After a quick look at your program, I notice that you are only incrementing the variable i if the entity encountered is a LINE entity.
Since you haven't used a filter list with the ssget selection, there is a good chance that the user could inadvertently select an object which is not a line.
In this case, the if statements within your repeat loop would not evaluate to T, and so the counter variable i would never be incremented.
Hence, in the next iteration of the repeat loop, the same entity is processed (since i has not been incremented), and again i will not be incremented, and so on and so forth.
Meanwhile, the program is still writing the values of variables c and cbert to the file, and, since the values of these variables have not been altered since the previous iteration, the same values are written over and over - that is, if a LINE has been encountered first, since otherwise these variables would be nil and you would receive an error from supplying a null value to the rtos function.
Here is an alternative approach for you to study, I have included comments to help you see what the code is doing at each stage:
Ask if you have any questions about any part of the above code.
(defun c:pt2txt ( / counter endpoint entitydata filename openfile selection startpoint )
;; Define the function and localise the variables.
;; If the following expression returns a non-nil value...
;; All of the enclosed expressions must return a non-nil
;; value for the AND function to return T.
(setq selection (ssget '((0 . "LINE"))))
;; Provide the SSGET function with a filter list so
;; that only LINEs may be selected.
(setq filename (getfiled "Specify Output File" "" "txt" 1))
;; The IF statement is required since the user could press
;; 'cancel' when prompted to specify an output file.
;; If 'cancel' is pressed, the GETFILED function would
;; return nil.
;; (open nil "w") will result in an error.
(setq openfile (open filename "w"))
;; A check that the file was able to be opened for writing.
;; This is to account for the possibility that the user
;; has specified an output file in a location for which they
;; do not have write permissions.
;; In such a case (open <filename> "w") will return nil.
) ;; End AND
;; The IF function can only accept at most three parameters:
;; a TEST expression, a THEN expression to be evaluated if the
;; TEST expression evaluates to a non-nil value, and an ELSE expression
;; to be evaluated if TEST evaluates to nil.
;; Hence, if we wish to evaluate multiple expressions within the THEN
;; parameter, we need to wrap these expressions into a single PROGN
;; expression to be passed to the IF function.
;; The PROGN function does nothing more than evaluate the expressions
;; passed to it, then return the result of the last expression evaluated.
(setq counter 0)
;; Initialise the counter variable at zero
;; (since the Selection Set index is zero based).
(repeat (sslength selection)
;; Repeat evaluation of the following expressions a number
;; of times equal to the number of objects in the selection set.
(setq entitydata (entget (ssname selection counter)))
;; The entity at index 'counter' in the selection set is returned by
;; the SSNAME function, and its corresponding DXF entity data is returned
;; by the ENTGET function.
(setq startpoint (cdr (assoc 10 entitydata))
endpoint (cdr (assoc 11 entitydata))
;; Collect the start and end points of the Line entity from the DXF data list
;; We know that is must be a LINE entity since the SSGET filter list has
;; only allowed the user to select lines.
(rtos (car startpoint)) ";"
(rtos (cadr startpoint)) ";"
(rtos (caddr startpoint))
(rtos (car endpoint)) ";"
(rtos (cadr endpoint)) ";"
(rtos (caddr endpoint))
;; Write the x;y;z values of the start and end points of the Line entity
;; to the open file.
;; RTOS is not supplied with units or precision parameters, so the values
;; of the LUNITS and LUPREC System Variables will be used.
(setq counter (1+ counter))
;; Increment the counter to move on to the next line in the Selection Set
) ;; End REPEAT
;; Close the open file.
(princ (strcat "\nThe data from " (itoa counter) " line(s) has been written to file."))
;; Print a message to inform the user that the data has been written successfully.
) ;; End PROGN
) ;; End IF
;; (princ) is used to 'exit cleanly', since otherwise the
;; result of the last expression evaluated (the IF expression) would
;; be returned.
) ;; End DEFUN