Migrating script from C to PHP

Question

I'm migrating a script from C to PHP (http://svn.stellman-greene.com/mgrs_to_utm/trunk/) and I have a problem with this concept in mgrs_to_utm.c:

Letters[0] = (toupper(MGRS[j]) - (long)'A');
if ((Letters[0] == LETTER_I) || (Letters[0] == LETTER_O))

MGRS[j] is a part of string, but WTF I can substract a (long)'A' to a LETTER??

LETTER_I is an integer (defined in mgrs_to_utm.h).

I have in mind PHP and I can't found the logic to this operation.

Thanks a lot for your help :)

that looks like an ASCii substraction to adjust a value in the array letter — MimiEAM
– MimiEAM, Commented Aug 20, 2012 at 10:16
I don't know about the typecasting to LONG but this is actually very common to convert a letter from ASCII to the number of the letter in the alphabet. Characters are just normal integers, so having a char c it's okay to do toupper(c) - 'A'. — Some programmer dude
– Some programmer dude, Commented Aug 20, 2012 at 10:16
ord(strtoupper(MGRS[j])) - ord('A') I think, this php is equivalent to that c code. But note from manual, it won't always return US-ASCII value. — Leri
– Leri, Commented Aug 20, 2012 at 10:19

SpacedMonkey · Accepted Answer · 2012-08-20 10:16:00Z

3

In ASCII, the character 'A' has value 65, so Letters[0] effectively contains an offset into the alphabet (A being 0).

If MGRS[j] is 'I' (73) then we take 'A' (65) from it to leave 8

A B C D E F G H I J K...
0 1 2 3 4 5 6 7 8 9 10

The code is pretty much the same as:

if ( MGRS[j] == 'I' || MGRS[j] == 'O' || MGRS[j] == 'i' || MGRS[j] == 'o')

answered Aug 20, 2012 at 10:16

SpacedMonkey

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2 Comments

Lito Over a year ago

Ok :) A PHP implement would be $Letters[0] = ord(strtoupper($MGRS[$j])) - ord('A'); if (($Letters[0] == $LETTER_I) || ($Letters[0] == $LETTER_O)) {... Using ord PHP function php.net/manual/en/function.ord.php. I can understand now the rest of code :)

cleong Over a year ago

Beware that the value of the letter is actually used in calculation in a couple places:

grid_northing = (double)(letters[2]) * ONEHT + false_northing; grid_easting = (double)((letters[1]) - ltr2_low_value + 1) * ONEHT;

. To get the value, I would do intval($letters[2], 36) - 10.

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