C Newbie, ascii control function - c

Question

I have written a program that works well in C that converts non-readable ASCII to their character values. I would appreciate if a C master? would show me a better way of doing it that I have currently done, mainly this section:
if (isascii(ch)) {
switch (ch) {
case 0:
printControl("NUL");
break;
case 1:
printControl("SOH");
break;
.. etc (32 in total)
case default:
putchar(ch);
break;
}
}
Is it normal to make a switch that big? Or should I be using some other method (input from an ascii table?)

Answer 1

If you're always doing the same operation (e.g., putchar), you can just statically initialize an array that maps to what each character should map. You could then access the proper mapping value by smartly accessing the array per the offset of the incoming character.
For example, (in pseudo-code -- it's been awhile since I wrote in C), you would define:
const char* [] map = {"NUL", "SOH, ...};
and then index that smartly via something like:
const char* val = map[((int)ch)];
to get your value.
You would not be able to use this if your "from" values are not sequential; in that case, you would need to have some conditional blocks. But if you can leverage the sequentiality, you should.

Answer 2

Too many years ago when assembly languages for 8-bit micros were how I spent my time, I would have written something like
printf("%3.3s",
("NULSOHSTXETXEOTENQACKBELBS HT LF VT FF CR SO SI "
"DLEDC1DC2DC3DC4NAKSYNETBCANEM SUBESCFS GS RS US ")[3*ch]);
but not because its particularly better. And the multiply by three is annoying because 8-bit micros don't multiply so it would have required both a shift and an add, as well as a spare register.
A much more C-like result would be to use a table with four bytes per control, with the NUL bytes included. That allows each entry to be referred to as a string constant, but saves the extra storage for 32 pointers.
const char *charname(int ch) {
if (ch >= 0 && ch <= 0x20)
return ("NUL\0" "SOH\0" "STX\0" "ETX\0" /* 00..03 */
"EOT\0" "ENQ\0" "ACK\0" "BEL\0" /* 04..07 */
"BS\0\0" "HT\0\0" "LF\0\0" "VT\0\0" /* 08..0B */
"FF\0\0" "CR\0\0" "SO\0\0" "SI\0\0" /* 0C..0F */
"DLE\0" "DC1\0" "DC2\0" "DC3\0" /* 10..13 */
"DC4\0" "NAK\0" "SYN\0" "ETB\0" /* 14..17 */
"CAN\0" "EM\0\0" "SUB\0" "ESC\0" /* 18..1B */
"FS\0\0" "GS\0\0" "RS\0\0" "US\0\0" /* 1C..1F */
"SP\0\0") + (ch<<2); /* 20 */
if (ch == 0x7f)
return "DEL";
if (ch == EOF)
return "EOF";
return NULL;
}
I've tried to format the main table so its organization is clear. The function returns NULL for characters that name themselves, or are not 7-bit ASCII. Otherwise, it returns a pointer to a NUL-terminated ASCII string containing the conventional abbreviation of that control character, or "EOF" for the non-character EOF returned by C standard IO routines on end of file.
Note the effort taken to pad each character name slot to exactly four bytes. This is a case where building this table with a scripting language or a separate program would be a good idea. In that case, the simple answer is to build a 129-entry table (or 257-entry) containing the names of all 7-bit ASCII (or 8-bit extended in your preferred code page) characters with an extra slot for EOF.
See the sources to the functions declared in <ctype.h> for a sample of handling the extra space for EOF.

Answer 3

You can make a switch this big but it does become a bit difficult to manage.
The way I would approach this is to build an array with char c; char* ctrl; for each item. Then you could just loop through the array. This would make it a little easier to maintain the data.
Note that if you use every character in a particular range (for example, character 0 through 32), then your array would only need the name and it wouldn't be necessary to store the character value.

Answer 4

I would say build a table with the vals (0-32) and their corresponding control string ("NUL", "SOH"). (In this case the table requires just an array)
Then you can just check if it is in range an index into the table to get the string to pass to your printControl() function.