ChatGPT解决这个技术问题 Extra ChatGPT

How to declare an array in Python?

How do I declare an array in Python?

I can't find any reference to arrays in the documentation.

For some unfathomable reason, Python calls arrays "lists". The "everyone-knows-what-this-is-called-so-we're-going-to-call-it-something-else" school of language design. It's a particularly bad choice of name, since it looks like a linked list rather than an array.
@Glenn Maynard: probably because in C-like languages arrays are fixed length while Python lists are not. Its more like STL vector in C++ or ArrayList in Java.
It's called a list, because it's a list. [A(), 1, 'Foo', u'öäöäö', 67L, 5.6]. A list. An array is "an arrangement of items at equally spaced addresses in computer memory" (wikipedia).
Nothing about the universally-understood term "array" suggests a fixed length or anything about the content; those are just limitations of C's particular implementation of arrays. Python lists are equally spaced (pointers to objects, internally), or else __getitem__ wouldn't be O(1).
@Glenn, from en.wikipedia.org/wiki/Array_data_structure : "the elements of an array data structure are required to have the same size" (true for Python's arrays, not true for Python lists) and "set of valid index tuples and the addresses of the elements (and hence the element addressing formula) are usually fixed while the array is in use" (not true in Python for either list or array).

Z
Zanon
variable = []

Now variable refers to an empty list*.

Of course this is an assignment, not a declaration. There's no way to say in Python "this variable should never refer to anything other than a list", since Python is dynamically typed.

*The default built-in Python type is called a list, not an array. It is an ordered container of arbitrary length that can hold a heterogenous collection of objects (their types do not matter and can be freely mixed). This should not be confused with the array module, which offers a type closer to the C array type; the contents must be homogenous (all of the same type), but the length is still dynamic.


Would it be possible to initialize the contents of the array, as in JavaScript? (e.g., as variable = ["Hi", "Hello"];?)
How would you declare a multidimensional array, then (e. g., a 2D array?)
@AndersonGreen As I said there's no such thing as a variable declaration in Python. You would create a multidimensional list by taking an empty list and putting other lists inside it or, if the dimensions of the list are known at write-time, you could just write it as a literal like this: my_2x2_list = [[a, b], [c, d]]. Depending on what you need multi-dimensional arrays for, you also might consider using numpy, which defines array types for multi-dimensional, homogeneous, unboxed arrays that can be much more efficient where applicable, so they're preferable for numeric computations.
@IfanIqbal Yes, if it contains at least one element, you can.
It is formally known as list
A
Anton

This is surprisingly complex topic in Python.

Practical answer

Arrays are represented by class list (see reference and do not mix them with generators).

Check out usage examples:

# empty array
arr = [] 

# init with values (can contain mixed types)
arr = [1, "eels"]

# get item by index (can be negative to access end of array)
arr = [1, 2, 3, 4, 5, 6]
arr[0]  # 1
arr[-1] # 6

# get length
length = len(arr)

# supports append and insert
arr.append(8)
arr.insert(6, 7)

Theoretical answer

Under the hood Python's list is a wrapper for a real array which contains references to items. Also, underlying array is created with some extra space.

Consequences of this are:

random access is really cheap (arr[6653] is same to arr[0])

append operation is 'for free' while some extra space

insert operation is expensive

Check this awesome table of operations complexity.

https://i.stack.imgur.com/B2Q7K.png


Just to add, there is a really cool way to cut arrays in python: for [1, 2, 3, 4, 5, 6, 7, 8, 9][1:-2] result will be [2, 3, 4, 5, 6, 7]
I see some down votes for this post time to time. It would be great if someone could post a word why. Thanks.
You da real MVP. Need to know actual chosen design of 'list' in order to make reasonable programming decisions. Basically it's like a 'vector' in C++. Thanks!
This is some real long answer with some great explanation!
May I add the practical notation for comparison, for instance: a == b[:2] returns True if the first 2 elements of b equals the values of array a
N
Nicholas Riley

You don't actually declare things, but this is how you create an array in Python:

from array import array
intarray = array('i')

For more info see the array module: http://docs.python.org/library/array.html

Now possible you don't want an array, but a list, but others have answered that already. :)


