locations:
surnames = { 1: 'kent', 3:'parker', 5:'wayne' }

# The thing I couldn't figure out is, after I insert a
# surname the rest of the indices are not valid.
# That is, the following won't work:
for i, x in surnames.iteritems():
names.insert(i,surnames)

This seems to work (tested only with what you see below)::

>>> names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']
>>> surnames = {1:'kent', 3:'parker', 5:'wayne'}
>>> for index in sorted(surnames, reverse=True):

... names.insert(index, surnames[index])
... ['clark', 'kent', 'super', 'peter', 'parker', 'spider', 'bruce',
'wayne', 'bat']

I just did the inserts from right to left, that is, starting at the end.
That way, after an insert, I don't have to adjust any indices.

You may also find that if you do a lot of inserts into the list, it may
be more efficient to create a new list, e.g.::

>>> names = ['clark', 'super', 'peter', 'spider', 'bruce', 'bat']
>>> surnames = {1:'kent', 3:'parker', 5:'wayne'}
>>> new_names = []
>>> for i, name in enumerate(names):

... if i in surnames:
... new_names.append(surnames)
... new_names.append(name)
... ['clark', 'kent', 'super', 'peter', 'parker', 'spider', 'bruce',
'wayne', 'bat']


STeVe

 

[levent]

Thanks for the answers. Enumerating in reverse is indeed quite a smart
idea.

The fact is though, I overly simplified the task in the super-hero
example. In the real case, the dictionary keys are not necessarily the
indices for inserts; that is to say, the inserts do not necessarily
take place in some sorted order.

I think I was thinking more of a linked-list idea, where you do not
store the indices as integers to some random access array but rather as
pointers into list's nodes. Then the subsequent inserts would not hurt
previously stored pointers. For those who know a bit C++/STL here is a
sketch of the idea:

name_list heros;
heros.push_back("clark");
// ... add the rest
indexed_name_list surnames;
surnames.push_back(
make_pair( find( heros.begin(), heros.end(), "clark"), "kent") )
); // the find function returns an iterator to appropriate location
// ... add the rest

for_each(surnames.begin(), surnames.end(), insert_surnames)
// insert_surnames is a callback that receives a single indexed surname
// at a time and does the job, without affecting outer iterators.


I was wondering how to make indices as *robust* in Python... Any ideas?


PS: following is the compilable code-let of the task in C++

// ====================================================
#include <iostream>
#include <list>
#include <vector>
#include <map>
#include <string>
#include <algorithm>

using namespace std;
typedef string name;
typedef pair<list<name>::iterator, string> indexed_name;

void insert_name( list<name>* into, indexed_name in ) {
into->insert(in.first, in.second);
}

int main() {
using namespace std;
// Define super-heros (fathers ignored)
list<name> heros;
heros.push_back("super");
heros.push_back("clark");
heros.push_back("spider");
heros.push_back("peter");
heros.push_back("bat");
heros.push_back("bruce");

// Assign father names to proper son
list<indexed_name> surnames;
surnames.push_back( // ++ is to have surname inserted _after_ the
name
make_pair(++find(heros.begin(),heros.end(),"clark"),
string("kent"))
);
surnames.push_back(
make_pair(++find(heros.begin(),heros.end(),"peter"),
string("parker"))
);
surnames.push_back(
make_pair(++find(heros.begin(),heros.end(),"bruce"),
string("wayne"))
);

// Insert surnames succeeding appropriate heros
for_each(surnames.begin(), surnames.end(),
bind1st(ptr_fun(insert_name), &heros) );

// Salute the heros
copy(heros.begin(),heros.end(),ostream_iterator<string>(cout," "));
cout << endl;
return 0;
}

// =====================================================

 

[Peter Otten]

Thanks for the answers. Enumerating in reverse is indeed quite a smart
idea.

The fact is though, I overly simplified the task in the super-hero
example. In the real case, the dictionary keys are not necessarily the
indices for inserts; that is to say, the inserts do not necessarily
take place in some sorted order.

I think I was thinking more of a linked-list idea, where you do not
store the indices as integers to some random access array but rather as
pointers into list's nodes. Then the subsequent inserts would not hurt
previously stored pointers. For those who know a bit C++/STL here is a
sketch of the idea:

name_list heros;
heros.push_back("clark");
// ... add the rest
indexed_name_list surnames;
surnames.push_back(
make_pair( find( heros.begin(), heros.end(), "clark"), "kent") )
); // the find function returns an iterator to appropriate location
// ... add the rest

for_each(surnames.begin(), surnames.end(), insert_surnames)
// insert_surnames is a callback that receives a single indexed surname
// at a time and does the job, without affecting outer iterators.


