在python中有一个列表，使用lambda和map / filter生成新列表

Question

i have a list:

seq = ['soup','dog','salad','cat','great']

As per the definition of filter, below code fetches the correct result:

list(filter(lambda w: w[0]=='s',seq))

['soup','salad']

ie returning the list containing only words starting with 's'

but if i am using map function, it is returning the list as true/false:

list(map(lambda w: w[0]=='s',seq))`

[True, False, True, False, False]

please explain the map function wrt to the above example

Answer 1

map applies a function to a sequence and returns a generator.

Example:

k = list(map(int,["1","2","3"]))

int() is a function string->int hence k becomes:

k ==  [1,2,3] # (a list of ints)

Your lambda is a fuction string->bool that takes a string and evaluates the first char to be 's' or not:

lambda w: w[0]=='s'

As a function of string->bool , your result is a list of bool s when using list(map(lambda w: w[0]=='s', seq)) to apply your lambda to your sequence.

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Btw. you could also have done it as list comprehension:

s_seq = [x for x in seq if x[0]=='s'] # which is closer to what filter does...

This might shed more light on map() : Understanding the map function