How to Multiply All Items in a List Together with Python

How do I multiply all the elements inside a list by each other?

You're initializing total to 1 every iteration of the loop.

The code should be (if you actually want to do it manually):

a = [2, 3, 4]
total = 1
for i in a:
    total *= i

That solves your immediate problem but, if you're using Python 3.8 or higher, this functionality is in the math library:

import math
a = [2, 3, 4]
total = math.prod(a)

How do I multiply each element in a list by a number?

You can just use a list comprehension:

my_list = [1, 2, 3, 4, 5]
my_new_list = [i * 5 for i in my_list]

>>> print(my_new_list)
[5, 10, 15, 20, 25]

Note that a list comprehension is generally a more efficient way to do a for loop:

my_new_list = []
for i in my_list:
    my_new_list.append(i * 5)

>>> print(my_new_list)
[5, 10, 15, 20, 25]

As an alternative, here is a solution using the popular Pandas package:

import pandas as pd

s = pd.Series(my_list)

>>> s * 5
0     5
1    10
2    15
3    20
4    25
dtype: int64

Or, if you just want the list:

>>> (s * 5).tolist()
[5, 10, 15, 20, 25]

Finally, one could use map, although this is generally frowned upon.

my_new_list = map(lambda x: x * 5, my_list)

Using map, however, is generally less efficient. Per a comment from ShadowRanger on a deleted answer to this question:

The reason "no one" uses it is that, in general, it's a performance
pessimization. The only time it's worth considering map in CPython is
if you're using a built-in function implemented in C as the mapping
function; otherwise, map is going to run equal to or slower than the
more Pythonic listcomp or genexpr (which are also more explicit about
whether they're lazy generators or eager list creators; on Py3, your
code wouldn't work without wrapping the map call in list). If you're
using map with a lambda function, stop, you're doing it wrong.

And another one of his comments posted to this reply:

Please don't teach people to use map with lambda; the instant you
need a lambda, you'd have been better off with a list comprehension
or generator expression. If you're clever, you can make map work
without lambdas a lot, e.g. in this case, map((5).__mul__, my_list), although in this particular case, thanks to some
optimizations in the byte code interpreter for simple int math, [x * 5 for x in my_list] is faster, as well as being more Pythonic and simpler.

Multiply a list by the elements of other list

Try this:

factors = [1, 2, 3]
values = [[1, 2, 3], [1, 2, 3], [1, 2, 3]]

multiply = []
for idx, lst in enumerate(values):
    multiply.append([factors[idx] * x for x in lst])

print(multiply)

For a list comprehension version of the above code, see @Hommes answer

How to multiply each elements in a list in Python

You are multiplying strings. Instead multiply integers.

list = ['123', '456', '789']
my_new_list = []
for i in list:
    my_new_list.append(int(i)*2)

print (my_new_list)

Or just make every number in list an integer.
Also here is a list comprehension version of your code

list = ['123', '456', '789']
my_new_list = [int(i)*2 for i in list]

List Comprehension you should look into it. What does "list comprehension" mean? How does it work and how can I use it?

How to multiply all integers inside list

Try a list comprehension:

l = [x * 2 for x in l]

This goes through l, multiplying each element by two.

Of course, there's more than one way to do it. If you're into lambda functions and map, you can even do

l = map(lambda x: x * 2, l)

to apply the function lambda x: x * 2 to each element in l. This is equivalent to:

def timesTwo(x):
    return x * 2

l = map(timesTwo, l)

Note that map() returns a map object, not a list, so if you really need a list afterwards you can use the list() function afterwards, for instance:

l = list(map(timesTwo, l))

Thanks to Minyc510 in the comments for this clarification.

How to perform element-wise multiplication of two lists?

Use a list comprehension mixed with zip():.

[a*b for a,b in zip(lista,listb)]

How to multiply all elements in a list with each other in python?

There's a number of issues with your approach. First, you're attempting to iterate over the value len(C) in your for-loop. Instead, you'd want to use range(len(C)) as this returns a generator of integers from 0 to len(C), which you can iterate over. Second, we don't make changes to loop variables in Python because the variable is overwritten at the start of every iteration of the loop so any changes would be ignored. Third, your calculation of answ will yield errors because you're trying to cast array slices to integers. You're also indexing them backwards. What you want to do is:

sum(a * b for a, b in zip(C, C[1:]))

This will get you a sum of the multiplication of every element in the list with the element in the position to the right of it. This doesn't consider the edge of the list however.

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