Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>As others have mentioned,</p> <pre><code>char *s = "hello ppl."; </code></pre> <p>points to read-only memory because it is a string literal. It should be </p> <pre><code>char s[] = "hello ppl."; </code></pre> <p>which creates an array in read-write memory and copies the string into it.</p> <p>Ignoring non-ASCII character sets, the problem can be solved most efficiently like this:</p> <pre><code>void Convert(char *s) { for(char *sp = s; *sp; sp++) if(*sp &gt;= 'a' &amp;&amp; *sp &lt;= 'z') *sp = (*sp - 'a' + 2) % 26 + 'a'; } </code></pre> <p>If you're dealing with EBCDIC or any other charset that doesn't have contiguous alphabetic characters, you can use a map:</p> <pre><code>char *from = "abcdefghijklmnopqrstuvwxyz"; char *to = "cdefghijklmnopqrstuvwxyzab"; char map[CHAR_MAX+1]; void Initialize() { for(int i=0; from[i]; i++) map[from[i]] = to[i]; } void Convert(char *s) { for(char *sp = s; *sp; sp++) if(map[*sp]) *sp = map[*sp]; } </code></pre> <p>The compiler will optimize each of these to nearly optimal assembly language.</p> <p><strong>Update</strong> In the original problem there was no separate Initialize() call, so I optimized the code to make "Initialize(); Convert(s);" as fast as possible. If you are able to call Initialize() ahead of time and only care about how fast "Convert(s);" runs, the optimal code will fill the array first, like this:</p> <pre><code>char *from = "abcdefghijklmnopqrstuvwxyz"; char *to = "cdefghijklmnopqrstuvwxyzab"; char map[CHAR_MAX+1]; void Initialize() { int i; for(i=0; i&lt;=CHAR_MAX; i++) // New code here fills the array map[i] = i; for(i=0; from[i]; i++) map[from[i]] = to[i]; } void Convert(char *s) { for(char *sp = s; *sp; sp++) // 'if' removed *sp = map[*sp]; } </code></pre> <p>This modified code is 375% slower if you are calling "Initialize(); Convert(s);", but it is 3% faster if you have already called Initialize() and you are only timing "Convert(s);".</p>
    singulars
    1. This table or related slice is empty.
    1. POWhats wrong with this program?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USRay Burns
    1. USRay Burns
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COOther than the problem that a character set may have more than 256 characters, that's not bad. But why wouldn't you change Initialize to first set map[i] = i for i= 0..255, then change the mappings for the specific characters in from and to? That way, you're lookup code can ditch the 'if' altogether.
      singulars
      1. PO
      1. USpaxdiablo
    2. CO@paxdiablo: This solution is already much faster than yours (2.38ns vs 3.32ns on an Intel Core Duo T7400 VC++ 9 using the given input data). Unfortunately your suggestion would slow it down, not speed it up. I tried code filling the whole map in advance and omitting the 'if', and it turned out to MUCH slower at 11.31ns, almost 5x as slow. That's why I used the 'if'. Of course if the string were much longer or the initialization were a separate step, the complete map would be faster (I timed it at 0.35ns with the initialization step done in advance).
      singulars
      1. PO
      1. USRay Burns
    3. COFixed the hard-coded 256 to make it portable even to machines with 9 bit and 16 bit char types
      singulars
      1. PO
      1. USRay Burns
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload