B

Task/Arithmetic-evaluation/BBC-BASIC/arithmetic-evaluation.bbc

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+      Expr$ = "1 + 2 * (3 + (4 * 5 + 6 * 7 * 8) - 9) / 10"
+      PRINT "Input = " Expr$
+      AST$ = FNast(Expr$)
+      PRINT "AST =   " AST$
+      PRINT "Value = " ;EVAL(AST$)
+      END
+
+      DEF FNast(RETURN in$)
+      LOCAL ast$, oper$
+      REPEAT
+        ast$ += FNast1(in$)
+        WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
+        oper$ = LEFT$(in$,1)
+        IF oper$="+" OR oper$="-" THEN
+          ast$ += oper$
+          in$ = MID$(in$,2)
+        ELSE
+          EXIT REPEAT
+        ENDIF
+      UNTIL FALSE
+      = "(" + ast$ + ")"
+
+      DEF FNast1(RETURN in$)
+      LOCAL ast$, oper$
+      REPEAT
+        ast$ += FNast2(in$)
+        WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
+        oper$ = LEFT$(in$,1)
+        IF oper$="*" OR oper$="/" THEN
+          ast$ += oper$
+          in$ = MID$(in$,2)
+        ELSE
+          EXIT REPEAT
+        ENDIF
+      UNTIL FALSE
+      = "(" + ast$ + ")"
+
+      DEF FNast2(RETURN in$)
+      LOCAL ast$
+      WHILE ASC(in$)=32 in$ = MID$(in$,2) : ENDWHILE
+      IF ASC(in$)<>40 THEN = FNnumber(in$)
+      in$ = MID$(in$,2)
+      ast$ = FNast(in$)
+      in$ = MID$(in$,2)
+      = ast$
+
+      DEF FNnumber(RETURN in$)
+      LOCAL ch$, num$
+      REPEAT
+        ch$ = LEFT$(in$,1)
+        IF INSTR("0123456789.", ch$) THEN
+          num$ += ch$
+          in$ = MID$(in$,2)
+        ELSE
+          EXIT REPEAT
+        ENDIF
+      UNTIL FALSE
+      = num$