Lambda with if statement

Question

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

• Loocid: You can use list comprehension for this y_pred=[i if i >= 0 else 0 for i in y_pred]

Answer 1

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.

• flakes: Even simpler: [max(0, y) for y in y_pred]
• Tomalak: @flakes True, but this was not meant to be code golf. :) I wanted to keep the OP's ternary.
• flakes: Just pointing it out! I did +1 the answer!

Answer 2

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])

Answer 3

An easy way to do this with list comprehension:

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