Modify Quick Sort in Commodore Basic

archive/c/commodore-basic/quick-sort.bas

@@ -1,8 +1,10 @@
-10 DIM A(100)
+5 REM 
+10 DIM A(99)
+15 DIM SK(299): REM Need to potentially stack 3 values for each array element
 20 GOSUB 2000: REM Get array
 25 REM Error if invalid, not end of input/value, or less that 2 items
 30 IF V = 0 OR C >= 0 OR NA < 2 THEN GOTO 200
-40 GOSUB 3000: REM Perform quick sort
+40 SP = -1: GOSUB 3000: REM Perform quick sort
 50 GOSUB 3500: REM Show array
 60 END
 200 Q$ = CHR$(34): REM quote
@@ -65,48 +67,45 @@
 2010 NA = 0
 2020 GOSUB 1000: REM Read input value
 2030 IF V = 0 THEN RETURN: REM invalid
-2040 NA = NA + 1
-2050 A(NA) = NR
+2040 A(NA) = NR
+2050 NA = NA + 1
 2060 IF C < 0 THEN RETURN: REM end of input or value
 2070 IF C = 44 THEN GOTO 2020: REM comma, get next value
 2080 V = 0
 2090 RETURN
 3000 REM Quick sort
-3001 REM Commodore Basic does not really support recursion because
-3002 REM everything is a global variable. However, recursion can be simulated
+3001 REM Commodore Basic does not really support recursion because everything
+3002 REM is a global variable. However, recursion can be simulated with
 3003 REM a "stack". This "stack" is just an array, SK, and a stack index, SP.
 3004 REM Source:
 3005 REM https://en.wikipedia.org/wiki/Quicksort#Lomuto_partition_scheme
 3006 REM Inputs:
 3007 REM - A contains array to sort
 3008 REM - NA contains size of array
 3009 REM Outputs: A contains sorted array
-3010 REM Need to potentially stack 3 values for each array element
-3020 DIM SK(NA * 3)
-3030 SP = 0
-3040 LO = 1
-3050 HI = NA
-3060 REM Recursive portion of algorithm
-3061 REM Inputs:
-3062 REM - A contains array to sort
-3063 REM - LO contains low index
-3064 REM - HI contains high index
-3065 REM Outputs: A contains partially sorted array
-3070 IF LO >= HI OR LO < 1 THEN RETURN
-3080 GOSUB 3300: REM Parition array and get pivot index (P)
-3090 SP = SP + 1: SK(SP) = LO: REM Push LO
-3100 SP = SP + 1: SK(SP) = HI: REM Push HI
-3110 SP = SP + 1: SK(SP) = P: REM Push P
-3120 HI = P - 1: GOSUB 3060: REM Sort left side of partition
-3130 P = SK(SP): SP = SP - 1: REM Pop P
-3140 HI = SK(SP): SP = SP - 1: REM Pop HI
-3150 SP = SP + 1: SK(SP) = HI: REM Push HI
-3160 SP = SP + 1: SK(SP) = P: REM Push P
-3170 LO = P + 1: GOSUB 3060: REM Sort right side of partition
-3180 SP = SP - 1: REM Pop P (don't store)
-3190 HI = SK(SP): SP = SP - 1: REM Pop HI
-3200 LO = SK(SP): SP = SP - 1: REM Pop LO
-3210 RETURN
+3010 LO = 0
+3020 HI = NA - 1
+3030 REM Recursive portion of algorithm
+3031 REM Inputs:
+3032 REM - A contains array to sort
+3033 REM - LO contains low index
+3034 REM - HI contains high index
+3035 REM Outputs: A contains partially sorted array
+3040 IF LO >= HI OR LO < 0 THEN RETURN
+3050 GOSUB 3300: REM Parition array and get pivot index (P)
+3060 SP = SP + 1: SK(SP) = LO: REM Push LO
+3070 SP = SP + 1: SK(SP) = HI: REM Push HI
+3080 SP = SP + 1: SK(SP) = P: REM Push P
+3090 HI = P - 1: GOSUB 3030: REM Sort left side of partition
+3100 P = SK(SP): SP = SP - 1: REM Pop P
+3110 HI = SK(SP): SP = SP - 1: REM Pop HI
+3120 SP = SP + 1: SK(SP) = HI: REM Push HI
+3130 SP = SP + 1: SK(SP) = P: REM Push P
+3140 LO = P + 1: GOSUB 3030: REM Sort right side of partition
+3150 SP = SP - 1: REM Pop P (don't store)
+3160 HI = SK(SP): SP = SP - 1: REM Pop HI
+3170 LO = SK(SP): SP = SP - 1: REM Pop LO
+3180 RETURN
 3300 REM Partition array
 3301 REM Inputs:
 3302 REM - A contains array to partition
@@ -135,11 +134,11 @@
 3501 REM A contains array
 3502 REM NA contains size of array
 3510 IF NA < 1 THEN GOTO 3590
-3520 FOR I = 1 TO NA
+3520 FOR I = 0 TO NA - 1
 3530    S$ = STR$(A(I))
 3540    IF A(I) >= 0 THEN S$ = MID$(S$, 2): REM strip leading space
 3550    PRINT S$;
-3560    IF I < NA THEN PRINT ", ";
+3560    IF I < (NA - 1) THEN PRINT ", ";
 3570 NEXT I
 3580 PRINT
 3590 RETURN