• bachmeier (16/16) Aug 11 2023 I'm trying to understand why this code returns an empty string
• Steven Schveighoffer (26/47) Aug 11 2023 In a slicing operation, the second index is one index *beyond* the data
• Salih Dincer (5/10) Aug 11 2023 Something goes wrong is indeed a 2nd degree problem. The

bachmeier <no spam.net> writes:

I'm trying to understand why this code returns an empty string 
rather than an out of bounds error:

```
writeln("this"[4..$]); // Empty string
```

But

```
writeln("this"[4]);
```

won't compile. I thought maybe it had something to do with the 
`$` or the `..` operator, but `"this"[4..4]` compiles and runs 
while `"this"[5..5]` does not. Therefore it's not the fact that 
there are no elements in the slice.

On the other hand, `"this"[3..3]` also returns an empty string. 
That still doesn't explain why there is no out of bounds error 
for `"this"[4..4]`.

Steven Schveighoffer <schveiguy gmail.com> writes:

On 8/11/23 12:04 PM, bachmeier wrote:
 I'm trying to understand why this code returns an empty string rather 
 than an out of bounds error:
 
 ```
 writeln("this"[4..$]); // Empty string
 ```
 
 But
 
 ```
 writeln("this"[4]);
 ```
 
 won't compile. I thought maybe it had something to do with the `$` or 
 the `..` operator, but `"this"[4..4]` compiles and runs while 
 `"this"[5..5]` does not. Therefore it's not the fact that there are no 
 elements in the slice.
 
 On the other hand, `"this"[3..3]` also returns an empty string. That 
 still doesn't explain why there is no out of bounds error for 
 `"this"[4..4]`.

In a slicing operation, the second index is one index *beyond* the data 
you are looking to slice. In other words, the upper bound is *exclusive* 
of the data.

The data looks like this:

```
data: [ t h i s ]
index:  0 1 2 3 4
```

So "this"[1 .. 3] will do it like this:

```
data: [ t h i s ]
index:  0 1 2 3 4
slice:   [h i ]
```

If you slice 4 .. 4, then you get:

```
data: [ t h i s ]
index:  0 1 2 3 4
slice:         []
```

This is allowed, because the pointer is still within bounds of the array.

If you slice 5 .. 5, that is out of bounds of the entire array, and even 
though you aren't getting any data, the pointer will point beyond the 
scope of the array, and isn't allowed.

-Steve

Salih Dincer <salihdb hotmail.com> writes:

On Friday, 11 August 2023 at 16:04:21 UTC, bachmeier wrote:
 I'm trying to understand why this code returns an empty string 
 rather than an out of bounds error:

 ```d
 writeln("this"[4..$]); // Empty string
 ```

Something goes wrong is indeed a 2nd degree problem.  The 
important thing is that slicing a string might be wrong if it 
contains UTF.

SDB 79