How to create generators
How to create generators
The generator is just like a function. Rather than return value it yields value. Any function that contains a yield statement is called a generator.
We can yield several values one by one. Each time a value is yielded with yield statement, the function freezes and waits to be reawakened. When it is called again, it resumes its execution at the point where it stopped.
We can make use of all the values by iterating over the generator:
nested = [[1, 2], [3, 4], [5]] # ww w . ja va2 s . c o m
def flatten(nested):
for sublist in nested:
for element in sublist:
yield element
for num in flatten(nested):
print num
list(flatten(nested))
Recursive generator
def flatten(nested):
try: # w w w. jav a 2 s .co m
for sublist in nested:
for element in flatten(sublist):
yield element
except TypeError:
yield nested
list(flatten([[[1],2],3,4,[5,[6,7]],8]))
When flatten is called, there are two possibilities: the base case and the recursive case. In the base case, the function is told to flatten a single element, in which case the for loop raises a TypeError, and the generator simply yields the element.
If it is a list, we go through all the sublists and call flatten on them. Then you yield all the elements of the flattened sublists by using another for loop.
Fibonacci sequences using generators
def fibonacci(max):
a, b = 0, 1# from w w w. j a v a 2 s . c o m
while a < max:
yield a
a, b = b, a+b
for n in fibonacci(1000):
print n,
The code above generates the following result.
yield statement works with for loop
def gensquares(N):
for i in range(N):
yield i ** 2 # resume here later
# ww w.j a v a2 s . com
for i in gensquares(5): # resume the function
print i, ':', # print last yielded value
x = gensquares(10)
x.next()
x.next()
x.next()
def buildsquares(n):
res = []
for i in range(n): res.append(i**2)
return res
for x in buildsquares(5): print x, ':',
for x in [n**2 for n in range(5)]:
print x, ':',
for x in map((lambda x:x**2), range(5)):
print x, ':',
The code above generates the following result.
