Python: simple list modify task - list

I need to remove the unique elements of the list, the first thought is:
def cut_uniq(data):
for x in data:
if data.count(x) == 1:
data.remove(x)
print(data)
cut_uniq([1, 2, 3, 4, 5,])
return
[2, 4]
please, tell me why?


Look at each iteration:
i x data
0 1 [1,2,3,4,5]
1 3 [2,3,4,5]
2 5 [2,4,5]
[2,4]
You can iterate over a different list than you are modifying. This returns a copy of the list
def cut_uniq(data):
return [x for x in data if data.count(x) > 1]
or more efficiently
from collection import Counter
def cut_uniq(data):
return [x for x, count in Counter(data) if count > 1]
If you really do want to modify the original list, and not return a copy
def cut_uniq(data):
i = 0
while i < len(data):
if data.count(data[i]) == 1:
del data[i]
else:
i += 1
or
from collections import Counter
def cut_uniq(data):
for x, count in Counter(data):
if count == 1:
data.remove(x)
95% of the time that you modify the same list as you're iterating over, you'll have problems.


When you use
for x in data:
it translates to
for i in [0,1,2,3,4]:
x = data[i]
So in the first loop, i = 0 data[i]=1. you remove 1 from data, the data is [2,3,4,5]
on the second loop , i = 1, because now data is [2,3,4,5], data[i] = 3. So 2 is left in the data list and never been visited.
Same as the number 4.
So when you finished your loop, the [2,4] leted in the list.

Related

how iterate through lists inside of a list?

I want to iterate through the list and through every list inside of it and subtract the the first item from the second item and at the end I want to add the returned sum together, heres what i have:
def number(bus_stops):
for i in range(len(bus_stops)):
return sum(bus_stops[i][0] - bus_stops[i][1])
print(number([[10,0], [4,5], [3,2]]))
this looks pretty sense to me but it doesn't work any help would be appreciated (also if you can't tell I'm a beginner)

You can use this example how to iterate the list, unpack it and compute the total sum:
def number(bus_stops):
total = 0
for a, b in bus_stops:
total += b - a # <-- subtract first item from second, add to total
return total
print(number([[10, 0], [4, 5], [3, 2]]))
Prints:
-10
Or using sum():
def number(bus_stops):
return sum(b - a for a, b in bus_stops)

how do you draw random numbers from a list and put them in another

I do not know how to draw 2 or more different numbers and put them in another list, so far I have tried this:
import random
def jugar(m):
m = list(range(1, 14, 1)) * m
random.shuffle(m)
player1 = []
for n in m:
random.choice(m)
player1.append(n)
if n + n == 21:
print("Nano jack")
elif n + n < 21:
random.choice(m)
player1.append(n)
elif n + n > 21:
print("Loser")
return player1
jugar(1)
but this returns me 2 equal numbers, it is similar to the game of blackjack, I want it to keep adding random numbers until it reaches 21 or more, thanks for the help in advance

You can use choice method to randomly select an item from a given list.
Use it in a for loop to randomly select more items.
import random
the_list = [1, 2, 3, 4]
new_list = []
for i in range(3):
value = random.choice(the_list)
new_list.append(value)

How to find 2nd largest number in a list in python

How to find 2nd highest number in list.element in list can repeat.
when all elements in list are same it should give element not present

Create a function taking a list as argument:
def find_second(l):
# Take a set to remove duplicates and check the length
if len(set(l)) <= 1:
return "Not present"
else:
return sorted(l)[1]
Run some tests:
l1 = [1]
r1 = find_second(l1)
# Prints Not present
print(r)
l2 = [1, 3, 2]
r2 = find_second(l2)
# Prints 2
print(r)

python compare items in 2 list of different length and sequence/ duplicates should be considered

I'm trying to compare two lists of unequal length
list1=['a','b','d','b','c','d','e','f']
list2=['a','b','d','d']
list1 should be compared until the last element in list2(which is 'd') is found in list1.
Below is the desired output
output = ['b','c']
below is the code which i have
i = 0
j = 0
output = []
while(True):
if(list1[i] == list2[j]):
i += 1
j += 1
if (j == len(list2)):
break
else:
output.append(list1[i])
i = i + 1
is there any better way of doing the same?
Thanks for helping!

