Iterate through characters of a string?

Code:
dim printwidth=28

Let a$="PART 1 Bruce Hall, a slightly built youth 16 years of age, fair haired and blue eyed, stood 4' 8 in shoes and despite that slight assistance still could not see over the brick wall behind which he believed stood a flying machine. At least that was what he'd overheard two delivery boys discussing outside the local butchers shop."

REM pRINT a(1 to 10)
dim stringpos as integer
stringpos=0
dim lines as integer
dim sentenceEnd as integer
while stringpos<len(a)
sentenceEnd=stringpos+printwidth

if sentenceEnd<len(a) then

FOR stringposIter=sentenceEnd TO stringpos STEP -1
if code(a(stringposIter to stringposIter))=code(" ") then

print at lines,0;a(stringpos to stringposIter)

stringpos=stringposIter
exit for
end if
next

end if

if sentenceEnd>len(a) then
print at lines,0;a(stringpos to sentenceEnd)
exit while
end if

lines=lines+1

wend