Simple Caesar cypher program

/*
* Simple Caesar cypher algorithm
* Preserves case and non-alpha characters
* by The Lightning Stalker
*/

#include <stdio.h>
#include <ctype.h>

#define ALPHA 26                    // letters in alphabet
#define CONVBASE 7                  // conversion base
#define COMPLEMENT ALPHA - CONVBASE // conversion complement
#define CHARLMIN *"a" + CONVBASE    // smallest char without rollover
#define CHARUMIN *"A" + CONVBASE    // . . .

static const char stringin[] = "Joyvtl Jvbuayf";  // string to convert

int main (int argc, char **argv)
{
    int c;
    char stringout[256];

    for (c = 0; c < sizeof(stringin); c++)
    {
        if (isalpha(stringin[c]))
            if (islower(stringin[c]))
                if (stringin[c] >= CHARLMIN)
                    stringout[c] = stringin[c] - CONVBASE;
                else
                    stringout[c] = stringin[c] + COMPLEMENT;
            else
                if (stringin[c] >= CHARUMIN)
                    stringout[c] = stringin[c] - CONVBASE;
                else
                    stringout[c] = stringin[c] + COMPLEMENT;
        else
        stringout[c] = stringin[c];
    }
    printf("%s\n", stringout);
    return(0);
}