Lambda with if statement-CodePudding

Let say I have a list called "y_pred", I want to write a lambda function to change the value to 0 if the value is less than 0.

Before: y_pred=[1,2,3,-1]

After: y_pred=[1,2,3,0]

I wrote something like this, and return an error message

y_pred=list(lambda x: 0 if y_pred[x]<0 else y_pred[x])    
TypeError: 'function' object is not iterable

CodePudding user response：

You want an expression (a if cond else b) mapped over your list:

y_pred_before = [1, 2, 3, -1]
y_pred_after = list(map(lambda x: 0 if x < 0 else x, y_pred_before))
# => [1, 2, 3, 0]

A shorter form of the same thing is a list comprehension ([expr for item in iterable]):

y_pred_after = [0 if x < 0 else x for x in y_pred_before]

Your error "TypeError: 'function' object is not iterable" comes from the fact that list() tries to iterate over its argument. You've given it a lambda, i.e. a function. And functions are not iterable.

You meant to iterate over the results of the function. That's what map() does.

CodePudding user response：

You can use numpy (assuming that using lambda is not a requirement):

import numpy as np
y_pred = np.array(y_pred)
y_pred[y_pred < 0] = 0
y_pred

Output:

array([1, 2, 3, 0])

CodePudding user response：

An easy way to do this with list comprehension:

y_pred=[x if x>0 else 0 for x in y_pred_before]