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digitalmars.D.learn - Indexed foreach on struct?

reply Matt Soucy <msoucy csh.rit.edu> writes:
So I was messing around with some code I've been writing recently, and I 
wanted to use a foreach on a struct as if it were an associative array. 
The problem is, I can't find any information on how to do that.
I can't use something like "alias this", because the class I'm writing 
acts as a wrapper that lets me use string offsets for something that's 
not string-indexed by default, so there's nothing to alias it TO.
I don't see any sort of opApply or similar to do this, and the foreach 
section of dlang.org doesn't help. Is there a way to do it, or do I need 
to do a workaround?
Thank you,
-Matt Soucy
Jan 23 2012
next sibling parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 23, 2012 at 03:32:07PM -0500, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently,
 and I wanted to use a foreach on a struct as if it were an
 associative array. The problem is, I can't find any information on
 how to do that.


[...]

 I don't see any sort of opApply or similar to do this, and the
 foreach section of dlang.org doesn't help. Is there a way to do it,
 or do I need to do a workaround?


[...]

You can use opApply. Sample test program:

	struct Test {
		int opApply(int delegate(ref int) dg) {
			auto ret = 0;
			for (auto i=0; ret==0 && i<5; i++) {
				ret = dg(i);
			}
			return ret;
		}
	}

	import std.stdio;
	void main() {
		Test t;

		foreach (n; t) {
			writeln(n);
		}
	}

Program prints:

	0
	1
	2
	3
	4

Hope that helps.


T

-- 
If you look at a thing nine hundred and ninety-nine times, you are perfectly
safe; if you look at it the thousandth time, you are in frightful danger of
seeing it for the first time. -- G. K. Chesterton
Jan 23 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 01/23/2012 01:16 PM, H. S. Teoh wrote:

 On Mon, Jan 23, 2012 at 03:32:07PM -0500, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently,
 and I wanted to use a foreach on a struct as if it were an
 associative array. The problem is, I can't find any information on
 how to do that.


 [...]

 I don't see any sort of opApply or similar to do this, and the
 foreach section of dlang.org doesn't help. Is there a way to do it,
 or do I need to do a workaround?


 [...]

 You can use opApply.


And potentially more than one overload to match the foreach loop variables.


 Sample test program:

 	struct Test {
 		int opApply(int delegate(ref int) dg) {
 			auto ret = 0;
 			for (auto i=0; ret==0&&  i<5; i++) {
 				ret = dg(i);


Add:

     if (ret) {
        break;
     }

ret is non-zero if the foreach body contains a break statement.


 			}
 			return ret;
 		}
 	}

 	import std.stdio;
 	void main() {
 		Test t;

 		foreach (n; t) {
 			writeln(n);
 		}
 	}

 Program prints:

 	0
 	1
 	2
 	3
 	4

 Hope that helps.


 T


Ali
Jan 23 2012
parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 23, 2012 at 01:26:44PM -0800, Ali Çehreli wrote:

 On 01/23/2012 01:16 PM, H. S. Teoh wrote:


[...]

 	struct Test {
 		int opApply(int delegate(ref int) dg) {
 			auto ret = 0;
 			for (auto i=0; ret==0&&  i<5; i++) {
 				ret = dg(i);


 
 Add:
 
     if (ret) {
        break;
     }
 
 ret is non-zero if the foreach body contains a break statement.


[...]

Actually I already put that into the loop condition. But yes, the return
value needs to be checked.


T

-- 
If Java had true garbage collection, most programs would delete
themselves upon execution. -- Robert Sewell
Jan 23 2012
prev sibling next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 01/23/2012 12:32 PM, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently, and I
 wanted to use a foreach on a struct as if it were an associative array.


bearophile and I had replied to your earlier post.

Ali
Jan 23 2012
parent reply Matt Soucy <msoucy csh.rit.edu> writes:
On 01/23/2012 04:28 PM, Ali Çehreli wrote:

 On 01/23/2012 12:32 PM, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently, and I
 wanted to use a foreach on a struct as if it were an associative array.


 bearophile and I had replied to your earlier post.

 Ali


Eek, I'm sorry about that. For some reason, my old sent post wasn't 
showing up at 
http://www.digitalmars.com/d/archives/digitalmars/D/learn/index.html or 
in Thunderbird, so I thought there had been some sort of problem. I had 
to repair the folder in Thunderbird in order for the posts to show up. 
I'll make sure this doesn't happen again.
But between replies to both posts, I have enough information to do what 
I need to. So thank you.
-Matt
Jan 23 2012
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 01/23/2012 03:27 PM, Matt Soucy wrote:

 On 01/23/2012 04:28 PM, Ali Çehreli wrote:

 On 01/23/2012 12:32 PM, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently, 






and I

 wanted to use a foreach on a struct as if it were an associative array.


 bearophile and I had replied to your earlier post.

 Ali


 Eek, I'm sorry about that. For some reason, my old sent post wasn't
 showing up at
 http://www.digitalmars.com/d/archives/digitalmars/D/learn/index.html or
 in Thunderbird, so I thought there had been some sort of problem. I had
 to repair the folder in Thunderbird in order for the posts to show up.
 I'll make sure this doesn't happen again.


No problem at all. :) Just making sure that posts don't get lost.


 But between replies to both posts, I have enough information to do what
 I need to. So thank you.
 -Matt


Great! :)

Ali
Jan 23 2012
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Mon, Jan 23, 2012 at 01:16:29PM -0800, H. S. Teoh wrote:

 On Mon, Jan 23, 2012 at 03:32:07PM -0500, Matt Soucy wrote:

 So I was messing around with some code I've been writing recently,
 and I wanted to use a foreach on a struct as if it were an
 associative array. The problem is, I can't find any information on
 how to do that.




[...]

Oops, missed the part about associative arrays. For that, you can simply
add more arguments to the delegate, for example:

	struct Test {
		int opApply(int delegate(ref int, ref int) dg) {
			auto ret = 0;
			for (auto i=0; ret==0 && i<5; i++) {
				auto j = i+10;
				ret = dg(i, j);
			}
			return ret;
		}
	}

	import std.stdio;
	void main() {
		Test t;

		foreach (n, m; t) {
			writeln(n, " ", m);
		}
	}


T

-- 
"I suspect the best way to deal with procrastination is to put off the
procrastination itself until later. I've been meaning to try this, but
haven't gotten around to it yet. " -- swr
Jan 23 2012