Here's the simplified code
"set a" changed since 2nd loop while we were expecting it as a consistent value.
From the output we got some information
- memory address of variable delta was re-allocated in every loop -> line#14,18,22
- item we changed 3 times in line #9, but they affected on same object because the id remains same line#15,19,23. And that's why the problem occur.
Analysis result: from the memory address we printed, we found that the object we changed(item, from delta), is no doubt a member in "set a".
The item is a
object reference
of an element of "set a"We can created a copy of "delta" instead of reference to "set a", but there're 2 kinds of copy in Python. Shallow copy and deep copy.
Shallow copy
Deep copy
Pseudo code
Python code | Pseudo code | |
a = [1,2] | Node** pa=new Node*[3]; pa[0] = new Node(1); pa[1] = new Node(2); pa[2] = new Node*[2](1,2); | |
reference operation | b = a b is a #True id(b) == id(a) #True id(b[0]) == id(a[0]) #True | Node**& pb = pa; //C++ reference printf("%p", pa) printf("%p", pb) |
shallow copy | b=list(a) b is a #False id(b) == id(a) #False id(b[0]) == id(a[0]) #True | Node** pb=new Node*[3]; pb[0] = pa[0]; pb[1] = pa[1]; pb[2] = pa[2]; |
deep copy | import copy b = copy.deepcopy(a) id(b) == id(a) #False id(b[0]) == id(a[0]) #False | Node** pb=new Node*[3]; pb[0] = new Node(pa[0]); pb[1] = new Node(pa[1]); pb[2] = new Node(pa[2]); |