I think you want itertools.takewhile
from itertools import takewhile
def taker(l1, l2):
it = iter(l1)
for j in l2:
yield from takewhile(lambda x: x!=j, it)
list(taker(list1, list2)) is ['b', 'c']

Pythonic way to convert a list of integers into a string of comma-separated ranges

I have a list of integers which I need to parse into a string of ranges.
For example:
[0, 1, 2, 3] -> "0-3"
[0, 1, 2, 4, 8] -> "0-2,4,8"
And so on.
I'm still learning more pythonic ways of handling lists, and this one is a bit difficult for me. My latest thought was to create a list of lists which keeps track of paired numbers:
[ [0, 3], [4, 4], [5, 9], [20, 20] ]
I could then iterate across this structure, printing each sub-list as either a range, or a single value.
I don't like doing this in two iterations, but I can't seem to keep track of each number within each iteration. My thought would be to do something like this:
Here's my most recent attempt. It works, but I'm not fully satisfied; I keep thinking there's a more elegant solution which completely escapes me. The string-handling iteration isn't the nicest, I know -- it's pretty early in the morning for me :)
def createRangeString(zones):
rangeIdx = 0
ranges = [[zones[0], zones[0]]]
for zone in list(zones):
if ranges[rangeIdx][1] in (zone, zone-1):
ranges[rangeIdx][1] = zone
else:
ranges.append([zone, zone])
rangeIdx += 1
rangeStr = ""
for range in ranges:
if range[0] != range[1]:
rangeStr = "%s,%d-%d" % (rangeStr, range[0], range[1])
else:
rangeStr = "%s,%d" % (rangeStr, range[0])
return rangeStr[1:]
Is there a straightforward way I can merge this into a single iteration? What else could I do to make it more Pythonic?

>>> from itertools import count, groupby
>>> L=[1, 2, 3, 4, 6, 7, 8, 9, 12, 13, 19, 20, 22, 23, 40, 44]
>>> G=(list(x) for _,x in groupby(L, lambda x,c=count(): next(c)-x))
>>> print ",".join("-".join(map(str,(g[0],g[-1])[:len(g)])) for g in G)
1-4,6-9,12-13,19-20,22-23,40,44
The idea here is to pair each element with count(). Then the difference between the value and count() is constant for consecutive values. groupby() does the rest of the work
As Jeff suggests, an alternative to count() is to use enumerate(). This adds some extra cruft that needs to be stripped out in the print statement
G=(list(x) for _,x in groupby(enumerate(L), lambda (i,x):i-x))
print ",".join("-".join(map(str,(g[0][1],g[-1][1])[:len(g)])) for g in G)
Update: for the sample list given here, the version with enumerate runs about 5% slower than the version using count() on my computer

Whether this is pythonic is up for debate. But it is very compact. The real meat is in the Rangify() function. There's still room for improvement if you want efficiency or Pythonism.
def CreateRangeString(zones):
#assuming sorted and distinct
deltas = [a-b for a, b in zip(zones[1:], zones[:-1])]
deltas.append(-1)
def Rangify((b, p), (z, d)):
if p is not None:
if d == 1: return (b, p)
b.append('%d-%d'%(p,z))
return (b, None)
else:
if d == 1: return (b, z)
b.append(str(z))
return (b, None)
return ','.join(reduce(Rangify, zip(zones, deltas), ([], None))[0])
To describe the parameters:
deltas is the distance to the next value (inspired from an answer here on SO)
Rangify() does the reduction on these parameters
b - base or accumulator
p - previous start range
z - zone number
d - delta

To concatenate strings you should use ','.join. This removes the 2nd loop.
def createRangeString(zones):
rangeIdx = 0
ranges = [[zones[0], zones[0]]]
for zone in list(zones):
if ranges[rangeIdx][1] in (zone, zone-1):
ranges[rangeIdx][1] = zone
else:
ranges.append([zone, zone])
rangeIdx += 1
return ','.join(
map(
lambda p: '%s-%s'%tuple(p) if p[0] != p[1] else str(p[0]),
ranges
)
)
Although I prefer a more generic approach:
from itertools import groupby
# auxiliary functor to allow groupby to compare by adjacent elements.
class cmp_to_groupby_key(object):
def __init__(self, f):
self.f = f
self.uninitialized = True
def __call__(self, newv):
if self.uninitialized or not self.f(self.oldv, newv):
self.curkey = newv
self.uninitialized = False
self.oldv = newv
return self.curkey
# returns the first and last element of an iterable with O(1) memory.
def first_and_last(iterable):
first = next(iterable)
last = first
for i in iterable:
last = i
return (first, last)
# convert groups into list of range strings
def create_range_string_from_groups(groups):
for _, g in groups:
first, last = first_and_last(g)
if first != last:
yield "{0}-{1}".format(first, last)
else:
yield str(first)
def create_range_string(zones):
groups = groupby(zones, cmp_to_groupby_key(lambda a,b: b-a<=1))
return ','.join(create_range_string_from_groups(groups))
assert create_range_string([0,1,2,3]) == '0-3'
assert create_range_string([0, 1, 2, 4, 8]) == '0-2,4,8'
assert create_range_string([1,2,3,4,6,7,8,9,12,13,19,20,22,22,22,23,40,44]) == '1-4,6-9,12-13,19-20,22-23,40,44'

