Questions tagged [immutability]
Immutability is the inability to modify data after it has been created. Modifications are instead made by copying the data. A property of immutable data is that it is *referentially transparent*.
immutability
3,703
questions
715
votes
27
answers
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Remove specific characters from a string in Python
I'm trying to remove specific characters from a string using Python. This is the code I'm using right now. Unfortunately, it appears to do nothing to the string.
for char in line:
if char in "...
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663
votes
13
answers
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views
Why are exclamation marks used in Ruby methods?
In Ruby some methods have a question mark (?) that ask a question like include? that ask if the object in question is included, this then returns a true/false.
But why do some methods have ...
- 10.7k
555
votes
16
answers
107k
views
Why are mutable structs “evil”?
Following the discussions here on SO I already read several times the remark that mutable structs are “evil” (like in the answer to this question).
What's the actual problem with mutability and ...
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422
votes
17
answers
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views
What is meant by immutable?
What exactly does immutable mean - that is, what are the consequences of an object being mutable or immutable? In particular, why are Java's Strings immutable?
My understanding is that the ...
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417
votes
16
answers
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Is a Java string really immutable?
We all know that String is immutable in Java, but check the following code:
String s1 = "Hello World";
String s2 = "Hello World";
String s3 = s1.substring(6);
System.out.println(s1); // Hello ...
- 3,246
389
votes
18
answers
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views
Const in JavaScript: when to use it and is it necessary?
I've recently come across the const keyword in JavaScript. From what I can tell, it is used to create immutable variables, and I've tested to ensure that it cannot be redefined (in Node.js):
const x = ...
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295
votes
13
answers
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views
What is the difference between shallow copy, deepcopy and normal assignment operation?
import copy
a = "deepak"
b = 1, 2, 3, 4
c = [1, 2, 3, 4]
d = {1: 10, 2: 20, 3: 30}
a1 = copy.copy(a)
b1 = copy.copy(b)
c1 = copy.copy(c)
d1 = copy.copy(d)
print("immutable - id(a)==id(a1)", id(a) ==...
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290
votes
16
answers
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views
Correct way to push into state array
I seem to be having issues pushing data into a state array.
I am trying to achieve it this way:
this.setState({ myArray: this.state.myArray.push('new value') })
But I believe this is incorrect way ...
- 45.2k
276
votes
14
answers
71k
views
Why is immutability so important (or needed) in JavaScript?
I am currently working on React JS and React Native frameworks. On the half way road I came across Immutability or the Immutable-JS library, when I was reading about Facebook's Flux and Redux ...
- 12.4k
255
votes
27
answers
140k
views
How to make an immutable object in Python?
Although I have never needed this, it just struck me that making an immutable object in Python could be slightly tricky. You can't just override __setattr__, because then you can't even set attributes ...
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238
votes
26
answers
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Immutability of Strings in Java
Consider the following example.
String str = new String();
str = "Hello";
System.out.println(str); //Prints Hello
str = "Help!";
System.out.println(str); //Prints Help!
Now, ...
- 3,977
233
votes
76
answers
17k
views
What's the best name for a non-mutating "add" method on an immutable collection? [closed]
Sorry for the waffly title - if I could come up with a concise title, I wouldn't have to ask the question.
Suppose I have an immutable list type. It has an operation Foo(x) which returns a new ...
229
votes
19
answers
315k
views
String is immutable. What exactly is the meaning? [duplicate]
I wrote the following code on immutable Strings.
public class ImmutableStrings {
public static void main(String[] args) {
testmethod();
}
private static void testmethod() {
...
- 7,669
229
votes
14
answers
146k
views
What would a "frozen dict" be?
A frozen set is a frozenset.
A frozen list could be a tuple.
What would a frozen dict be? An immutable, hashable dict.
I guess it could be something like collections.namedtuple, but that is ...
- 12.9k
221
votes
19
answers
192k
views
Enums in Javascript with ES6
I'm rebuilding an old Java project in Javascript, and realized that there's no good way to do enums in JS.
The best I can come up with is:
const Colors = {
RED: Symbol("red"),
BLUE: Symbol("...
- 2,391
215
votes
11
answers
96k
views
Immutable vs Unmodifiable collection [duplicate]
From the Collections Framework Overview:
Collections that do not support modification operations (such as add, remove and clear) are referred to as unmodifiable. Collections that are not ...
