Altering elements in a python array using the for loop

Question

This is for python and the for loop! I am aware you can use other iterations, I would like to know if there is a solution to this problem using the FOR loop. I apologise if this a duplicate, I did not know how to phrase the problem to search for it. It is a simple question. I like for loops and use them often, but when it comes time to alter the elements as they are iterated, I always seem to have to create a new array with the edited elements. Here is an example:

for item in list:
     if item > 7:
          item += 1

the item in this case is not returned to the list. I find that I have do something like this:

newlist = []
    for item in list:
         if item > 7:
             item += 1
        newlist.append(item)

It is not a huge problem, but each time I write out code similar to this I wonder if there is not a simpler way, and one that doesn't create new variables.

The example above uses built in variable names, it is only an example, and not how anyone should implement a for loop or declare variables

And don't use the names of built-in types like list, str, int', dict`, etc, as variable names, either. This recent question is an example of what can happen if you don't follow this rule. — PM 2Ring
– PM 2Ring, Commented Mar 6, 2015 at 9:30

lisu · Accepted Answer · 2015-03-06 09:19:01Z

5

One of the things you can do

for i, item in enumerate(my_list):
  if item > 7:
    my_list[i] += 2

answered Mar 6, 2015 at 9:19

lisu

2,31315 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Padraic Cunningham · Accepted Answer · 2015-03-06 09:58:47Z

1

You can use a generator expression or a list comprehension to change the original list:

lst = [1,3,5,7,8,9,10]

lst[:] = (i + 1 if i > 7 else i for i in lst)

print(lst)
[1, 3, 5, 7, 9, 10, 11]

Using a list comp:

 lst[:] = [i + 1 if i > 7 else i for i in lst]

The [:] syntax changes the original list.

It is also more efficient:

In [18]: %%timeit
my_list = list(range(10000))
for i, item in enumerate(my_list):
  if item > 7:
    my_list[i] += 1
   ....: 
100 loops, best of 3: 2.04 ms per loop

In [19]: %%timeit
lst = list(range(10000))
lst[:] = (i + 1 if i > 7 else i for i in lst)
   ....: 

1000 loops, best of 3: 1.55 ms per loop

In [20]: %%timeit
lst = list(range(10000))
lst[:] = [i + 1 if i > 7 else i for i in lst]
   ....: 
1000 loops, best of 3: 1.28 ms per loop

edited Mar 6, 2015 at 9:58

answered Mar 6, 2015 at 9:36

Padraic Cunningham

181k30 gold badges264 silver badges327 bronze badges

3 Comments

Dale Ludwig Over a year ago

That works thanks. I have a follow up question, when you say more efficient, you are referring to the use of resources to complete the task right?

Padraic Cunningham Over a year ago

@DaleLudwig, list comprehensions are optimised, a special LIST_APPEND bytecode is generated so we avoid the overhead of calling the list.append method.

Padraic Cunningham Over a year ago

There is some interesting performance enhancing ideas here wiki.python.org/moin/PythonSpeed/PerformanceTips

Collectives™ on Stack Overflow

Altering elements in a python array using the for loop

2 Answers 2

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related