Returning pointer to the letter in string - arrays

Question

This is my code to search ih some string consists of one letter:
selectedWords BYTE "BICYCLE"
inputLetter BYTE 'Y'
cld
mov ecx, LENGTHOF selectedWords
mov edi, offset selectedWords
mov al, inputLetter ;Load character to find
repne scasb ;Search
jne notfound
But how to return the pointer to the letter in string?
If I want after to change one leter with some other. Its easy to do if you have pointer to the letter in string.

Answer 1

If you read the documentation for REP and SCASB you'll see that SCAS updates edi. Thus the location of the matching char is stored in EDI.
All you have to do is return EDI if ZF=1 and return 0 if ZF<>1.
cld
mov ecx, LENGTHOF selectedWords
mov edi, offset selectedWords
mov al, inputLetter ;Load character to find
repne scasb ;Search
jne notfound
found:
mov eax,edi ;return the address of the match.
ret
notfound:
xor eax,eax ;return 0 aka not found as address.
ret

Answer 2

If repne scasb finds the element, EDI points to the element after the first match. You have to decrement it to get the pointer to the desired element.
You don't need to clear the direction flag (cld). It's very very unlikelikely that the direction flag is set without any involvement of your part. And if so, you should seit it back to the former status.
INCLUDE Irvine32.inc
.DATA
selectedWords BYTE "BICYCLE"
inputLetter BYTE 'Y'
err_msg BYTE "Not found.", 0
.CODE
main PROC
mov ecx, LENGTHOF selectedWords
mov edi, offset selectedWords
mov al, inputLetter ; Load character to find
repne scasb ; Search
jne notfound
dec edi
mov al, [edi]
call WriteChar ; Irvine32: Write a character in AL
exit ; Irvine32: ExitProcess
notfound:
lea edx, err_msg
call WriteString ; Irvine32: Write a null-terminated string pointed to by EDX
exit ; Irvine32: ExitProcess
main ENDP
END main
If you don't like repne scasb you can scan the word with a "normal" comparison loop
INCLUDE Irvine32.inc
.DATA
selectedWords BYTE "BICYCLE"
inputLetter BYTE 'Y'
err_msg BYTE "Not found.", 0
.CODE
main PROC
mov edi, offset selectedWords
mov ecx, LENGTHOF selectedWords
mov al, inputLetter
##:
cmp [edi], al ; Compare memory/immediate value
je found ; JE = jump if equal
inc edi ; Increment pointer
dec ecx ; Decrement counter
jne #B ; Jump back to the last ##, if ECX == 0
jmp notfound
found:
mov al, [edi]
call WriteChar ; Irvine32: Write a character in AL
exit ; Irvine32: ExitProcess
notfound:
lea edx, err_msg
call WriteString ; Irvine32: Write a null-terminated string pointed to by EDX
exit ; Irvine32: ExitProcess
main ENDP
END main