Why does this JavaScript code make the entire array point to the same object? -

-

in book javascript: parts, there code on page 64:

for (i = 0; < n; += 1) {  my_array[i] = []; } // note: array.dim(n, []) not work here. // each element reference same // array, bad.

why each element in array point same object? seems me line my_array[i] = []; reevaluated each loop, causing new empty object created.

what're rules of getting reference same object versus new one? how make different empty array every loop?

edit: reading people's responses, read paragraph wrong. answers!

this loop new reference each time. run it, mutate 1 of arrays , you'll see 1 changes.

the problem more array.dim method. imagine it's implemented this:

array.dim = function(n, init) {   var arr = [];   var i;    (i = 0; < n; += 1) {     arr[i] = init;   }    return arr; }

using literal array syntax create new reference array, method uses same reference populate each value of resulting array.

it might easiest compare 2 way.

var init = []; (i = 0; < n; += 1) {   arr[i] = init; }

the first approach creates 1 array uses populate each element.

for (i = 0; < n; += 1) {   arr[i] = []; }

the second approach creates new reference each iteration of loop.


Comments

Post a Comment

Popular posts from this blog

c# - How Configure Devart dotConnect for SQLite Code First? -

-
can configure following sample code ? when run following code, error : no entity framework provider found ado.net provider invariant name 'devart.data.sqlite' setting in machine.config <system.data> <dbproviderfactories> <add name="dotconnect sqlite" invariant="devart.data.sqlite" description="devart dotconnect sqlite" type="devart.data.sqlite.sqliteproviderfactory, devart.data.sqlite, version=4.6.287.0, culture=neutral, publickeytoken=09af7300eec23701" /> </dbproviderfactories> </system.data> after add below block code in app.config <?xml version="1.0"?> <configuration>   <configsections> <!-- more information on entity framework configuration, visit http://go.microsoft.com/fwlink/?linkid=237468 --> <section name="entityframework" type="system.data.entity.internal.configfile.entityframeworksection, entit...
Read more

java - Copying object fields -

-
what best way of copying classes when need have 2 independent variables? have simple class: public class myclass { boolean = false; string b= "not empty"; } do need make method : assign(myclass data ) { a= data.a; b= data.b; } is there automatic method copy (duplicate) objects in java? do need make method : pretty close. instead of making method, should make constructor. such constructors called copy constructor , , create them this: myclass(myclass data) { = data.a; b = data.b; } and create copy of instance, use constructor this: myclass obj1 = new myclass(); myclass obj2 = new myclass(obj1); copy constructor can tedious : using copy constructor create deep-copy can tedious, when class has mutable fields. in case, assignment create copy of reference, , not object itself. have create copy of fields (if want deep copy). can go recursive. a better way create deep copy serialize , deserialize object. why not use clone()? ...
Read more

c++ - Clear the memory after returning a vector in a function -

-
i have function processes , stores lot of data in , returns results vector of class. amount of data stored in function tremendous , want clear storage memory of function after finished job. necessary (does function automatically clear memory) or should clear memory function? update: vector<customers> process(char* const *filename, vector<int> id) { vector<customers> list_of_customers; (perform actions) return list_of_customers; } variables defined locally within function automatically released @ end of function scope. destructors objects called, should free memory allocated objects (if objects coded correctly). example of such object std::vector . anything you've allocated new must have corresponding delete release storage. try avoid doing own allocation , use raii instead, i.e. containers or smart pointers.
Read more