A simple arithmetic expression is stored as a string in the format: <Value1><Operator><Value2> An operator character is one of the following: '+' '-' '*' '/' Example arithmetic expression strings: "803+1904" "34/7" (a) A procedure Calculate() will: • take an arithmetic expression string as a parameter • evaluate the expression • output the result. Assume: • the string contains only numeric digits and a single operator character • Value1 and Value2 represent integer values • Value1 and Value2 are unsigned (they will not be preceded by '+' or '-').

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [1] (a)(i) Declaration of local variables for Par1 Par2 and Par3
2. [1] (a)(i) Loop to end of (parameter) string // until operator is found
3. [1] (a)(i) Extract a character in a loop...
4. [1] (a)(i) Attempt at extraction of three parts of expression using substring functions
5. [1] (a)(i) Completely correct extraction of all three parts of expression
6. [1] (a)(i) Convert string to Integer using STR_TO_NUM(<something sensible>)
7. [1] (a)(i) Attempt to interpret at least two operators (Par2): + - * /
8. [1] (a)(i) Corresponding correct calculation (all operators) and final Output of result
9. PROCEDURE Calculate(Expression : STRING) DECLARE Val1, Val2, Index : INTEGER DECLARE Result : REAL DECLARE Par1, Par2, Par3 : STRING CONSTANT PLUS = '+' CONSTANT MINUS = '-' CONSTANT MULTIPLY = '*' CONSTANT DIVIDE = '/' FOR Index <- 1 TO LENGTH(Expression) //search for operator ThisChar <- MID(Expression, Index, 1) IF IS_NUM(ThisChar) = FALSE THEN Par1 <- LEFT(Expression, Index - 1) Par2 <- ThisChar Par3 <- RIGHT(Expression, LENGTH(Expression) - Index) // Added EXIT FOR or similar in explanation for clarity ENDIF NEXT Index Val1 <- STR_TO_NUM(Par1) Val2 <- STR_TO_NUM(Par3) CASE OF Par2 PLUS : Result <- Val1 + Val2 MINUS : Result <- Val1 - Val2 MULTIPLY : Result <- Val1 * Val2 DIVIDE : Result <- Val1 / Val2 ENDCASE OUTPUT Result ENDPROCEDURE
10. [1] (a)(ii) FUNCTION Calculate(Expression : STRING) RETURNS REAL
11. [1] (b) Example string: "23/0" (Any divide by zero example)
12. [1] (b) Reason: The result is infinity / cannot be represented / is undefined // will cause the program to crash