I am trying to print this list in descending order however this is not working any ideas why?
l = ["Germany",3,2,10,"Italy",7,9,1,"canada",4,5,3,"china",4,3,9]
d_list = []
for i in range(len(l)-2, -1, -3):
d_list.append(l[i-1])
d_list.append(l[i])
d_list.append(l[i+1])
print (d_list)
print [i for i in reversed(l)]
because reversed returns an iterator, so it wouldn't be enough to just print reversed.
Or what does descended order mean here for you?
Another option would be to reverse the entire list, then print it.
l[::-1]
for x in range(len(l)):
print(l[x-1])
Related
It seems so "dirty" emptying a list in this way:
while len(alist) > 0 : alist.pop()
Does a clear way exist to do that?
This actually removes the contents from the list, but doesn't replace the old label with a new empty list:
del lst[:]
Here's an example:
lst1 = [1, 2, 3]
lst2 = lst1
del lst1[:]
print(lst2)
For the sake of completeness, the slice assignment has the same effect:
lst[:] = []
It can also be used to shrink a part of the list while replacing a part at the same time (but that is out of the scope of the question).
Note that doing lst = [] does not empty the list, just creates a new object and binds it to the variable lst, but the old list will still have the same elements, and effect will be apparent if it had other variable bindings.
If you're running Python 3.3 or better, you can use the clear() method of list, which is parallel to clear() of dict, set, deque and other mutable container types:
alist.clear() # removes all items from alist (equivalent to del alist[:])
As per the linked documentation page, the same can also be achieved with alist *= 0.
To sum up, there are four equivalent ways to clear a list in-place (quite contrary to the Zen of Python!):
alist.clear() # Python 3.3+
del alist[:]
alist[:] = []
alist *= 0
You could try:
alist[:] = []
Which means: Splice in the list [] (0 elements) at the location [:] (all indexes from start to finish)
The [:] is the slice operator. See this question for more information.
it turns out that with python 2.5.2, del l[:] is slightly slower than l[:] = [] by 1.1 usec.
$ python -mtimeit "l=list(range(1000))" "b=l[:];del b[:]"
10000 loops, best of 3: 29.8 usec per loop
$ python -mtimeit "l=list(range(1000))" "b=l[:];b[:] = []"
10000 loops, best of 3: 28.7 usec per loop
$ python -V
Python 2.5.2
lst *= 0
has the same effect as
lst[:] = []
It's a little simpler and maybe easier to remember. Other than that there's not much to say
The efficiency seems to be about the same
list = []
will reset list to an empty list.
Note that you generally should not shadow reserved function names, such as list, which is the constructor for a list object -- you could use lst or list_ instead, for instance.
Another simple code you could use (depending on your situation) is:
index=len(list)-1
while index>=0:
del list[index]
index-=1
You have to start index at the length of the list and go backwards versus index at 0, forwards because that would end you up with index equal to the length of the list with it only being cut in half.
Also, be sure that the while line has a "greater than or equal to" sign. Omitting it will leave you with list[0] remaining.
I´ve been trying for a while to select an item from a list with the variable of the for loop. But I keep getting this error:
TypeError: list indices must be integers or slices, not str
The issue dissapears when I change the i for a number, but that's not what I want to do. I´ve been looking for similar issues but couldn't manage to get it working. Advise please.
I want this to result as: ['p1', 'q1', 'p2', 'q2', 'p3', 'q3', 'p4', 'q4', 'p5', 'q5']
listcont=[]
cont=0
while cont<=5:
for i in list:
listcont.append(list[i]+str(cont))
cont+=1
return listcont
n=5
list=['q','p']
print(concat(list,n))´´´
First, when you write for i in list you're already iterating over the elements of the list, not the indices. So you can use the item directly:
listcont.append(i + str(cont))
Second, you shouldn't name things list since it shadows the built-in of that name and will cause all kinds of trouble.
Third, the while loop would be better written as a for with a range
n = 5
my_list = ['q', 'p']
listcont = []
for counter in range(1, n+1):
for item in my_list:
listcont.append(item + str(counter))
Finally, you can simplify all of this into a list comprehension and make it look neater with an f-string:
def make_list(my_list, limit):
return [f'{item}{counter}' for counter in range(1, limit+1) for item in my_list]
make_list(['p', 'q'], 5)
When you use for loop, you must know that if you are using for i in list it means that i here is the element of the list, and the loop will traverse each element of the list.
