Qbasic code for a simple calculator

CLS

SCREEN 13

PRINT "CALCULATOR-"

PRINT "ENTER YOUR EXPRESSION"

PRINT "EXAMPLE-58 * 20"

PRINT "THE TERMS SHOULD BE SEPARATED BY SPACE OR ERROR WILL OCCUR"



INPUT "ENTER YOUR EXPRESSION-"; EXP$



EXP$ = EXP$ + " "

INDEX = 1

DIM TERMS(3) AS INTEGER



FOR I = 1 TO LEN(EXP$)

    CURRENTTOKEN$ = MID$(EXP$, I, 1)

    IF CURRENTTOKEN$ = " " THEN

        IF INDEX = 2 AND OPERATION$ = "" THEN

            OPERATION$ = TERMCURRENT$

        ELSE

            TERMS(INDEX) = VAL(TERMCURRENT$)

            INDEX = INDEX + 1

        END IF

        TERMCURRENT$ = ""

    ELSE

        TERMCURRENT$ = TERMCURRENT$ + CURRENTTOKEN$

    END IF

NEXT I







SELECT CASE OPERATION$

    CASE "+"

        R = TERMS(1) + TERMS(2)

    CASE "*"

        R = TERMS(1) * TERMS(2)

    CASE "-"

        R = TERMS(1) - TERMS(2)

    CASE "/"

        R = TERMS(1) / TERMS(2)

    CASE "%"

        R = TERMS(1) MOD TERMS(2)

    CASE "^"

        R = TERMS(1) ^ TERMS(2)

END SELECT

PRINT TERMS(1); OPERATION$; TERMS(2); "="; R