CODEWITHSUNDEEP
pythonmath
Håkan Mjölk
Håkan Mjölk

Reputation: 73

How to calculate the dot product of two lists without importing modules?

I have this task to do the dot product of two vectors in the form of lists. I tried this method but it gives me the wrong answer, not quite sure what the issue is.

def scalar_product(vec1,vec2):
    new_list = []
    a_zip = zip(vec1, vec2)
    result = 1
    zipped_list = list(a_zip)
    for lists in zipped_list:
        for integer in lists:
            result = result * integer
            new_list.append(result)
    return sum(new_list)

print(scalar_product([1,1,1],[2,2,2]))

So my thought process here was to first zip the lists together so I get one big list containing sublists. Then I looped through the lists and the sublist to multiply the values in the sub-lists and append that to a new_list. And then to sum up the new_list. But the output here is 21 when it actually should be 6.

I don't want to import any modules or such for this.

Upvotes: 1

Views: 916

Answers (4)

Aven Desta
Aven Desta

Reputation: 2443

def scalar_product(vec1,vec2):
  total_sum = 0
  for x, y in zip(vec1, vec2):
    total_sum = total_sum + (x*y)
  return total_sum

print(scalar_product([1,1,1],[2,2,2]))
# 6

Here is what each step is doing. Take the example: vec1 = [1,2,3], vec2=[5,6,7]

zip(vec1,vec2) # is an iterator with value [(1,5),(2,6),(3,7)]
total_sum = 0 # the total sum is currently initialized to zero.
for x,y in zip(vec1,vec2) # assigns x and y from the zipped iterator.
# On the first round x = 1, y = 5
# On the second round x = 2, y = 6
# On the final round x = 3, y = 7
  total_sum = total_sum + (x*y)
# On the first round total_sum = 0 + 1*5 = 5
# On the second round total_sum = 5 + 2*6 = 17
# On the final round total_sum = 17 + 3*7 = 38
  return tota_sum # total_sum is now 38.

Upvotes: 2

Leonardo Scholl Sternberg
Leonardo Scholl Sternberg

Reputation: 11

There are many ways to make obtain the answer, one that is easy to read is as follows: (it iterates number of times equal to the length of the vectors and sum the multiplications)

def scalar_product(vec1,vec2):
    sum=0
    for count in range(0,len(vec1)): # could be len(vec2), it doesn't matter
        sum=sum+vec1[count]*vec2[count]
    return sum
print(scalar_product([1,1,1],[2,2,2]))

Upvotes: 1

chepner
chepner

Reputation: 531693

def scalar_product(vec1, vec2):
    return sum(map(lambda x, y: x*y, vec1, vec2))

A trivial import from the standard library makes this simpler:

from operator import mul

def scalar_product(vec1, vec2):
    return sum(map(mul, vec1, vec2))

If you are really averse to an import, and you know the type of value in the vectors, you can hard-code an unbound method instead. (But really, just use the trivial import.) For example,

def scalar_product(vec1, vec2):
    return sum(map(float.__mul__, vec1, vec2))

You can also use a generator expression in place of map:

def scalar_product(vec1, vec2):
    return sum(x*y for x, y in zip(vec1, vec2))

Upvotes: 2

Ade_1
Ade_1

Reputation: 1486

you can try this one liner

a,b= [1,1,1],[2,2,2]
c= sum([(lambda x,y: x*y)(x,y)for x,y in zip(a,b)])

Upvotes: 2

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