This is sort of funny, but not really a good answer for a question tagged "beginner". Just to make it clear: In Python you usually use a data type called a list. Python has a special-purpose data type called an array which is more like a C array and is little used.
No, but everyone else already used a list. I thought it would be a good answer to point out that there are arrays too.
super mad props for this answer. I've been programming in Python for years and only recently realized that there was an actual Python array object that is different from list objects. While the data struct is very similar, arrays limit what type of objects the array can hold. Great answer @LennartRegebro!
This should be the right answer List and Arrays are two different things @LennartRegebro thanks
There's a specific reason you may want to use arrays instead of lists. Specifically, if you have an array called myarray full of numbers, you can perform mathematical operations against it, and those operations are applied to all items inside of it. So, if you execute myarray/3, every number inside gets divided by 3. If you attempt to do the same thing with a list, you will get an error. So, arrays are more efficient for large datasets of numbers.
l
limitcracker

I think you (meant)want an list with the first 30 cells already filled. So

   f = []

   for i in range(30):
       f.append(0)

An example to where this could be used is in Fibonacci sequence. See problem 2 in Project Euler


That's a rather baroque way of initialising a list. Try f = [0] * 30 instead.
Is not the same than a = range(10) from @slehar anwer? Love python, it's syntax and it's Zen.
@erm3nda nope: $ python3 Python 3.4.2 [...] >>> a = range(10) >>> print (a) range(0, 10) >>>
p
pradyunsg

This is how:

my_array = [1, 'rebecca', 'allard', 15]

Since this is not a python array but python list, would it not be less confusing to call it "my_list"?
That creates a list, not an array. They are different and have different properties in python. Specifically, you can perform numerical operations against an array, but not against a list.
This statement does not answer the question posted.
G
Gavriel Cohen

For calculations, use numpy arrays like this:

import numpy as np

a = np.ones((3,2))        # a 2D array with 3 rows, 2 columns, filled with ones
b = np.array([1,2,3])     # a 1D array initialised using a list [1,2,3]
c = np.linspace(2,3,100)  # an array with 100 points beteen (and including) 2 and 3

print(a*1.5)  # all elements of a times 1.5
print(a.T+b)  # b added to the transpose of a

these numpy arrays can be saved and loaded from disk (even compressed) and complex calculations with large amounts of elements are C-like fast.

Much used in scientific environments. See here for more.


c
csabinho

JohnMachin's comment should be the real answer. All the other answers are just workarounds in my opinion! So:

array=[0]*element_count

agree. except careful to call even the variable "array", or get purist wrath. to this i would also add that you can similarly create "multidimensional arrays": x=[[0] * 10] * 10
Yes, this is still a list. Use the type() function to determine for your self.
h
hussam

A couple of contributions suggested that arrays in python are represented by lists. This is incorrect. Python has an independent implementation of array() in the standard library module array "array.array()" hence it is incorrect to confuse the two. Lists are lists in python so be careful with the nomenclature used.

list_01 = [4, 6.2, 7-2j, 'flo', 'cro']

list_01
Out[85]: [4, 6.2, (7-2j), 'flo', 'cro']

There is one very important difference between list and array.array(). While both of these objects are ordered sequences, array.array() is an ordered homogeneous sequences whereas a list is a non-homogeneous sequence.


b
bayer

You don't declare anything in Python. You just use it. I recommend you start out with something like http://diveintopython.net.


Some times you must declare the type of a variable: if you don't use it before, a control structure, it doesn't exist outside the control structure and you will then be constructing a new variable. The assumption is then that the variable is an int, which clashes if you use it as a more complex type.
@Clearer yes, using functions sometimes need to declare it, and sometimes to play some globals when using functions and don't wanna write too much arguments on functions.
It's not just function; a simple if statement could give you the same problem.
Programming is all about declaring, no matter which language do you use. Type declaring is whole different story
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.
n
non sequitor

I would normally just do a = [1,2,3] which is actually a list but for arrays look at this formal definition


C
Community

To add to Lennart's answer, an array may be created like this:

from array import array
float_array = array("f",values)

where values can take the form of a tuple, list, or np.array, but not array:

values = [1,2,3]
values = (1,2,3)
values = np.array([1,2,3],'f')
# 'i' will work here too, but if array is 'i' then values have to be int
wrong_values = array('f',[1,2,3])
# TypeError: 'array.array' object is not callable

and the output will still be the same:

print(float_array)
print(float_array[1])
print(isinstance(float_array[1],float))

# array('f', [1.0, 2.0, 3.0])
# 2.0
# True

Most methods for list work with array as well, common ones being pop(), extend(), and append().

Judging from the answers and comments, it appears that the array data structure isn't that popular. I like it though, the same way as one might prefer a tuple over a list.

The array structure has stricter rules than a list or np.array, and this can reduce errors and make debugging easier, especially when working with numerical data.

