Multiplying strings in a list by numbers from another list, element by element
Question:
I have two lists, ['A', 'B', 'C', 'D']
and [1, 2, 3, 4]
. Both lists will always have the same number of items. I need to multiply each string by its number, so the final product I am looking for is:
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
Answers:
I would use itertools.repeat
for a nice, efficient implementation:
>>> letters = ['A', 'B', 'C', 'D']
>>> numbers = [1, 2, 3, 4]
>>> import itertools
>>> result = []
>>> for letter, number in zip(letters, numbers):
... result.extend(itertools.repeat(letter, number))
...
>>> result
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
>>>
I also think it is quite readable.
The code is pretty straight forward, see inline comments
l1 = ['A', 'B', 'C', 'D']
l2 = [1, 2, 3, 4]
res = []
for i, x in enumerate(l1): # by enumerating you get both the item and its index
res += x * l2[i] # add the next item to the result list
print res
OUTPUT
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
You use zip()
to do it like this way:
a = ['A', 'B', 'C', 'D']
b = [1, 2, 3, 4]
final = []
for k,v in zip(a,b):
final += [k for _ in range(v)]
print(final)
Output:
>>> ['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
Or you can do it, too, using zip()
and list comprehension
:
a = ['A', 'B', 'C', 'D']
b = [1, 2, 3, 4]
final = [k for k,v in zip(a,b) for _ in range(v)]
print(final)
Output:
>>> ['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
Nested list comprehension works too:
>>> l1 = ['A', 'B', 'C', 'D']
>>> l2 = [1, 2, 3, 4]
>>> [c for c, i in zip(l1, l2) for _ in range(i)]
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
In above zip
returns (char, count)
tuples:
>>> t = list(zip(l1, l2))
>>> t
[('A', 1), ('B', 2), ('C', 3), ('D', 4)]
Then for every tuple the second for
loop is executed count
times to add the character to the result:
>>> [char for char, count in t for _ in range(count)]
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
You can use NumPy and then convert the NumPy array to a list:
letters = ['A', 'B', 'C', 'D']
times = [1, 2, 3, 4]
np.repeat(letters, times).tolist()
#output
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
I have two lists, ['A', 'B', 'C', 'D']
and [1, 2, 3, 4]
. Both lists will always have the same number of items. I need to multiply each string by its number, so the final product I am looking for is:
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
I would use itertools.repeat
for a nice, efficient implementation:
>>> letters = ['A', 'B', 'C', 'D']
>>> numbers = [1, 2, 3, 4]
>>> import itertools
>>> result = []
>>> for letter, number in zip(letters, numbers):
... result.extend(itertools.repeat(letter, number))
...
>>> result
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
>>>
I also think it is quite readable.
The code is pretty straight forward, see inline comments
l1 = ['A', 'B', 'C', 'D']
l2 = [1, 2, 3, 4]
res = []
for i, x in enumerate(l1): # by enumerating you get both the item and its index
res += x * l2[i] # add the next item to the result list
print res
OUTPUT
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
You use zip()
to do it like this way:
a = ['A', 'B', 'C', 'D']
b = [1, 2, 3, 4]
final = []
for k,v in zip(a,b):
final += [k for _ in range(v)]
print(final)
Output:
>>> ['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
Or you can do it, too, using zip()
and list comprehension
:
a = ['A', 'B', 'C', 'D']
b = [1, 2, 3, 4]
final = [k for k,v in zip(a,b) for _ in range(v)]
print(final)
Output:
>>> ['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
Nested list comprehension works too:
>>> l1 = ['A', 'B', 'C', 'D']
>>> l2 = [1, 2, 3, 4]
>>> [c for c, i in zip(l1, l2) for _ in range(i)]
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
In above zip
returns (char, count)
tuples:
>>> t = list(zip(l1, l2))
>>> t
[('A', 1), ('B', 2), ('C', 3), ('D', 4)]
Then for every tuple the second for
loop is executed count
times to add the character to the result:
>>> [char for char, count in t for _ in range(count)]
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']
You can use NumPy and then convert the NumPy array to a list:
letters = ['A', 'B', 'C', 'D']
times = [1, 2, 3, 4]
np.repeat(letters, times).tolist()
#output
['A', 'B', 'B', 'C', 'C', 'C', 'D', 'D', 'D', 'D']