Programming Forums > Script Programming > Python

commodore · May 2nd, 2006, 3:41 PM

I have a list with 5 elements. I want to change the 3rd element and to move the elements forward (3>4, 4>5, 5>6) at the same time. Is there a simple way to do this or do I have to do something mad?

I'm sorry if this is stupid, but I'm a beginner.

andro · May 2nd, 2006, 3:49 PM

I think it's as easy as

(Toggle Plain Text)

list.insert(i, x)

Arevos · May 2nd, 2006, 4:34 PM

What andro says.

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list = range(5)     # create a list of 5 elements, from 0 to 4
list.insert(2, 9)   # change the 3rd element 9 and move the rest forward
# (Indicies start from 0, so the index of 2 refers to the 3rd element)

Sane · May 2nd, 2006, 4:37 PM

Also, it may be a good idea to become familiar with list indexing. Yes, Andro's code works perfectly for what you need to do. However, there will be times when you need to use an alternative method (for other purposes, eg. replacing the third element), list indexing...

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my_list = ['a', 'b', 'c', 'd', 'e']
new_list = my_list[:3] + ['new'] + my_list[3:] 
# essentially: ['a', 'b', 'c'] + ['new'] + ['d', 'e']
print new_list

Any questions, feel free to ask.

commodore · May 3rd, 2006, 8:19 AM

Thanks everyone! The python sub-forum seems a lot friendlier than the HTML forum

Sane that's a great tip!

hydroxide · May 8th, 2006, 11:25 AM

Quote:

Originally Posted by Sane

Also, it may be a good idea to become familiar with list indexing. Yes, Andro's code works perfectly for what you need to do. However, there will be times when you need to use an alternative method (for other purposes, eg. replacing the third element), list indexing...

(Toggle Plain Text)

my_list = ['a', 'b', 'c', 'd', 'e']
new_list = my_list[:3] + ['new'] + my_list[3:] 
# essentially: ['a', 'b', 'c'] + ['new'] + ['d', 'e']
print new_list

Any questions, feel free to ask.

IIRC for replacement you can also do "mylist[startrange:endrange] = somelist", eg: "mylist[3:4] = ['new']", making startrange and endrange identical if you want insertion.

-T. (no, I can't test it ;-P)

Dietrich · May 8th, 2006, 12:13 PM

hydroxide, you are right

(Toggle Plain Text)

# this inserts 'new' at index 3
my_list = ['a', 'b', 'c', 'd', 'e']
my_list[3:3] = ['new']
print my_list  # ['a', 'b', 'c', 'new', 'd', 'e']

Sane · May 8th, 2006, 4:34 PM

Ah, that's interesting. Very abstract sort of logic, would have never though of that to work.

Dietrich · May 9th, 2006, 12:36 AM

Actually I found it in one of the Python Reference Manuals:

Quote:

s.insert(i, x) same as s[i:i] = [x] if i >= 0
also i == -1 inserts before the last element

Didn't give it much attention until hydroxide mentioned it.

May 2nd, 2006, 3:41 PM	#1
commodore Programmer Join Date: Nov 2005 Location: Estonia Posts: 97 Rep Power: 0	Appending to a certain index in a list I have a list with 5 elements. I want to change the 3rd element and to move the elements forward (3>4, 4>5, 5>6) at the same time. Is there a simple way to do this or do I have to do something mad? I'm sorry if this is stupid, but I'm a beginner.

May 2nd, 2006, 3:49 PM	#2
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 316 Rep Power: 4	I think it's as easy as (Toggle Plain Text) list.insert(i, x) list.insert(i, x)

May 2nd, 2006, 4:34 PM	#3
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	What andro says. (Toggle Plain Text) list = range(5) # create a list of 5 elements, from 0 to 4 list.insert(2, 9) # change the 3rd element 9 and move the rest forward # (Indicies start from 0, so the index of 2 refers to the 3rd element) list = range(5) # create a list of 5 elements, from 0 to 4 list.insert(2, 9) # change the 3rd element 9 and move the rest forward # (Indicies start from 0, so the index of 2 refers to the 3rd element)

May 2nd, 2006, 4:37 PM	#4
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,013 Rep Power: 6	Also, it may be a good idea to become familiar with list indexing. Yes, Andro's code works perfectly for what you need to do. However, there will be times when you need to use an alternative method (for other purposes, eg. replacing the third element), list indexing... (Toggle Plain Text) my_list = ['a', 'b', 'c', 'd', 'e'] new_list = my_list[:3] + ['new'] + my_list[3:] # essentially: ['a', 'b', 'c'] + ['new'] + ['d', 'e'] print new_list my_list = ['a', 'b', 'c', 'd', 'e'] new_list = my_list[:3] + ['new'] + my_list[3:] # essentially: ['a', 'b', 'c'] + ['new'] + ['d', 'e'] print new_list Any questions, feel free to ask.

May 8th, 2006, 12:13 PM	#7
Dietrich Professional Programmer Join Date: Feb 2005 Posts: 434 Rep Power: 4	hydroxide, you are right (Toggle Plain Text) # this inserts 'new' at index 3 my_list = ['a', 'b', 'c', 'd', 'e'] my_list[3:3] = ['new'] print my_list # ['a', 'b', 'c', 'new', 'd', 'e'] # this inserts 'new' at index 3 my_list = ['a', 'b', 'c', 'd', 'e'] my_list[3:3] = ['new'] print my_list # ['a', 'b', 'c', 'new', 'd', 'e'] __________________ I looked it up on the Intergnats!

May 3rd, 2006, 8:19 AM	#5
commodore Programmer Join Date: Nov 2005 Location: Estonia Posts: 97 Rep Power: 0	Thanks everyone! The python sub-forum seems a lot friendlier than the HTML forum Sane that's a great tip!

May 8th, 2006, 4:34 PM	#8
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,013 Rep Power: 6	Ah, that's interesting. Very abstract sort of logic, would have never though of that to work.

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Programming Forums > Script Programming > Python

Appending to a certain index in a list