Why does my nested loop append only the last thing in python?-CodePudding

I am working on a simple math problem that requires me to find a list of Sval values per k per w value. I have this nested for loops.

w = [0,1/3,-1]
k = [0,-1]
S = 0.001

Sval = [S]*30
Sval_list = []

for wval in w:
    for kval in k:
        
        for i in range(1, 30):
            Sval[i] = Sval[i-1] wval-kval
 
        print("{},{}".format(wval,kval),Sval)
        Sval_list.append(Sval)

The loops is expected to loop for six times, each loop would output a list of Sval. I expected an outcome of a list with shape(6,30), which I got. But it is appending the output list from 6th loop for six times, instead of appending one output list per loop. Any help to solve this question or to make my codes neater is welcomed!

CodePudding user response：

At the end of the program, Sval_list contains six copies of Sval (a list), since you appended it 6 times. Each time you run through the inner most for loop and modify Sval, it modifies all copies of Sval in Sval_list since that's how lists work in python, and that's why all lists in Sval_list are the same at the end.

You can fix this by effectively detaching each Sval when you append it to Sval_list. Instead of writing Sval_list.append(Sval), write Sval_list.append(list(Sval)). The list will create a new list that no longer changes when you change Sval.

CodePudding user response：

your mistake is in using the reference, by appending the Sval reference to the Sval_list.
you get a shallow copy of Sval as value and with change of Sval, all your data would be changed. appending a hard copy of Sval to the Sval_list would do the job:

Sval_list.append(Sval[:])