CODEWITHSUNDEEP
python
Ayush Joshi
Ayush Joshi

Reputation: 23

Create a list of repeated elements from 2 lists in python

Being new to python, here is my problem. Given two lists: 

a = [3, 4] 
b = [5, 2]

I want to create a new list which consists of items of first list repeated n number of times, where n is the corresponding element in second list. So I would like an output like:

c = [3,3,3,3,3,4,4]

Where 3 is repeated 5 times, 4 is repeated 2 times and so on. My current code looks like this 

for item,no in zip(a,b):
    print(str(item)*no)

which gets me: 

33333
44

I am trying to figure out how to get from my current stage to the output that I want. Any help would be appreciated.

I was also thinking that maybe this could be done with list comprehension but I could not come up with the python code for it.

Upvotes: 2

Views: 842

Answers (7)

Noah
Noah

Reputation: 76

You said you were new to Python, so here's a simple way to do it, just using basic loops:

a = [3 , 4]
b = [5 , 2]
c = []
index = 0
for i in a:
     for i in range(0 , b[index]):
         c.append(i)
     index += 1

Upvotes: 0

hfz
hfz

Reputation: 421

Something a bit different

c = [a[b.index(j)] for i,j in enumerate(b) for i in range(j)]

Upvotes: 0

Miles Gehm
Miles Gehm

Reputation: 1

c = []
for i in range(len(b)):
    for j in range(b[i]):
        c.append(a[i])
print c

Upvotes: 0

ettanany
ettanany

Reputation: 19806

You can use list comprehension like this:

[i for i,j in zip(a, b) for k in range(j)]

The above line is equivalent to the following nested for loops:

c = []
for i,j in zip(a, b):
    for k in range(j):
        c.append(i)

Output:

>>> a = [3, 4]
>>> b = [5, 2]
>>> c = [i for i,j in zip(a, b) for k in range(j)]
>>> c
[3, 3, 3, 3, 3, 4, 4]

Upvotes: 0

AChampion
AChampion

Reputation: 30258

A simple list comprehension would work:

>>> a = [3, 4]
>>> b = [5, 2]
>>> [c for n, m in zip(a, b) for c in [n]*m]
[3, 3, 3, 3, 3, 4, 4]

If you are concerned on the memory efficiency (e.g. for large m) then you can use itertools.repeat() to avoid the intermediate lists:

>>> import itertools as it
>>> [c for n, m in zip(a, b) for c in it.repeat(n, m)]
[3, 3, 3, 3, 3, 4, 4]

Upvotes: 2

Shaun
Shaun

Reputation: 3895

You just need to use extend

c = []
for item, no in zip(a, b):
    c.extend([item] * no)

Upvotes: 0

cxw
cxw

Reputation: 17041

c=[]
for item, count in zip(a,b):
    c.extend([item]*count)
print(c)

should do it. Add to the list one element at a time, and then print all at once at the end. [item]*count is a count-element list([]) of item, and extend adds the contents of the list you provide to the list you call extend on.

Upvotes: 0

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