Attempts to insert/append a float to an int array will throw a TypeError:

values = [1,2,3]
int_array = array("i",values)
int_array.append(float(1))
# or int_array.extend([float(1)])

# TypeError: integer argument expected, got float

Keeping values which are meant to be integers (e.g. list of indices) in the array form may therefore prevent a "TypeError: list indices must be integers, not float", since arrays can be iterated over, similar to np.array and lists:

int_array = array('i',[1,2,3])
data = [11,22,33,44,55]
sample = []
for i in int_array:
    sample.append(data[i])

Annoyingly, appending an int to a float array will cause the int to become a float, without throwing an exception.

np.array retain the same data type for its entries too, but instead of giving an error it will change its data type to fit new entries (usually to double or str):

import numpy as np
numpy_int_array = np.array([1,2,3],'i')
for i in numpy_int_array:
    print(type(i))
    # <class 'numpy.int32'>
numpy_int_array_2 = np.append(numpy_int_array,int(1))
# still <class 'numpy.int32'>
numpy_float_array = np.append(numpy_int_array,float(1))
# <class 'numpy.float64'> for all values
numpy_str_array = np.append(numpy_int_array,"1")
# <class 'numpy.str_'> for all values
data = [11,22,33,44,55]
sample = []
for i in numpy_int_array_2:
    sample.append(data[i])
    # no problem here, but TypeError for the other two

This is true during assignment as well. If the data type is specified, np.array will, wherever possible, transform the entries to that data type:

int_numpy_array = np.array([1,2,float(3)],'i')
# 3 becomes an int
int_numpy_array_2 = np.array([1,2,3.9],'i')
# 3.9 gets truncated to 3 (same as int(3.9))
invalid_array = np.array([1,2,"string"],'i')
# ValueError: invalid literal for int() with base 10: 'string'
# Same error as int('string')
str_numpy_array = np.array([1,2,3],'str')
print(str_numpy_array)
print([type(i) for i in str_numpy_array])
# ['1' '2' '3']
# <class 'numpy.str_'>

or, in essence:

data = [1.2,3.4,5.6]
list_1 = np.array(data,'i').tolist()
list_2 = [int(i) for i in data]
print(list_1 == list_2)
# True

while array will simply give:

invalid_array = array([1,2,3.9],'i')
# TypeError: integer argument expected, got float

Because of this, it is not a good idea to use np.array for type-specific commands. The array structure is useful here. list preserves the data type of the values.

And for something I find rather pesky: the data type is specified as the first argument in array(), but (usually) the second in np.array(). :|

The relation to C is referred to here: Python List vs. Array - when to use?

Have fun exploring!

Note: The typed and rather strict nature of array leans more towards C rather than Python, and by design Python does not have many type-specific constraints in its functions. Its unpopularity also creates a positive feedback in collaborative work, and replacing it mostly involves an additional [int(x) for x in file]. It is therefore entirely viable and reasonable to ignore the existence of array. It shouldn't hinder most of us in any way. :D


k
kontinuity

How about this...

>>> a = range(12)
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> a[7]
6

c
camh

Following on from Lennart, there's also numpy which implements homogeneous multi-dimensional arrays.


C
Colonel Panic

Python calls them lists. You can write a list literal with square brackets and commas:

>>> [6,28,496,8128]
[6, 28, 496, 8128]

P
Pedro A. Aranda

I had an array of strings and needed an array of the same length of booleans initiated to True. This is what I did

strs = ["Hi","Bye"] 
bools = [ True for s in strs ]

I am here because I wanted the C declaration: int count[26]={0}; There is probably a better way but this variant of bools from above worked count=[0 for ii in range(26)] Later, I then changed it to count=[0]*26 which seems preferable.
S
Stephen Rauch

You can create lists and convert them into arrays or you can create array using numpy module. Below are few examples to illustrate the same. Numpy also makes it easier to work with multi-dimensional arrays.

import numpy as np
a = np.array([1, 2, 3, 4])

#For custom inputs
a = np.array([int(x) for x in input().split()])

You can also reshape this array into a 2X2 matrix using reshape function which takes in input as the dimensions of the matrix.

mat = a.reshape(2, 2)

t
tonyb
# This creates a list of 5000 zeros
a = [0] * 5000  

You can read and write to any element in this list with a[n] notation in the same as you would with an array.

It does seem to have the same random access performance as an array. I cannot say how it allocates memory because it also supports a mix of different types including strings and objects if you need it to.