프로그래머스(Programmers) LEVEL2 - 행렬의곱셈

X = [[1, 2], [4, 5], [3, 6]]
3 x 2 matrix이다. 
첫번째 row는 X[0] , 첫번째 row의 첫번째 element는 X[0][0]

Y = [[1,2,3],[4,5,6]]
2 x 3 matrix이다

두 행렬을 곱한다고 했을 때 X의 coolumns와 Y의 rows가 같아야 한다

arr1 = [[12,7,3], [4,5,6],[7 ,8,9]] # 3x3 matrix 
arr2 = [[5,8,1,2],[6,7,3,0],[4,5,9,1]] # 3x4 matrix 



# 1st solution  
def solution(arr1, arr2):
    answer = [[0 for _ in range(len(arr2[0]))] for _ in range(len(arr1))]
    for i in range(len(arr1)):  # iterate through rows of arr1
        for j in range(len(arr2[0])): # iterate through columns of arr2
            for k in range(len(arr2)): # iterate through rows of arr
                answer[i][j] += arr1[i][k]  * arr2[k][j]                  
    return answer
    
    
# 2nd solution 
def solution(arr1, arr2): 
    final = []
    for x_row in arr1: 
        answer = []
        for y_col in zip(*arr2):
            inner = []
            for x, y  in zip(x_row, y_col):
                inner.append(x*y)
            answer.append(sum(inner))
        final.append(answer)
    return final
    
# list comprehension 
def solution(arr1, arr2):  
    return [[sum(x*y for x, y in zip(x_row, y_col)) for y_col in zip(*arr2)] for x_row in arr1]

 

# 3x3 matrix
X = [[12,7,3],
    [4 ,5,6],
    [7 ,8,9]]

# 3x4 matrix
Y = [[5,8,1,2],
    [6,7,3,0],
    [4,5,9,1]]   


for i in range(len(X)):
    for j in range(len(Y[0])):
        print(X[i], Y[j])
                   
# IndexError: list index out of range가 난다. i는 0,1,2 3개까지이지만 j는 0,1,2,3,4  이기 때문 Y[0]의 원소 갯수가 3or 이하였으면 index error가 안 난다