(* AS OF 10/21/79 *)
(*$C3*)

PROGRAM CALCULATE;

CONST
  RCONST = -2.5;
  RCONST1= 65535.5;

VAR R1,R2,TEMP:REAL;
    X : ARRAY [1..2] OF REAL;
    CH1,OP:CHAR;



FUNCTION SUBREAL(R1,R2:REAL) : REAL;

BEGIN
  SUBREAL := R1 - R2
END;



PROCEDURE ADDREAL(VAR R1:REAL; R2:REAL);
BEGIN
  R1 := R1  +  R2
END;

PROCEDURE TF(B:BOOLEAN);
BEGIN
  IF B THEN
    WRITELN('TRUE')
  ELSE
    WRITELN('FALSE')
END;

PROCEDURE CALC;
BEGIN
    CASE OP OF
      'S': WRITELN(SIN(R1));
      'C': WRITELN(COS(R1));
      'A': WRITELN(ARCTAN(R1));
      'L': WRITELN(LN(R1));
      'E': WRITELN(EXP(R1));
      '+': BEGIN ADDREAL(X[1],X[2]); WRITELN(X[1]:10:3) END;
      '-': WRITELN(SUBREAL(X[1],X[2]):10:2);
      '*': WRITELN(R1 * R2);
      '/': WRITELN(R1 / R2);
      'M': WRITELN(-R1);
      '=': TF(R1 = R2);
      'N': TF(R1 <> R2);
      '$': WRITELN(SQRT(R1):10:3,SQRT(R2):10:3);
      '<': TF(R1 < R2);
      '>': TF(R1 > R2);
      'Z': TF(R1 <= R2);
      'G': TF(R1 >=R2);
      '1': WRITELN(SQR(R1),' ',SQR(R2));
      '2': WRITELN(R1 + 1);
      '3': WRITELN(1+R1);
      '4': WRITELN(TRUNC(R1));
      '5': WRITELN(ROUND(R1));
      '6': WRITELN(RCONST);
      '7': WRITELN(RCONST1);
      '8': BEGIN R1 := -2.234; X[1] := 3.456; WRITELN(R1,' ',X[1]); END;

    END;
END; (* CALCULATOR *)

PROCEDURE MENU;
BEGIN
      WRITE('S:SIN   ');
      WRITE('C:COS   ');
      WRITE('A:ARCTAN   ');
      WRITE('L:LN   ');
      WRITE('E:EXP   ');
      WRITE('1:SQR   ');
      WRITELN('$:SQRT  ');
      WRITELN('+, -, *, / ARITHMETIC OPERATORS');
      WRITELN('M:NEGATE');
      WRITE('= : EQUAL   ');
      WRITELN('N : NOT EQUAL');
      WRITE('<:LESS THAN   ');
      WRITELN('>:GREATER THAN ');
      WRITELN('Z:LESS THAN OR EQUAL TO');
      WRITELN('G:GREATER THAN OR EQUAL TO');
      WRITE('4:TRUNC   ');
      WRITELN('5:ROUND');
END;

BEGIN (* MAIN PROGRAM *)
  REPEAT
    WRITE('ENTER FIRST OPERAND? ');
    READ(R1);
    X[1] := R1;
    WRITELN('R1=',R1); WRITELN;
    WRITE('ENTER SECOND OPERAND? ');
    READ(R2);
    X[2] := R2;
    WRITELN('R2=',R2); WRITELN;
    WRITELN('ENTER OPERATOR:');
    MENU;
    WRITE('? ');
    READ(OP);
    WRITELN;
    CALC;
    WRITELN('TYPE <ESCAPE> TO STOP');
    READ(CH1);
  UNTIL CH1 = CHR(27)
END.
