copy a list to list

[Mackjan]

Hi
I try to copy a list to another list but it dosent work

def kopi(x,y):
    i=0
    for i in range (1,len(x)-1):
        
        x[i]= y[i-1]
        i+=1
    print y

I dont know what is the problem with my code.
thanks alot for your help.

[woooee]

1) You don't return x from the function so it is not visible outside the function.
2) You have multiple variables named "i" (Please don't use, i, l, or o as they look like numbers and there as 23 other letters that work just as well) for i in range (1,len(x)-1): is one, and i=0, i+=1 is the second.
3) The proper way to add to a list is to use append. Following your original idea

def kopi(x,y):
    for j in range (0, len(x)):
        x.append(y[j])        
    print y

You can also use x=y[:]
If you want x[0] + y then you can do
new_x= []
new_x,append(x[0])
new_x += y
If you want x to be y[1]-->y[len(y)], omitting the first element, you can use z=y[1:].
If this doesn't help, then you will have to be more explicit about "doesn't work". That doesn't say much.

[Mackjan]

Hi
Thank you very much for your replay.
yes, I use i, very often but it is the same i, everywhere. When i=0 I mean the first element in my list then i++ the second ..etc. x(i) I mean the content of the element.
I don’t want to make a exact copy of the x, I want to copy all element from x-1 to y.
for example if I have( 9,6,2,4 )in may x then I want to move this element minus one to y then in y I will have (8,5,1,3).
x and y are two parameters in my function when I call my function I send my original lists to function instead of x and y.
when I print y I get only (None,None,…)
Thanks again

1) You don't return x from the function so it is not visible outside the function.
2) You have multiple variables named "i" (Please don't use, i, l, or o as they look like numbers and there as 23 other letters that work just as well) for i in range (1,len(x)-1): is one, and i=0, i+=1 is the second.
3) The proper way to add to a list is to use append. Following your original idea

def kopi(x,y):
    for j in range (0, len(x)):
        x.append(y[j])        
    print y

You can also use x=y[:]
If you want x[0] + y then you can do
new_x= []
new_x,append(x[0])
new_x += y
If you want x to be y[1]-->y[len(y)], omitting the first element, you can use z=y[1:].
If this doesn't help, then you will have to be more explicit about "doesn't work". That doesn't say much.

[woooee]

First, the following code will explain what I mean by 2 variables named "i". The second x prints out a different value and a different element for x_list. If this code changed for loop value for x to 1, then the loop would never end in this example because x would never get to 5. It, in fact, repeatedly gives a another block of memory named "x" the value of 1.

x=0
x_list=[ 1, 2, 3, 4, 5 ]
for x in range(0, 5):
   print "\nfor loop x =", x, x_list[x]
   x = 1
   print "     2nd x =", x, x_list[x]
   
##-----  output
for loop x = 0 1
     2nd x = 1 2
     
for loop x = 1 2
     2nd x = 1 2
          
for loop x = 2 3
     2nd x = 1 2
               
for loop x = 3 4
     2nd x = 1 2
                    
for loop x = 4 5
     2nd x = 1 2

If you want to subtract one from each element in x, then it would be something like this, assuming that each element is appended/added to y instead of replacing an existing element

def kopi(x,y):
    for each_el in x:
        print "before subtracting", each_el,
        y.append(each_el-1)
        print "  after subtracting", each_el-1
    print y
    return y

If every element in x is not an integer or float, you will get an error when you try to subtract.

[ZZucker]

You have to be careful not to create a list alias. Something you do inadvertantly with a nested list:

# make a true copy of a nested list

import copy

nested_list = [1, [2, 3], 4]
copied_list = copy.deepcopy(nested_list)

# a regular list copy looks alright at first
copied_list2 = nested_list[:]

print nested_list   # [1, [2, 3], 4]
print copied_list   # [1, [2, 3], 4]
print copied_list2  # [1, [2, 3], 4]

print '-'*20

# change the orignal list
nested_list[1][0] = 99

# but the test shows a surprise!
print nested_list   # [1, [99, 3], 4]
print copied_list   # [1, [2, 3], 4] 
print copied_list2  # [1, [99, 3], 4]  oops!

[Mackjan]

Hi
Thank you very much it was exact what I needed to know, in this line:

y.append(each_el-1)

Did we create a new list y? Which is very important for my program.
Can we use extend instead of append?

[vegaseat]

The function extend() adds the contents of another list to your list ...

listA = [1, 2, 3]
listB = [4, 5, 6]
# make a simple non-alias copy
listC = listA[:]

listA.append(listB)
print listA   # [1, 2, 3, [4, 5, 6]]

listC.extend(listB)
print listC   # [1, 2, 3, 4, 5, 6]