Description
Write a Unix shell program (i.e. script) called “calc” to simulate a simple calculator.
Key functional requirements:
● Begin by displaying a brief set of instructions on how to use the calculator.
● Prompt the user for an operand followed by an operator and then the second operand.
● Display the two numbers, the operation and the answer (see example sequence below).
● As each subsequent operation will use the previous result, prompt the user for the next
operator and operand.
● Implement integer arithmetic only (no need for floating point support).
● Check for division by zero; display warning and continually ask the user to re-enter the
divisor.
● At the “Operand” prompt, the user can enter: Any digit, MR, MC, Clear (C), Off (X)
● At the “Operator” prompt, the user can enter: +, -, *, /, MS, M+, MC, Clear (C), Off (X)
● At both prompts (treat as operand or operator): MC, Clear (C), Off (X)
● Error checking: For the operand, assume valid input (no error checking required). For
the operator, check for an invalid operator, display a brief informative message, then
re-ask the user for input; do not “clear” the transaction.
● Initialize variables appropriately (hint: memory).
● Program (script) must be directly executable (i.e. run with “./calc” NOT “bash calc”)
● Do not implement operator precedence, or Reverse Polish Notation, or anything
complicated: Think dollar store calculator.
● Work towards a solution which minimizes code duplication.
● Match your code behaviour to the sample dialog below.
Comprehensive section (mandatory):
● Add a section of comments in the file containing your code (The # (hash/number sign)
character introduces a comment in Unix scripts) in which you answer the following
questions:
1. Name two Unix or programming concepts which this assignment helped you
learn or become more comfortable.
2. Describe two difficulties or problems which you encountered in completing the
assignment. These could be academic, technical, or non-technical.
● Answer each question in a few sentences.
● See sample comment block below. You can paste this and fill in your own words.
# This is a comment in Unix
#
# Comprehensive section:
#
# 1. This assignment helped …
#
# 2. My two difficulties were …
#
Implement the following operations using the following keys or key sequences:
User input Function Required action
digits operand entry accept entry
+ Addition arithmetic
– Subtraction arithmetic
* Multiplication arithmetic
/ Division arithmetic
C Clear clear result; keep memory
MS Memory Store stores result in memory
M+ Memory Add adds result to memory
MR Recall can be 1st or 2nd operand
MC Memory Clear puts zero in memory
X Off exit to Unix
Example sequence: (not exhaustive testing)
$ ./calc
[ *** User instructions displayed *** ]
Enter operand: 3
Enter operator: +
Enter operand: 4
3 + 4 = 7
Enter operator: –
Enter operand: 12
7 – 12 = -5
Enter operator: MS
-5 -> M
Enter operator: *
Enter operand: 3
-5 * 3 = -15
Enter operator: C
results cleared
Enter operand: MR
M -> -5
Enter operator: *
Enter operand: MR
M -> -5
-5 * -5 = 25
Enter operator: M+
25 + -5 = 20 -> M
Enter operator: /
Enter operand: 0
Can’t divide by zero. Please re-enter divisor.
Enter operand: 0
Can’t divide by zero. Please re-enter divisor.
Enter operand: -6
25 / -6 = -4
Enter operator: +
Enter operand: C
results cleared
Enter operand: -9
Enter operator: k
Sorry, k, is not a valid operator. Please re-enter operator.
Enter operator: MC
0 -> M
Enter operator: +
Enter operand: 3
-9 + 3 = -6
Enter operator: X
Good bye.
$
Suggestions on programming in an unfamiliar environment (General
comments):
Good Bad
● write 2 or 3 lines and get that working
● add a few more lines
● experiment by changing a line or two
● gradually add functionality
● build confidence
● type in an entire program without ever
testing a single line
● stumble on syntax error on line 2
● stumble again every few lines with
syntax errors
● discover that your whole approach
was wrong, but you are too heavily
invested to re-write
● become frustrated and discouraged
Suggestions for getting started on this assignment:
Begin with the looping and control structure. Focus on the control flow and sequencing
(operand vs operator, etc.). Initially ignore the arithmetic — implement that later. Use fake
output statements as placeholders (i.e. echo “2 + 3 = 5” with no computation yet).
Marking will be based on:
● Computation – correctness of results
● Logic – how accurately does it simulate a real calculator (control flow)
● Comments/Documentation
Don’t be too concerned about the number output format. I am mainly interested in the ability of
your program to perform input/output and correct control flow. Pay close attention to the
sequence of prompts in the above example.
Assig: calculator marking scheme
Comprehensive section: 2
● mandatory section: must be completed to earn any marks
● demonstrates awareness and reflection
Function and Logic: 8
● how accurately control flow simulates a calculator
● correct control sequencing (e.g. prompts for operand after operator; study after operand
4 in above sample)
● can press MC, Clear, Off anytime even at first prompt (-1 each if not supported)
● correct arithmetic
● checks for division by zero & re-prompt for operand
● MR for both operands
● works for negative numbers
Documentation & Style: 2
● follow the Widget Corporate Style Guide
● good use of variable names ($result, $operand, etc. NOT $a, $b, $c, etc.) (-1)
● answers question WHY? (-1)
● must achieve greater than 4/8 in function and logic for doc & style marks to count (i.e.
can’t pass if calculator not really working)
Penalties:
● missing or inadequate Comprehensive section – no marks (zero) for the assignment
● control sequencing error (e.g. prompting of operator/operand in wrong order) (-3)
● fail to clean up temp files: -1
● missing user instructions: -1
● script not directly executable -1
Please note: Programs are expected to be free from syntax errors and should not “bomb”.
Programs which generate syntax errors or terminate abnormally will not be eligible for partial
credit, even if a few parts “sort-of” work.
Grading Examples
Eg 1
Program is mostly working but generates syntax errors during execution. Result: Program is
below standard for submission. Mark = 0
Eg 2
Program is working perfectly but the mandatory Comprehensive Section is missing or
incomplete. Result: Program is below standard for submission. Mark = 0
Eg 3
Program prompts for “operator” when it should prompt for “operand” (or vice versa, etc.).
(Control flow error -3) Program has minor mistake of “Clear” not working at all prompts. (-1)
Result: Comprehensive 2/2; Function & Logic 4/8; Doc & Style 0/2 (not eligible because F&L
less than 5/8). Mark = 6/12
Eg 4
Program works perfectly. Missing title block (-1) and missing Originality Statement (Academic
Integrity Statement) (-1). Mark = 10/12
Submission Instructions
Please refer to the “Submission Instructions” in the D2L course shell.
