Why BASIC had numbered lines? [duplicate]

Question

Possible Duplicate:
Why did we bother with line numbers at all?

I'm curious about why early versions of the BASIC programming language had line numbering like in:

42 PRINT "Hello world!"

The text editors back then had no line numbering?

EDIT: Yes, I know they are used for GOTOs, but why? I mean having labels was too computationally expensive?

Answer 1

Many microcomputers had a BASIC interpreter in ROM that would start upon bootup. The problem was that there was no text editor or file system to speak of. You had an interactive prompt to do everything through. If you wanted to insert a line of code, you just typed it, starting with the line number. It would insert it into the correct spot in you code. e.g:

>10 print "hello"
>30 goto 10
>20 print "world"
>list
10 PRINT "hello"
20 PRINT "world"
30 GOTO 10
>

(In that example > is the BASIC prompt)

If you wanted to erase a line, you would type something like ERASE 20. Some really fancy systems gave you a line editor (i.e. EDIT 10) And if you didn't plan your line numbers and ran out (how do I insert a line between 10 and 11?) some systems gave you a RENUM command which would renumber your code (and adjust GOTOs and GOSUBs appropriately).

Fun Times!