Why is the alphabet split into multiple ranges in this C code?

In a custom library I saw an implementation:

inline int is_upper_alpha(char chValue)
{
    if (((chValue >= 'A') && (chValue <= 'I')) ||
        ((chValue >= 'J') && (chValue <= 'R')) ||
        ((chValue >= 'S') && (chValue <= 'Z')))
        return 1;
    return 0;
}

Is that an Easter egg or what are the advantages vs standard C/C++ method?

inline int is_upper_alpha(char chValue)
{
    return ((chValue >= 'A') && (chValue <= 'Z'));
}

The author of this code presumably had to support EBCDIC at some point, where the numeric values of the letters are non-contiguous (gaps exist between I, J and R, S, as you may have guessed).

It is worth noting that the C and C++ standards only guarantee that the characters 0 to 9 have contiguous numeric values for precisely this reason, so neither of these methods is strictly standard-conforming.

Looks like it attempts to cover both EBCDIC and ASCII. Your alternative method doesn't work for EBCDIC (it has false positives, but no false negatives)

C and C++ do require that '0'-'9' are contiguous.

Note that the standard library calls do know whether they run on ASCII, EBCDIC or other systems, so they're more portable and possibly more efficient.