How to subtract numbers from only a few elements in a list?

Question

I have a list that contains a few elements and I want to subtract 1 from only the first 3 elements from the list. I can't figure out what the correct code is to fulfill this task. I would appreciate it if someone could help me. Thank You!

thelist = [5,4,3,2,1]

I want it to become

[4,3,2,2,1]

Answer 1

One possible solution:

thelist = [5,4,3,2,1]

thelist = [v - 1 if i < 3 else v for i, v in enumerate(thelist)]

print(thelist)

Prints:

[4, 3, 2, 2, 1]

OR:

print(list(map(lambda k: k-1, thelist[:3])) + thelist[3:])

OR:

print([v - (i<3) for i, v in enumerate(thelist)])

Answer 2

In [95]: thelist = [5,4,3,2,1]                                                                                                                                                                                                                                                                                                

In [96]: [i-1 for i in thelist[:3]] + thelist[3:]                                                                                                                                                                                                                                                                             
Out[96]: [4, 3, 2, 2, 1]

Answer 3

You can modify the original list using a list comprehension as follows:

n = 3  # Number of first elements to modify.
modification_amount = -1  
thelist[:n] = [val + modification_amount for val in thelist[:n]]
>>> thelist
[4, 3, 2, 2, 1]

Answer 4

>>> thelist = [5,4,3,2,1]
>>> newlist=[x-1 if thelist.index(x) < 3 else x for x in thelist]
>>> newlist
[4, 3, 2, 2, 1]

Answer 5

just another solution if you wanna try it you can use numpy

import numpy as np

thelist = [5,4,3,2,1]   

newnp = np.array(thelist)

newnp[0:3] -= 1

print(list(newnp))

Answer 6

You can use a combination of list comprehensions and array slicing

Array Slicing

First we need to split the array into 2 components, the first 3 elements: first_three =x[:3] and the remainder remainder = x[3:]

List Comprehension

We can now subtract 1 from each of the items in first_three and save the result in a new array one_subtracted = [num - 1 for num in first_three]

Final

We can then create the result by adding the arrays back together result = one_subtracted + remainder

Shorthand

You can also do this as part of a single expression

the_list = [5,4,3,2,1]
result = [num - 1 for num in the_list[:3]] + the_list[3:]