changing elements in array and List

Question

0.00/5 (No votes)

See more:

Hi,
I write the following code:

#include "stdafx.h"

using namespace System;
using namespace System::Collections::Generic;

int main(array<System::String ^> ^args)
{
    array<String^>^ arr = gcnew array<String^>
    int i = 0;
 
    arr->SetValue("a", 0);
    arr->SetValue("b", 1);
    for each(String^% s in arr)
        s = i++.ToString();   //change elements in array
    for each(String^% s in arr)	           
        Console::WriteLine(L"{0}", s);

    List<String^>^ list= (gcnew List<String^>());
    list->Add("a");
    list->Add("b");
    for each(String^% s in list)    //change elements in List
	s=i.ToString();
    for each(String^% s in list)  
	Console::WriteLine(L"{0}", s);

    Console::ReadLine();
    return 0;
}

The output is:
0
1
a
b

I tried to change all elements in arr and list by the same way,
but got different results. I don't understand why the elements in arr are changed but not in list. Could someone explain it to me?
Thanks.

Posted 28-Dec-09 19:18pm

jhchou803

Add a Solution

2 solutions

Solution 1

Try changing your "for each" loops to regular "for" loops. I'm not sure about C++/CLI, but I know changing the elements being iterated in a "for each" loop in C# will throw an exception.

Posted 6-Jan-10 3:04am

Rick Shaub

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

N a v a n e e t h · Accepted Answer · 2010-01-10T06:46:00

Interesting question.

wrote:
I don't understand why the elements in arr are changed but not in list. Could someone explain it to me?

To understand why the results are different, you need to understand what is happening when foreach is compiled. Compiler translates foreach depending on it's usage.

When foreach is used to iterate over an array, compiler compiles it into normal for loop.
When the type is not an array, compiler checks for GetEnumerator method and use it. The enumerator object returned by GetEnumerator method will be used for the processing..

Considering the above facts, your code for changing elements in an array compiles as a normal for loop and changing elements in list compiles as(written on this editor directly).

C++

List<String^>^::Enumerator^ enumerator = list->GetEnumerator();
while(true)
{
   if(!enumerator->MoveNext())
   {
      break;
   }
   String^ str = enumerator.Current; // str will be a different object with same value
   str = /* */ // Your new value assignment will come here
}

So whatever object you are changing is a copy of real object available in the list. Hence the changes won't be seen.

However, I am surprised to see the code compiles on C++/CLI. In C#, changing a foreach variable will yield into an error.

Hope that helps.