Problem with Sorting

[Jonathan M Davis]

On Thursday 27 January 2011 20:30:47 Mandeep Singh Brar wrote:
> Hi,
> 
> I am facing the following problems with sorting. .
> 
> module testSort;
> 
> import std.stdio;
> import std.algorithm;
> 
> class testcl {
> 	public string name;
> 	this(string na) { name=na; }
> 	public string toString() {
> 		return name;
> 	}
> 	public int opCmp(testcl b) {
> 
> 		return name < b.name ?  -1: 1;
> 	}
> 
> }
> 
> int main(string[] args) {
> 	testcl a = new testcl(args[1]);
> 	testcl b = new testcl(args[2]);
> 	testcl[] arr = [a,b];
> 	writeln(arr);
> 	sort!("a>b")(arr);
> 	writeln(arr);
> 	return 0;
> }
> 
> I get the following input when i run this program as:
> 
> ./testSort te2 test2
> [te2, test2]
> [test2, te2]
> 
> 
> ./testSort test1 test2
> [test1, test2]
> [test2, test1]
> 
> which is as desired.. But randomly with a few strings it gives me a range
> violation as:
> 
> 
> ./testSort test2 te1
> [test2, te1]
> core.exception.RangeError at std.algorithm(5293): Range violation
> ----------------
> ./testSort() [0x80599d6]
> ./testSort() [0x8050ca6]
> ./testSort() [0x8052b26]
> ./testSort() [0x804d39d]
> ./testSort() [0x804d2a8]
> ./testSort() [0x804ce3a]
> ./testSort() [0x8049919]
> ./testSort() [0x8050e66]
> ./testSort() [0x8050dc0]
> ./testSort() [0x8050eaa]
> ./testSort() [0x8050dc0]
> ./testSort() [0x8050d66]
> /lib/libc.so.6(__libc_start_main+0xe7) [0x139ce7]
> ./testSort() [0x8049751]
> 
> The problem gets corrected if i write the comparison function as :
> 
> 	public int opCmp(testcl b) {
> 		if(name == b.name) return 0;
> 		return name < b.name ?  -1: 1;
> 	}
> 
> I could not get the problem with the above code.

Well, of course the first version doesn't work. opCmp is supposed to return 0 if 
the values are equal, and it doesn't do that. opCmp returns a value < 0 if the 
first value is less than the second, 0 if they're equal, and a value > 0 if the 
first value is greater than the second value.

Also, the signature for opCmp on classes is supposed to be

int opCmp(Object o)

It's supposed to take Object. I'm surprised that it works at all the way that it 
is.

And since you're comparing strings, your opCmp can just use std.string.cmp to 
compare them.

- Jonathan M Davis