It's most useful I would say for maybe passwords or something, making sure that what you enter is only a number, or only a letter, for example:
sync on
do
ent$=entry$()
if isNum(ent$) then nstring$=nstring$+ent$
text 10,10,nstring$
if isLetter(ent$) then lstring$=lstring$+ent$
text 10,100,lstring$
clear entry buffer
sync
cls
loop
function isLetter(text$)
if asc(text$)>=asc("a") and asc(text$)<=asc("z") then alpha=1
if asc(text$)>=asc("A") and asc(text$)<=asc("Z") then alpha=1
endfunction alpha
function isNum(text$)
if asc(text$)>=asc("0") and asc(text$)<=asc("9") then num=1
endfunction num