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213
votes
8
answers
44k
views
Why can tuples contain mutable items?
If a tuple is immutable then why can it contain mutable items?
It is seemingly a contradiction that when a mutable item such as a list does get modified, the tuple it belongs to maintains being ...
- 26.2k
209
votes
20
answers
288k
views
Immutable vs Mutable types
I'm confused on what an immutable type is. I know the float object is considered to be immutable, with this type of example from my book:
class RoundFloat(float):
def __new__(cls, val):
...
- 2,261
202
votes
13
answers
115k
views
Why .NET String is immutable? [duplicate]
As we all know, String is immutable. What are the reasons for String being immutable and the introduction of StringBuilder class as mutable?
- 2,875
202
votes
12
answers
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views
Mutable vs immutable objects
I'm trying to get my head around mutable vs immutable objects. Using mutable objects gets a lot of bad press (e.g. returning an array of strings from a method) but I'm having trouble understanding ...
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198
votes
17
answers
122k
views
Remove a property in an object immutably
I am using Redux. In my reducer I'm trying to remove a property from an object like this:
const state = {
a: '1',
b: '2',
c: {
x: '42',
y: '43'
},
}
And I want to have ...
- 3,324
189
votes
8
answers
130k
views
Error: "Cannot modify the return value" c#
I'm using auto-implemented properties.
I guess the fastest way to fix following is to declare my own backing variable?
public Point Origin { get; set; }
Origin.X = 10; // fails with CS1612
Error ...
- 7,835
176
votes
7
answers
82k
views
Swift make method parameter mutable?
How can I deal with this error without creating additional variable?
func reduceToZero(x:Int) -> Int {
while (x != 0) {
x = x-1 // ERROR: cannot assign to 'let' value 'x'
...
- 1,897
176
votes
15
answers
133k
views
Immutable array in Java
Is there an immutable alternative to the primitive arrays in Java? Making a primitive array final doesn't actually prevent one from doing something like
final int[] array = new int[] {0, 1, 2, 3};
...
ignoramus
174
votes
13
answers
41k
views
How to avoid "too many parameters" problem in API design?
I have this API function:
public ResultEnum DoSomeAction(string a, string b, DateTime c, OtherEnum d,
string e, string f, out Guid code)
I don't like it. Because parameter order becomes ...
156
votes
10
answers
68k
views
Advantages of stateless programming?
I've recently been learning about functional programming (specifically Haskell, but I've gone through tutorials on Lisp and Erlang as well). While I found the concepts very enlightening, I still don't ...
- 129k
145
votes
7
answers
27k
views
What is the difference between immutable and const variables in Rust?
I learned that if a variable is not explicitly declared mutable using mut, it becomes immutable (it cannot be changed after declaration). Then why do we have the const keyword in Rust? Aren't they the ...
- 4,152
140
votes
10
answers
120k
views
Remove value from object without mutation
What's a good and short way to remove a value from an object at a specific key without mutating the original object?
I'd like to do something like:
let o = {firstname: 'Jane', lastname: 'Doe'};
let ...
- 5,714
140
votes
12
answers
34k
views
Why does C# disallow readonly local variables? [closed]
Having a friendly debate with a co-worker about this. We have some thoughts about this, but wondering what the SO crowd thinks about this?
- 48k
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votes
22
answers
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views
Aren't Python strings immutable? Then why does a + " " + b work?
My understanding was that Python strings are immutable.
I tried the following code:
a = "Dog"
b = "eats"
c = "treats"
print a, b, c
# Dog eats treats
print a + " " + b + " " + c
# Dog eats treats
...
- 5,431
136
votes
3
answers
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views
How do I work around mutability in moment.js?
I've run into a problem where I have to store the initial values of a moment object but I'm having some trouble preventing my variable from changing along with the original object.
Unfortunately ...
- 1,463
132
votes
9
answers
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views
Does Python have an immutable list?
Does python have immutable lists?
Suppose I wish to have the functionality of an ordered collection of elements, but which I want to guarantee will not change, how can this be implemented? Lists are ...
- 9,745
129
votes
7
answers
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views
Java Immutable Collections
From Java 1.6 Collection Framework documentation:
Collections that do not support any modification operations (such as add, remove and clear) are referred to as unmodifiable. [...] Collections that ...