I was wondering how to make indices as *robust* in Python... Any ideas?

Python's list is technically a vector. You can therefore either search for
the insertion point in a timely fashion:

heros = ["super", "clark", "spider", "peter", "bat", "bruce"]
names = [("clark", "kent"), ("peter", "parker"), ("bruce", "wayne")]

for first, last in names:
heros.insert(heros.index(first)+1, last)
print heros

or you have to snatch a real (linked) list from somewhere. Here's an ad-hoc
implementation:

class Node(object):
def __init__(self, value, next=None):
self.value = value
self.right = next
def __str__(self):
return "List(%s)" % ", ".join(repr(n.value) for n in self)
def __iter__(self):
item = self
while item:
yield item
item = item.right
def find(self, value):
for item in self:
if item.value == value:
return item
raise ValueError("%r not found" % (value,))
def insert_after(self, value):
self.right = Node(value, self.right)
@staticmethod
def from_list(items):
items = iter(items)
try:
first = items.next()
except StopIteration:
raise ValueError("empty lists not supported")
cur = head = Node(first)
for value in items:
node = Node(value)
cur.right = node
cur = node
return head

if __name__ == "__main__":
heros = ["super", "clark", "spider", "peter", "bat", "bruce"]
heros = Node.from_list(heros)
names = [("clark", "kent"), ("peter", "parker"), ("bruce", "wayne")]
surnames = [(heros.find(f), s) for f, s in names]
print heros
for node, value in surnames:
node.insert_after(value)
print heros

Peter

 

[Marc 'BlackJack' Rintsch]

I think I was thinking more of a linked-list idea, where you do not
store the indices as integers to some random access array but rather as
pointers into list's nodes. Then the subsequent inserts would not hurt
previously stored pointers. For those who know a bit C++/STL here is a
sketch of the idea:

name_list heros;
heros.push_back("clark");
// ... add the rest
indexed_name_list surnames;
surnames.push_back(
make_pair( find( heros.begin(), heros.end(), "clark"), "kent") )
); // the find function returns an iterator to appropriate location
// ... add the rest

for_each(surnames.begin(), surnames.end(), insert_surnames)
// insert_surnames is a callback that receives a single indexed surname
// at a time and does the job, without affecting outer iterators.


I was wondering how to make indices as *robust* in Python... Any ideas?

What about putting all information for each super hero into an object or
at least a list? And those objects/lists can then be stored into a
dictionary with the first name of the heroes as key. Something like this:

heroes = [['super', 'clark'], ['spider', 'peter'], ['bat', 'bruce']]

name2hero = dict((hero[1], hero) for hero in heroes)

fullnames = [['clark', 'kent'], ['peter', 'parker'], ['bruce', 'wayne']]
for name, surname in fullnames:
name2hero[name].append(surname)

for hero in heroes:
print 'Hello %s a.k.a %s %s' % tuple(hero)

IMHO you shouldn't try to program C++ in Python. Take a step back and
describe *what* you want to achieve and not *how* you do it in another
language. And then implement it in Python.

Ciao,
Marc 'BlackJack' Rintsch

 

[Steven Bethard]

Thanks for the answers. Enumerating in reverse is indeed quite a smart
idea.

The fact is though, I overly simplified the task in the super-hero
example. In the real case, the dictionary keys are not necessarily the
indices for inserts; that is to say, the inserts do not necessarily
take place in some sorted order.

I think I was thinking more of a linked-list idea, where you do not
store the indices as integers to some random access array but rather as
pointers into list's nodes. Then the subsequent inserts would not hurt
previously stored pointers. For those who know a bit C++/STL here is a
sketch of the idea:

Sorry, I don't know C++/STL, so I don't understand the example you gave.
If your dict doesn't already come with the indices, can't you just
create a dict that does?

>>> heros = ["super", "clark", "spider", "peter", "bat", "bruce"]
>>> names = dict(clark="kent", peter="parker", bruce="wayne")
>>> heros_indices = {}
>>> for index, hero_word in enumerate(heros):

.... if hero_word in names:
.... heros_indices[index + 1] = names[hero_word]
........ heros.insert(index, heros_indices[index])
....['super', 'clark', 'kent', 'spider', 'peter', 'parker', 'bat', 'bruce',
'wayne']

STeVe