While, what you want to do is for i in range(len(list)), this will traverse the list with i as a number which can gain a value, less than or equal to len(list) - 1.
You can learn this very basic thing about for loop here and hold yourself back from asking such questions.
Hope it helps, thanks.
You have a variable called list which is a bad idea because list is the type of a list in Python. But this isn't the issue. I'm guessing the function you have there, which is missing the declaration, is the function def concat(list, n), and you intended to write while cont <= n.
If all this is the case, when you do
for i in list:
i is going to be members of the list, so 'q', then 'p'. In this case list['p'] doesn't make any sense.
To get the output you're going for I would do (to be easy to read):
def concat(lst, n):
result = []
for i in range(n):
for v in lst:
result.append('{}{}'.format(v, i+1))
return result
You could do the whole thing in one line with:
['{}{}'.format(value, count + 1) for count in range(n) for value in lst]
i am trying to delete zero values from the list using the below code
for id,row in enumerate(list):
if row[0]=="0":
del list(id)
this works fine for input like
[0,1,3,0,9,10,0,3,0,6]
but doesn't work as expected for inputs like
[0,0,1,3,4,0,0,4,5,6,0,0].
output: [0,1,3,4,0,4,5,6,0]
I guess its because the element right after the deleted one gets the id of the deleted element and enumerate increments the id which leaves the element after the one which is deleted unchecked.
so what can be done to check all the elements ? is there a better way ?
I made a little change to your code:
mylist = [0,0,1,3,4,0,0,4,5,6,0,0]
for id,row in reversed(list(enumerate(mylist))):
if(row==0):
del mylist[id]
If you loop your list in the way you did (from start to end) and delete an element while doing it, you'll end up jumping indexes because python does not recognize that an element has been deleted from the list.
If you have an array with 10 elements inside and you delete the first (idx 0), in the next iteration you will be at index 1, but the array has been modified, so your idx 1 is the idx 2 of your array before the deletion, and the real idx 1 will be skipped.
So you just need to loop your array in reverse mode, and you won't miss indexes.
If you print the value of mylist, you'll get [1, 3, 4, 4, 5, 6].
This problem is documented on this python page under 8.3:
https://docs.python.org/3/reference/compound_stmts.html
They suggest doing it this way by using a slice. It works for me:
a = [-2,-4,3,4]
for x in a[:]:
if x < 0: a.remove(x)
print ('contents of a now: ')
print(*a)
enumerate returns an object called enumerate object and it is iterable not actually a list. second thing row is not a list it is not subscriptable.
for i,row in enumerate(l):
if row==0:
del(l[i])
you will not get result you want this way.
try this:
t=[] #a temporary list
for i in l:
if i!=0:
t.append(i)
t will contain sublist of l with non zero elements.
put the above inside a function and return the list t .
I just need a function that removes zeroes from an input list
def no_zero(a):
pos=0
while (pos+1)<=len(a):
if a[pos] == "0":
a.remove[pos]
pos= pos +1
return a
print(no_zero([0,1,0,2,0,3]))
I should be getting an output of 1,2,3 but instead it skips right to return a. Any pointers as to why? Cheers.
You can use a list comprehension:
def no_zero(a):
return [x for x in a if x != 0]
print(no_zero([0,1,0,2,0,3]))
Additionally, the reason your code currently isn't working is because you are comparing the items to a string ("0") instead of an integer (0). You are also attempting to modify a list as you iterate over it, which means that your indices don't correspond to the original indices of the list, and your result will be wrong.
I am trying to write a function that can print the element and index of a list. I want to do this without using the enumerate built in function and do it using for loops.
I was able to print out the element but I couldn't figure out a way to loop the index of my list.
Is there any good way I could work around this? Many thanks.
You could do this, simply iterating over the range of numbers regarding the length of your list:
def item_and_index(my_list):
for i in range(len(my_list)):
print(my_list[i], i)
This is exactly what you need, a function using for loops and not the enumerate function.
>>> L = ['a', 'b', 'c']
>>> for i in range(len(L)):
... print(i, L[i])
...
0 a
1 b
2 c
You could also try this:
i = 0
for elem in L:
print(i, elem)
i += 1