CODEWITHSUNDEEP
pythonpython-3.xfor-loopvariablesvariable-assignment
Ricardo
Ricardo

Reputation: 128

Variable assignment inside for in loop

Hope you're all well with the caotic world we're living...
This might be a very beginner level question, but I'd like to understand why It is like that.

Let's say I have a list of complex:
myList = [(1.231 +2.254j), (2.875 +23.543j), ...]

I've been trying to round the values with this function:

def round_complex(x, digits):
    return complex(round(x.real, digits), round(x.imag, digits))

And for doing so, I've tried this:

for item in myList:
  item = round_complex(item, 2)

Expecting that myList values get changed, for example:

myList = [(1.23 +2.25j), (2.88 +23.54j), ...]

But, It does not work. I've also tried with a more simple example, like a list of floats and the base round function from python. It also does not work.
Is there a way for me to change a value of an iterable object with this kind of for loop (for-in)?
Or do I really have to do this:

for i in range(len(myList)):
  myList[i] = round_complex(myList[i], 2)

Upvotes: 3

Views: 731

Answers (2)

atline
atline

Reputation: 31584

The simple answer is: NO.

Python uses a mechanism, which is known as "Call-by-Object", sometimes also called "Call by Object Reference" or "Call by Sharing" when pass function parameters.

If you pass immutable arguments like integers, strings or tuples to a function, the passing acts like call-by-value. The object reference is passed to the function parameters. They can't be changed within the function, because they can't be changed at all, i.e. they are immutable. It's different, if we pass mutable arguments. They are also passed by object reference, but they can be changed in place within the function.

So, after your iterate the list, the value (1.231 +2.254j) would be a immutable argument which your change won't affect the outside variable. But if you pass the value like [1.231 +2.254j] to function, then it will make effect like next:

test.py:

myList2 = [[(1.231 +2.254j)], [(2.875 +23.543j)]]
print(myList2)
def round_complex(x, digits):
    return complex(round(x.real, digits), round(x.imag, digits))

for item2 in myList2:
    item2[0] = round_complex(item2[0], 2)

print(myList2)

Execution:

$ python3 test.py
[[(1.231+2.254j)], [(2.875+23.543j)]]
[[(1.23+2.25j)], [(2.88+23.54j)]]

In a word, for you scenario, if you insist organize your input data as that & iterate with that way, you can't change the outside value directly inside the function.

You may refers to this to learn more.

Upvotes: 3

Living Gamer
Living Gamer

Reputation: 68

The thing I understand by reading for question that you want to Assign the values of i in myList. For doing so you can use append i.e

for i in mylist:
   mylist.append(round_complex(i, 2))

Upvotes: 0

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