RosettaCodeData/Task/Rot-13/IS-BASIC/rot-13.basic

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100 PROGRAM "Rot13.bas"
110 DO
120 LINE INPUT PROMPT "Line: ":LINE$
130 PRINT ROT13$(LINE$)
140 LOOP UNTIL LINE$=""
150 DEF ROT13$(TEXT$)
160 LET RESULT$=""
170 FOR I=1 TO LEN(TEXT$)
180 LET CH$=TEXT$(I)
190 SELECT CASE CH$
200 CASE "A" TO "M","a" TO "m"
210 LET CH$=CHR$(ORD(CH$)+13)
220 CASE "N" TO "Z","n" TO "z"
230 LET CH$=CHR$(ORD(CH$)-13)
240 CASE ELSE
250 END SELECT
260 LET RESULT$=RESULT$&CH$
270 NEXT
280 LET ROT13$=RESULT$
290 END DEF