- 7,473
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votes
8
answers
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views
How do I create an immutable Class?
I am working on creating an immutable class.
I have marked all the properties as read-only.
I have a list of items in the class.
Although if the property is read-only the list can be modified.
...
- 9,115
119
votes
10
answers
163k
views
Replace Multiple String Elements in C#
Is there a better way of doing this...
MyString.Trim().Replace("&", "and").Replace(",", "").Replace(" ", " ")
.Replace(" ", "-").Replace("'", "").Replace("/", "").ToLower();
I've ...
- 4,347
114
votes
12
answers
84k
views
Is Integer Immutable
I know this is probably very stupid, but a lot of places claim that the Integer class in Java is immutable, yet the following code:
Integer a=3;
Integer b=3;
a+=b;
System.out.println(a);
Executes ...
- 2,174
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votes
10
answers
41k
views
What does immutable mean?
If a string is immutable, does that mean that....
(let's assume JavaScript)
var str = 'foo';
alert(str.substr(1)); // oo
alert(str); // foo
Does it mean, when calling methods on a string, it will ...
- 485k
111
votes
5
answers
17k
views
val-mutable versus var-immutable in Scala
Are there any guidelines in Scala on when to use val with a mutable collection versus using var with an immutable collection? Or should you really aim for val with an immutable collection?
The fact ...
- 1,869
102
votes
4
answers
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views
If Python strings are immutable, why does it keep the same id if I use += to append to it?
Strings in Python are immutable, which means the value cannot be changed. However, when appending to the string in the following example, it looks like the original string memory is modified since the ...
- 1,245
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votes
9
answers
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views
Hashable, immutable [duplicate]
From a recent SO question (see Create a dictionary in python which is indexed by lists) I realized I probably had a wrong conception of the meaning of hashable and immutable objects in python.
What ...
- 84.3k
99
votes
3
answers
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views
Immutable numpy array?
Is there a simple way to create an immutable NumPy array?
If one has to derive a class from ndarray to do this, what's the minimum set of methods that one has to override to achieve immutability?
- 493k
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votes
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answers
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Why would one declare an immutable class final in Java?
I read that to make a class immutable in Java, we should do the following,
Do not provide any setters
Mark all fields as private
Make the class final
Why is step 3 required? Why should I mark ...
- 21k
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votes
2
answers
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views
Isn't Redux just glorified global state?
So I started learning React a week ago and I inevitably got to the problem of state and how components are supposed to communicate with the rest of the app. I searched around and Redux seems to be the ...
- 11.5k
96
votes
9
answers
13k
views
Building big, immutable objects without using constructors having long parameter lists
I have some big (more than 3 fields) objects that can and should be immutable. Every time I run into that case I tend to create constructor abominations with long parameter lists.
It doesn't feel ...
- 9,516
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votes
20
answers
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views
Why do we need immutable class?
I am unable to get what are the scenarios where we need an immutable class.
Have you ever faced any such requirement? or can you please give us any real example where we should use this pattern.
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votes
12
answers
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In Python, how can I make unassignable attributes (like ones marked with `final` in Java)?
Is there anything in Python that works like the final keyword in Java - i.e., to disallow assigning to a specific attribute of the instances of a class, after those instances have been created? I ...
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votes
14
answers
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Immutable class?
How can one make a Java class immutable, what is the need of immutability and is there any advantage to using this?
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votes
7
answers
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Immutable/Mutable Collections in Swift
I was referring to Apple's Swift programming guide for understanding creation of Mutable/ immutable objects(Array, Dictionary, Sets, Data) in Swift language. But I could't understand how to create a ...
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votes
8
answers
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views
In C#, why can't I modify the member of a value type instance in a foreach loop?
I know that value types should be immutable, but that's just a suggestion, not a rule, right?
So why can't I do something like this:
struct MyStruct
{
public string Name { get; set; }
}
public ...
- 8,167
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votes
9
answers
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views
How can I create a new instance of ImmutableDictionary?
I would like to write something like this:
var d = new ImmutableDictionary<string, int> { { "a", 1 }, { "b", 2 } };
(using ImmutableDictionary from System.Collections.Immutable). It seems like ...
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