a$ := "12345678"

CLS

// Using substrings, print:
// - Nth character
// - 4 characters starting from N

FOR n := 1 TO LEN a$ DO
    PRINT a$ (n), a$ (n : n + 3)
NEXT n

PRINT

// Using substrings, print:
// - first N characters
// - last N characters

FOR n := 1 TO LEN a$ DO
    PRINT a$ (: n), a$ (- n :)
NEXT n