/Python/Nested loop in list comprehension

Nested loop in list comprehension

# This is the  syntax of a general list comprehension expression in Python
[expression 
 for x1 in it1 if cond1
 for x2 in it2 if cond2
 ...
 for xn in itn if condn
]

# hereby it1,..., itn are iterables and cond1, ..., condn boolean conditions
# expression can depend on x1,...,xn and
# itj and condj can depend on the variables x1,...,xj-1 quantified before it

# The list comprehension expression is equivalent to the following nested 
# for-loop statement
res = []
for x1 in it1:
	if cond1:
  		for x2 in it2: 
      		if cond2:
    			...
      			for xn in itn if condn:
                	if condn:
       					res.append(expression)

L = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]   
for list in L:
    for number in list:
        print(number, end=' ')
# Prints 1 2 3 4 5 6 7 8 9

Source: learnbyexample.org

# 2-D List 
matrix = [[1, 2, 3], [4, 5], [6, 7, 8, 9]] 
  
# Nested List Comprehension to flatten a given 2-D matrix 
flatten_matrix = [val for sublist in matrix for val in sublist] 
  
print(flatten_matrix)

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