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digitalmars.D.learn - Removing entries from AAs

reply useo6 <unknown unknown.com> writes:
Hi,

I've create a little example of my problem:

module example;

class ExampleClass {

	public {

		int mi;

		this(int i) {
			mi = i;
		}

	}

}

int main(string[] args) {

	ExampleClass[hash_t] exp;

	ExampleClass cex = new ExampleClass(1);
	exp[cex.toHash()] = cex;

	cex = new ExampleClass(2);
	exp[cex.toHash()] = cex;

	cex = new ExampleClass(3);
	exp[cex.toHash()] = cex;

	foreach (ExampleClass c; exp) {
		if (c.mi == 2) {exp.remove(c.toHash()); }
	}

	return 0;

}

When I run this small example-app, I get an "Access Violation". When
I insert a break into my foreach-loop like:

	foreach (ExampleClass c; exp) {
		if (c.mi == 2) {exp.remove(c.toHash()); break; }
	}

it works, but this means, I leave the foreach-loop. What I'm doing
wrong by using the foreach-block?

Thanks for any help!
Aug 19 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 08/19/2011 02:01 PM, useo6 wrote:
 Hi,

 I've create a little example of my problem:

 module example;

 class ExampleClass {

 	public {

 		int mi;

 		this(int i) {
 			mi = i;
 		}

 	}

 }

 int main(string[] args) {

 	ExampleClass[hash_t] exp;

 	ExampleClass cex = new ExampleClass(1);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(2);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(3);
 	exp[cex.toHash()] = cex;

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); }
 	}

 	return 0;

 }

 When I run this small example-app, I get an "Access Violation". When
 I insert a break into my foreach-loop like:

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); break; }
 	}

 it works, but this means, I leave the foreach-loop. What I'm doing
 wrong by using the foreach-block?

 Thanks for any help!

The problem is that you change the AA while iterating over it. You could eg iterate over it and save all keys you want to remove into an array and then iterate over the array to remove the keys from the AA. This works: module example; class ExampleClass { public { int mi; this(int i) { mi = i; } } } int main(string[] args) { ExampleClass[hash_t] exp; ExampleClass cex = new ExampleClass(1); exp[cex.toHash()] = cex; cex = new ExampleClass(2); exp[cex.toHash()] = cex; cex = new ExampleClass(3); exp[cex.toHash()] = cex; hash_t[] toRemove; foreach (c; exp) { if (c.mi == 2) { toRemove~=c.toHash(); } } foreach (h; toRemove) { exp.remove(h); } return 0; }
Aug 19 2011
next sibling parent useo6 <unknown unknown.com> writes:
== Auszug aus Timon Gehr (timon.gehr gmx.ch)'s Artikel
 On 08/19/2011 02:01 PM, useo6 wrote:
 Hi,

 I've create a little example of my problem:

 module example;

 class ExampleClass {

 	public {

 		int mi;

 		this(int i) {
 			mi = i;
 		}

 	}

 }

 int main(string[] args) {

 	ExampleClass[hash_t] exp;

 	ExampleClass cex = new ExampleClass(1);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(2);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(3);
 	exp[cex.toHash()] = cex;

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); }
 	}

 	return 0;

 }

 When I run this small example-app, I get an "Access Violation".


 I insert a break into my foreach-loop like:

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); break; }
 	}

 it works, but this means, I leave the foreach-loop. What I'm doing
 wrong by using the foreach-block?

 Thanks for any help!


 eg iterate over it and save all keys you want to remove into an

 and then iterate over the array to remove the keys from the AA.
 This works:
 module example;
 class ExampleClass {
 	public {
 		int mi;
 		this(int i) {
 			mi = i;
 		}
 	}
 }
 int main(string[] args) {
 	ExampleClass[hash_t] exp;
 	ExampleClass cex = new ExampleClass(1);
 	exp[cex.toHash()] = cex;
 	cex = new ExampleClass(2);
 	exp[cex.toHash()] = cex;
 	cex = new ExampleClass(3);
 	exp[cex.toHash()] = cex;
 	hash_t[] toRemove;
 	foreach (c; exp) {
 		if (c.mi == 2) { toRemove~=c.toHash(); }
 	}
 	foreach (h; toRemove) {
 		exp.remove(h);
 	}
 	return 0;
 }

I already thought about that situation, but some explained the removing/clearing of an array by iterate over each element and remove it from the AA. I just solved the problem by using the HashSet-class from the dcollection's.
Aug 19 2011
prev sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 19 Aug 2011 08:28:59 -0400, Timon Gehr <timon.gehr gmx.ch> wrote:

 On 08/19/2011 02:01 PM, useo6 wrote:
 Hi,

 I've create a little example of my problem:

 module example;

 class ExampleClass {

 	public {

 		int mi;

 		this(int i) {
 			mi = i;
 		}

 	}

 }

 int main(string[] args) {

 	ExampleClass[hash_t] exp;

 	ExampleClass cex = new ExampleClass(1);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(2);
 	exp[cex.toHash()] = cex;

 	cex = new ExampleClass(3);
 	exp[cex.toHash()] = cex;

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); }
 	}

 	return 0;

 }

 When I run this small example-app, I get an "Access Violation". When
 I insert a break into my foreach-loop like:

 	foreach (ExampleClass c; exp) {
 		if (c.mi == 2) {exp.remove(c.toHash()); break; }
 	}

 it works, but this means, I leave the foreach-loop. What I'm doing
 wrong by using the foreach-block?

 Thanks for any help!

The problem is that you change the AA while iterating over it. You could eg iterate over it and save all keys you want to remove into an array and then iterate over the array to remove the keys from the AA.

Note that dcollections http://www.dsource.org/project/dcollections supports removal of the currently iterated element during iteration. See here: http://www.dsource.org/projects/dcollections/browser/branches/d2/concepts.txt#L81 Sorry for the lack of D2 documentation, I really need to give some TLC to dcollections... -Steve
Aug 19 2011