This is more verbose, mainly because I have used generic functions that I have and that are minor variations of itertools functions and recipes:
from itertools import tee, izip_longest
def pairwise_longest(iterable):
"variation of pairwise in http://docs.python.org/library/itertools.html#recipes"
a, b = tee(iterable)
next(b, None)
return izip_longest(a, b)
def takeuntil(predicate, iterable):
"""returns all elements before and including the one for which the predicate is true
variation of http://docs.python.org/library/itertools.html#itertools.takewhile"""
for x in iterable:
yield x
if predicate(x):
break
def get_range(it):
"gets a range from a pairwise iterator"
rng = list(takeuntil(lambda (a,b): (b is None) or (b-a>1), it))
if rng:
b, e = rng[0][0], rng[-1][0]
return "%d-%d" % (b,e) if b != e else "%d" % b
def create_ranges(zones):
it = pairwise_longest(zones)
return ",".join(iter(lambda:get_range(it),None))
k=[0,1,2,4,5,7,9,12,13,14,15]
print create_ranges(k) #0-2,4-5,7,9,12-15

def createRangeString(zones):
"""Create a string with integer ranges in the format of '%d-%d'
>>> createRangeString([0, 1, 2, 4, 8])
"0-2,4,8"
>>> createRangeString([1,2,3,4,6,7,8,9,12,13,19,20,22,22,22,23,40,44])
"1-4,6-9,12-13,19-20,22-23,40,44"
"""
buffer = []
try:
st = ed = zones[0]
for i in zones[1:]:
delta = i - ed
if delta == 1: ed = i
elif not (delta == 0):
buffer.append((st, ed))
st = ed = i
else: buffer.append((st, ed))
except IndexError:
pass
return ','.join(
"%d" % st if st==ed else "%d-%d" % (st, ed)
for st, ed in buffer)

Here is my solution. You need to keep track of various pieces of information while you iterate through the list and create the result - this screams generator to me. So here goes:
def rangeStr(start, end):
'''convert two integers into a range start-end, or a single value if they are the same'''
return str(start) if start == end else "%s-%s" %(start, end)
def makeRange(seq):
'''take a sequence of ints and return a sequence
of strings with the ranges
'''
# make sure that seq is an iterator
seq = iter(seq)
start = seq.next()
current = start
for val in seq:
current += 1
if val != current:
yield rangeStr(start, current-1)
start = current = val
# make sure the last range is included in the output
yield rangeStr(start, current)
def stringifyRanges(seq):
return ','.join(makeRange(seq))
>>> l = [1,2,3, 7,8,9, 11, 20,21,22,23]
>>> l2 = [1,2,3, 7,8,9, 11, 20,21,22,23, 30]
>>> stringifyRanges(l)
'1-3,7-9,11,20-23'
>>> stringifyRanges(l2)
'1-3,7-9,11,20-23,30'
My version will work correctly if given an empty list, which I think some of the others will not.
>>> stringifyRanges( [] )
''
makeRanges will work on any iterator that returns integers and lazily returns a sequence of strings so can be used on infinite sequences.
edit: I have updated the code to handle single numbers that are not part of a range.
edit2: refactored out rangeStr to remove duplication.

how about this mess...
def rangefy(mylist):
mylist, mystr, start = mylist + [None], "", 0
for i, v in enumerate(mylist[:-1]):
if mylist[i+1] != v + 1:
mystr += ["%d,"%v,"%d-%d,"%(start,v)][start!=v]
start = mylist[i+1]
return mystr[:-1]