Calling a function twice but in second time, first list which was returned by the function goes as argument in python

user_14718428 0
Viewed 28

in function entery when the first time argument is going (n, a) it's working fine a goes as empty list initialized with 0. but in the second time when b matrix is going as argument (n, b), so at that time li list which was returned by the function before is going as matrix b rather than empty matrix b which was initialized by 0. someone please help me finding error in my code.

#funtion for taking input in matrix
def entery(n, li):
    print(li)
    print('enter the element of matrix: ')
    for a in range(n):
        for b in range(n):
            no = int(input())
            li[a][b]= no
    return li #returning the updated matrix

n = int(input('dimensions of square matrix:'))
a = []
c = []
result = []
#creating matrix
for i in range(n):
    b =[]
    for j in range(n):
        b.append(0)
    a.append(b)
    c.append(b)
    result.append(b)

print("a matrix",a)
print("c matrix",c)
fm = []
sm = []
#calling function
fm = entery(n,a)
sm = entery(n,c)

print(fm)
print(sm)
#addition of matrix
for x in range(n):
    for y in range(n):
        result[x][y] = fm[x][y] + sm[x][y]

print(result)

the output is shown below-

output:
dimensions of square matrix:2
a matrix [[0, 0], [0, 0]]
c matrix [[0, 0], [0, 0]]
[[0, 0], [0, 0]] #when called first time
enter the element of matrix: 
11
22
33
44
[[11, 22], [33, 44]] # but in second time...
enter the element of matrix: 
12 
23
34
45
[[12, 23], [34, 45]]
[[12, 23], [34, 45]]
[[24, 46], [68, 90]]
function-call memory-management matrix python
1 Answers

All parameters (arguments) in the Python language are passed by reference. It means if you change what a parameter refers to within a function, the change also reflects back in the calling function.

So when you pass a mutable list like a to the function, there is a chance that the a list to get changed, like in the entry() function which you're inserting numbers to the list.

However if you check the value of list c after the first function call, you'll notice that it has been changed ,too.

[...]
#calling function
fm = entery(n,a)
print(f"a: {a}")
print(f"c: {c}")
[...]

result:

dimensions of square matrix:2
a matrix [[0, 0], [0, 0]]    
c matrix [[0, 0], [0, 0]]    
[[0, 0], [0, 0]]
enter the element of matrix: 
11
22
33
44
a: [[11, 22], [33, 44]]
c: [[11, 22], [33, 44]]

The reason is that:
There is a reference between a and c. When you first initialized both a and c, you put same list b in both lists:

[...]
for i in range(n):
    b =[]
    for j in range(n):
        b.append(0)
    a.append(b)       # <------
    c.append(b)       # <------
    result.append(b)
[...]

As you know b is also a list so when you put it in both a and c lists, the lists in "a and c lists" will have same ids and references, therefor if we change one of those lists, the other will be also changed.

You can prevent this, by using a copy of b when initiating the a and c lists, like this:

#creating matrix
for i in range(n):
    b =[]
    for j in range(n):
        b.append(0)
    a.append(b[:])    # <---- This makes a copy of list "b"
    c.append(b[:])
    result.append(b[:])

Therefor the result will be:

dimensions of square matrix:2
a matrix [[0, 0], [0, 0]]    
c matrix [[0, 0], [0, 0]]    
[[0, 0], [0, 0]]
enter the element of matrix: 
11
22
33
44
[[0, 0], [0, 0]]
enter the element of matrix:
12
23
34
45
[[11, 22], [33, 44]]
[[12, 23], [34, 45]]
[[23, 45], [67, 89]]
user_141592530
Related