CODEWITHSUNDEEP
pythonfor-loop
Daniel Milde
Daniel Milde

Reputation: 1184

Python equivalent of php's foreach($array as $key => &$value)

is there any equivalent to this PHP notation, which changes the original array (be aware of reference operator)?

// increase value of all items by 1
foreach ($array as $k => &$v) {
    $v++;
}

I know only this way, which is not so elegant:

for i in range(len(array)):
    array[i] += 1

Upvotes: 37

Views: 48802

Answers (4)

Daniel Kluev
Daniel Kluev

Reputation: 11315

Regarding references.

In python, every value is object, and every 'variable' is reference to the object. Assignment never copies value, it always assigns reference.

So v in for k,v in enumerate([1,2,3]) is reference too, by default. However most objects of basic 'types' are immutable, therefore when you do immutable_object_reference += 1 you create new instance of int and change immutable_object_reference to point to new instance.

When our values are of mutable types, references work same as in PHP:

>>> class mutable_pseudoint(object):
...     def __init__(self, d):
...         self.d = d
...     def __iadd__(self, v):
...         self.d += v
...     def __repr__(self):
...         return self.d.__repr__()
...     def __str__(self):
...         return self.d.__str__()
... 
>>> l = [mutable_pseudoint(1), mutable_pseudoint(2), mutable_pseudoint(3), mutable_pseudoint(4)]
>>> l
[1, 2, 3, 4]
>>> for k,v in enumerate(l):
...     v += 1
... 
>>> l
[2, 3, 4, 5]

Upvotes: 2

Acorn
Acorn

Reputation: 50507

When the built in enumerate() function is called on a list, it returns an object that can be iterated over, returning a count and the value returned from the list.

for i, val in enumerate(array):
    array[i] += 1

Upvotes: 68

Sahas
Sahas

Reputation: 11399

You could use list comprehension:

newArray = [i + 1 for i in array]

Upvotes: 5

nOw2
nOw2

Reputation: 666

I'm unaware of being able to get a pointer to a list item, but a cleaner way to access by index is demonstrated by http://effbot.org/zone/python-list.htm:

for index, object in enumerate(L):
    L[index] = object+1

Upvotes: 1

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