Python - How to change values in a list of lists?

Question

I have a list of lists, each list within the list contains 5 items, how do I change the values of the items in the list? I have tried the following:

    for [itemnumber, ctype, x, y, delay] in execlist:
        if itemnumber == mynumber:
            ctype = myctype
            x = myx
            y = myy
            delay = mydelay

Originally I had a list of tuples but I realized I cant change values in a tuple so I switched to lists but I still cant change any of the values. If I print ctype, x, y, delay, myctype, myx, myy, or mydelay from within the for loop it appears that everything is working but if I print execlist afterwards I see that nothing has changed.

Are `itemnumber`s unique? Or is there another reason why you want to iterate through the entire list, even after a replacement has been done? — Tim Pietzcker, Dec 06 '12 at 21:06

score 17 · Accepted Answer · edited Jun 30 '17 at 05:41

17

The problem is that you are creating a copy of the list and then modifying the copy. What you want to do is modify the original list. Try this instead:

for i in range(len(execlist)):
    if execlist[i][0] == mynumber:
         execlist[i][1] = myctype
         execlist[i][2] = myx
         execlist[i][3] = myy
         execlist[i][4] = mydelay

edited Jun 30 '17 at 05:41

tweetotaler

83
1
7

answered Dec 06 '12 at 21:09

Jeff Silverman

186
2

score 4 · Answer 2 · answered Dec 06 '12 at 21:08

You could use enumerate():

for index, sublist in enumerate(execlist):
   if sublist[0] == mynumber:
       execlist[index][1] = myctype
       execlist[index][2] = myx
       execlist[index][3] = myy
       execlist[index][4] = mydelay
       # break

You can remove the # if execlist only contains at most one sublist whose first item can equal mynumber; otherwise, you'll cycle uselessly through the entire rest of the list.

And if the itemnumbers are in fact unique, you might be better off with a dictionary or at least an OrderedDict, depending on what else you intend to do with your data.

score 3 · Answer 3 · answered Dec 06 '12 at 21:08

You need to assign via indexes. Let's say you've got a list of lists, where the inner lists each have 5 items like you describe. If you want to iterate through them and change the value of the second item in each inner list, you could do something like:

l = [[0, 1, 2, 3, 4], [5, 6, 7, 8, 9], [10, 11, 12, 13, 14]]
for i in l:
    i[1] = "spam"

print l
(output) [[0, "spam", 2, 3, 4], [5, "spam", 7, 8, 9], [10, "spam", 12, 13, 14]]

score 1 · Answer 4 · answered Dec 06 '12 at 21:08

Variable unpacking does not seem to pass the reference but copies the values. An solution would be to do it like this:

foo = [[1, "gggg"], [3, "zzzz"]]

for item in foo:
    item[0] = 2
    item[1] = "ffff"

print(foo)

>>>> [[2, 'ffff'], [2, 'ffff']]

score 0 · Answer 5 · answered Dec 06 '12 at 21:10

Don't assign local variables in lists. In the loop

for i in lis:
    i = 5

Just sets the variable i to 5 and leaves the actual contents of the list unchanged. Instead, you have to assign it directly:

for i in range(len(lis)):
     lis[i] = 5

The same applied for lists of lists, although in this case the local variable doesn't have to be assigned so you can use the for...in construct.

for i in listoflists:
     for i2 in range(len(i)):
          i[i2] = 5 #sets all items in all lists to 5

score 0 · Answer 6 · answered Dec 06 '12 at 21:21

Changing the variables assigned in the for does not change the list. Here's a fairly readable way to do what you want:

execlist = [
    #itemnumber, ctype, x, y, delay
    [1, 2, 3, 4, 5],
    [6, 7, 8, 9, 10],
    [11, 12, 13, 14, 15],
]

mynumber, myctype, myx, myy, mydelay = 6, 100, 101, 102, 104

for i, sublist in enumerate(execlist):
   if sublist[0] == mynumber:
        execlist[i] = [mynumber, myctype, myx, myy, mydelay]

print execlist

Output:

[[1, 2, 3, 4, 5], [6, 100, 101, 102, 104], [11, 12, 13, 14, 15]]

score 0 · Answer 7 · answered Dec 06 '12 at 21:22

The iterator variables are copies of the original (Python does not have a concept of a reference as such, although structures such as lists are referred to by reference). You need to do something like:

for item in execlist:
    if item[0] == mynumber:
        item[1] = ctype
        item[2] = myx
        item[3] = myy
        item[4] = mydelay

item itself is a copy too, but it is a copy of a reference to the original nested list, so when you refer to its elements the original list is updated.

This isn't as convenient since you don't have the names; perhaps a dictionary or class would be a more convenient structure.

score 0 · Answer 8 · answered Feb 28 '20 at 19:49

Here is an example I used recently, it was a real mind bender but hopefully it can help out some one!

pivot_peaks is a list of pandas data frames some of them are duplicates.

# Create a new list with only a single entry for each item
new_list = []

for obj_index, obj in enumerate(pivot_peaks):
    add_new_frame = False

    if len(new_list) == 0:
        new_list.append(obj)
    else:
        for item in new_list:
            if item.equals(obj):
                add_new_frame = False
                break
            else:
                add_new_frame = True

        if add_new_frame == True:
            new_list.append(obj)

Python - How to change values in a list of lists?

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