Seeking guidance to understand a lambda-map function. In the below, I see that the file "feedback" is read line by line and stored in a list "feedback". I'm unable to get my head around the variable x. I don't see the variable "x" declared anywhere. Can someone help me understand the statement?Thanks in advance
f = open('feedback.txt','r')
feedback = list(map(lambda x:x[:-1],f.readlines())
f.close()
The map function will execute the given function for every element in the list.
In your code the map function will get lambda x:x[:-1].
You can read that like: for every x in f.readlines() return everything except the last element of x.
So x will be every line the file. lambda x: you could see as def thing(x):.
I replaced lambda with a standard func:
def read_last(x): #x means a line
return x[:-1]
f = open('feedback.txt','r')
feedback = list(map(read_last, f.readlines())
f.close()
Maybe it will help.
lambda function is a simple anonymous function that takes any number of arguments, but has only one expression.
lambda arguments : expression
It is anonymous because we have not assigned it to an object, and thus it has no name.
example f and g are somewhat same:
def f(x):
# take a string and return all but last value
return x[:-1]
g = lambda x: x[:-1]
so:
f('hello') == g('hello') #True ->'hell'
But g is not how we would use lambda. The whole aim is to avoid assigning ;)
Now map takes in a function and applies it to an iteratable:it returns a generator in Python 3+ and thus a list is used to case that generator to a list
data = ['we are 101','you are 102','they are 103']
print(list(map(lambda x:x[:-1],data)))
#->['we are 10','you are 10','they are 10']
In principle, same as passing a function:
data = ['we are 101','you are 102','they are 103']
print(list(map(f,data)))
but often faster and awesome. I love lambdas
Keep in mind, while explaining lambda is solved here, it is not the implementation of choice for your particular example. Suggestion:
f = open('feedback.txt', 'r')
feedback = f.read().splitlines()
f.close()
