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Arithmetic conversions and a surprise with 'max'
Jun 11, 2010
Ali Çehreli
Jun 11, 2010
div0
Jun 11, 2010
div0
```The following program demonstrates a problem that I just hit. It is a known gotcha of arithmetic conversion rules.

The program is trying to center some text around an index of a char array. To avoid negative index values, it calls 'max' to limit the starting offset at 0.

import std.algorithm;

void main()
{
// An empty line
char[10] line = ' ';

// We want to center some text around the first quarter mark
int center_of_text = line.length / 4;
string text = "01234567";

// To be safe, we want to limit the starting index at 0.
int start = max(0, center_of_text - text.length / 2);

assert(start >= 0); // FAILS!
}

The problem is due to converting the second argument of max to unsigned.

Ali
```
```On 11/06/2010 20:00, Ali Çehreli wrote:
> The following program demonstrates a problem that I just hit. It is a
> known gotcha of arithmetic conversion rules.
>
> The program is trying to center some text around an index of a char
> array. To avoid negative index values, it calls 'max' to limit the
> starting offset at 0.
>
> import std.algorithm;
>
> void main()
> {
> // An empty line
> char[10] line = ' ';
>
> // We want to center some text around the first quarter mark
> int center_of_text = line.length / 4;
> string text = "01234567";
>
> // To be safe, we want to limit the starting index at 0.
> // (No negative index please!)
> int start = max(0, center_of_text - text.length / 2);
>
> assert(start >= 0); // FAILS!
> }
>
> The problem is due to converting the second argument of max to unsigned.
>
> Ali

It's not a 'gotcha of arithmetic conversion rules'.

import std.algorithm;
import std.stdio;

/*
T max(T)(T a, T b)
{
if(a > b)
return a;
return b;
}*/

void main(){

// An empty line
char[10] line = ' ';

// We want to center some text around the first quarter mark
int center_of_text = line.length / 4;
string text = "01234567";

// To be safe, we want to limit the starting index at 0.
auto	v0 = 0;
auto	v1 = center_of_text - text.length / 2;
int start = max(v0, v1);

writeln(typeid(typeof(v0)).toString);
writeln(typeid(typeof(v1)).toString);
writefln("v1: %d", v1);
writefln("start: %d", start);
assert(start >= 0); // FAILS!
}

If you uncomment the local max you get:

test.d(25): Error: template test.max(T) does not match any function template declaration
test.d(25): Error: template test.max(T) cannot deduce template function from argument types !()(int,uint)

There is special magic in std.algorithm.max to make it work.
Arguably the implementation in std.algorithm.max is wrong.

Allowing mixing of unsigned and signed seems like a bad idea to me.

--
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
```
```On 11/06/2010 21:02, div0 wrote:
> On 11/06/2010 20:00, Ali Çehreli wrote:
>> The following program demonstrates a problem that I just hit. It is a
>> known gotcha of arithmetic conversion rules.
>>
>> The program is trying to center some text around an index of a char
>> array. To avoid negative index values, it calls 'max' to limit the
>> starting offset at 0.
>>
>> import std.algorithm;
>>
>> void main()
>> {
>> // An empty line
>> char[10] line = ' ';
>>
>> // We want to center some text around the first quarter mark
>> int center_of_text = line.length / 4;
>> string text = "01234567";
>>
>> // To be safe, we want to limit the starting index at 0.
>> // (No negative index please!)
>> int start = max(0, center_of_text - text.length / 2);
>>
>> assert(start >= 0); // FAILS!
>> }
>>
>> The problem is due to converting the second argument of max to unsigned.
>>
>> Ali
>
> It's not a 'gotcha of arithmetic conversion rules'.

It is an all. (though max is wrong as well)

uint	a;
int	b;
b = a;

Compiles cleanly. I'm going nuts, I thought that had been disallowed.

> Write your own max function:
>
> import std.algorithm;
> import std.stdio;
>
> /*
> T max(T)(T a, T b)
> {
> if(a > b)
> return a;
> return b;
> }*/
>
> void main(){
>
> // An empty line
> char[10] line = ' ';
>
> // We want to center some text around the first quarter mark
> int center_of_text = line.length / 4;
> string text = "01234567";
>
> // To be safe, we want to limit the starting index at 0.
> // (No negative index please!)
> auto v0 = 0;
> auto v1 = center_of_text - text.length / 2;
> int start = max(v0, v1);
>
> writeln(typeid(typeof(v0)).toString);
> writeln(typeid(typeof(v1)).toString);
> writefln("v1: %d", v1);
> writefln("start: %d", start);
> assert(start >= 0); // FAILS!
> }
>
> If you uncomment the local max you get:
>
> test.d(25): Error: template test.max(T) does not match any function
> template declaration
> test.d(25): Error: template test.max(T) cannot deduce template function
> from argument types !()(int,uint)
>
> There is special magic in std.algorithm.max to make it work.
> Arguably the implementation in std.algorithm.max is wrong.
>
> Allowing mixing of unsigned and signed seems like a bad idea to me.
>
--
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
```
```On Fri, 11 Jun 2010 15:00:19 -0400, Ali Çehreli <acehreli@yahoo.com> wrote:

> The following program demonstrates a problem that I just hit. It is a known gotcha of arithmetic conversion rules.
>
> The program is trying to center some text around an index of a char array. To avoid negative index values, it calls 'max' to limit the starting offset at 0.
>
> import std.algorithm;
>
> void main()
> {
>      // An empty line
>      char[10] line = ' ';
>
>      // We want to center some text around the first quarter mark
>      int center_of_text = line.length / 4;
>      string text = "01234567";
>
>      // To be safe, we want to limit the starting index at 0.
>      // (No negative index please!)
>      int start = max(0, center_of_text - text.length / 2);
>
>      assert(start >= 0); // FAILS!
> }
>
> The problem is due to converting the second argument of max to unsigned.

These kinds of things can typically be converted into a different form that removes the subtraction, or uses it in a safe way:

int start = center_of_text - min(center_of_text, text.length / 2);

-Steve
```