Please explain this code to me in Python

Question

D = []
L = []
for i in range(4):
D.append(i)
L.append(D)

print(L)

Why the result [[0,1,2,3] ,[0,1,2,3], [0,1,2,3], [0,1,2,3]]? Thank you. All.

Answer 1

In your code, all lines of L are updated when moving to the next i, since D changes. It happens like this:
i = 0
L = [[0]]
i = 1
L = [[0, 1], [0, 1]]
and so on.

If you want to make a sequential addition (also note that in your example the variable D is not needed, you can get rid of it):
https://code.sololearn.com/cdz5OxB44j6f/?ref=app

Answer 2

This is how Python works. By updating D, the variable D will be updated in L.

D = []
L = []

for i = 0:
D = [0]
L = [[D]] = [[0]]

for i = 1:
D = [0, 1]
L = [[D][D]] = [[0, 1][0, 1]]

for i = 2:
D = [0, 1, 2]
L = [[D][D][D]] = [[0, 1, 2][0, 1, 2][0, 1, 2]]

for i = 3:
D = [0, 1, 2, 3]
L = [[D][D][D][D]] = [[0, 1, 2, 3][0, 1, 2, 3][0, 1, 2, 3][0, 1, 2, 3]]

Answer 3

In Python, everything is always passed by reference.
The assignment operation does not copy the object, it only creates a reference to the object.

Therefore, for example, here b will change if we change a:
https://code.sololearn.com/cQTaPMLic37r/?ref=app

In your case, the same thing happens. You are not adding D, but a reference to D, so there is a change in the entire list of L. L stores four references to the same object.

To solve this problem, you can also use the copy module:
https://code.sololearn.com/c1AcvOsWjPS5/?ref=app

Answer 4

As simple as this
Since D=[i] and L=[D]
Then when i=0
D=[[i]] which is (0)
L=[[D]] which is ([0])
So
If I=1
D=[0,1]
L=[[0,1]]
If I=2
D=[0,1,2]
L=[[0,1,2][0,1,2]]
This will keep going until it gets to the range of (i) which is (4)

Note: for every range of i, the places of(D) in (L) increases

Was this helpful?

Answer 5

created two empty lists to append, then a for loop on the range function for 4=0123.Append=adds items to the last. Therefore appending 0123 in D then this D list appended in L for 0123 times. thus python decodes line by line.
hope it helps

Answer 6

The order of the for loop in this code works like:
`
D.append(i) #Append i to D 4 times, where each time i is updated because of the for loop (This will keep executing until the condition is no longer met) => D = [0,1,2,3].

[*After done executing*]
||
* V
L.append(D) #Append D to L 4 times (This will keep executing until the condition is no longer met) => L = [[0,1,2,3],[0,1,2,3],[0,1,2,3],[0,1,2,3]].
`
Remember, the code order is from up to down.. so when a line of code is done executing, it will go to the next line of code, except for some cases, such as if-else statement..

Answer 7

What about this:
i = 0:
D = [0]
L = [[0]]

i = 1
D = [0, 1]
And L should be or supposed to be::
L = L.append(D) =[ [0],[0,1]]

i = 2
D = [0,1,2]
L = [ [0], [[0,1], [0,1,2]

i= 3
D = [0, 1, 2, 3]
L = [ [0], [0,1], [0,1,2], [0,1,2,3] ]

Stop Iteration.

What do you think ?

Answer 8

Oliver Pasaribu Yes, L is [[0],[0,1]] in this case.

Answer 9

Yes, it behaves like two different sequence of for loop:

D = []
L = []
for i in range(4):
D.append(i)
for j in range(4)
L.append(D)

Answer 10

Why? You know that [0], [[0]] and [], has a different meaning of one to each other.

Answer 11

I know that [0], [[0]] and [] have different meanings for each other. But I didn't know what kind of conclusion you want to get, so I offered some options.

Answer 12

Hi, this update invokes function append(), L.append(D) while D is [0] become [0,1], then [0,1,2] and finally [0,1,2,3]. so append it must be add new element to existing list, instead of overwrite the old content of the list.

Answer 13

Please observe this.

a = []

a.append([0])
print(a)

a.append([0,1])
print(a)

aappend([0,1,2])
print(a)

a.append([0,1,2,3])
print(a)

Answer 14

This is exactly the same to this I think:

D = []
L = []

for i in range(4):
#two statement nested
#inside for
D.append(i)
L.append(D)

// out from the for.. in loop:
print(L)

Answer 15

D = []
L = []
for i in range(4):
D.append(i)
L.append(D)
print(L)

First you should try this then you are easily understand the working of for loops

Answer 16

Emmy, how could when i=1, L =[[0][ then L += D = [ [0,1] ],
[[0]] then [[0]] +![0,1] should be
[ [0], [0,1] ] why [ [0,1] ]?

Do you think:
[ [0], [0,1] ] is identical to [ [0,1]]. Just different writing style: [ [] ] , [ [ [] ] ]. to [ [] ] , [ [], [] ] ?

Answer 17

Bhai code run hote nahe

Answer 18

I have tested, this original code:
D = []
L = []
for i in range(4):
D.append(i)
L.append(D)

give the same result as code below:

D = []
L = []
for i in range(4):
D.append(i)

for i in range(4):
L.append(D)