CMPT14x Lab 00: Template Lab Solution

Problem:

Compute the amount there will be in my bank account after one year if the interest is compounded quarterly and any withdrawals I make are entered immediately after the interest is paid.

Problem Suitability:

The bank account example given above is well within the reach of a simple calculator, and is therefore of somewhat marginal suitability for computer solution. However, the solution does illustrate how a number of the programming ideas discussed in chapter one are actually implemented in Modula-2.

Problem Restatement:

Libraries:

This problem has both input and output requirements. It will, therefore make use of the standard I/O Modula-2 libraries STextIO, SWholeIO, and SRealIO. Because of the withdrawals, four separate simple interest calculations will be made, rather than one compound interest calculation. No mathematical or financial libraries are required.

Problem Refinement (natural language pseudocode):

  1. Input Section:
  2. Computation:
  3. Output:

Second Refinement (natural language pseudocode):

  1. Input Section:
  2. Computation:
  3. Output:

Pseudocode (third refinement; a fourth may also be necessary):

In this refinement, only portions of the code that involve several steps, formulas, or computations (i.e. an algorithm) are detailed. In this example, the last natural language refinement is followed very closely, so this section might have been left out in this case, but it is included to make the model for students to follow as complete as possible.

Data Table:

Variables:

Constants:

Imports:

Sample I/O:

Input:

Output:

User Manual:

Description:

Bank Interest is a small application designed to maintain a simple bank account record. It allows the user to specify the opening balance, and annual interest rate at the beginning of the program, and then to make quarterly withdrawals. Interest is calculated on the balance before the withdrawal.

Operation:

If in an MS-DOS or UNIX environment, type the name of the program from the operating system prompt. If in a Graphics User Interface (GUI) environment, click twice in close succession on the icon representing the program. A banner will appear giving the name of the program and some information about the author. Following this the prompt 
What is the opening balance?
will appear on the screen. Type in the amount that the bank account had at the start of the year. Following this, the prompt 
What is the annual interest rate?
  Enter as a decimal; Example: 6.5% is 0.065.
will appear on the screen. Type in the interest rate in the form shown. Now, for each quarter, a message like 
How much do you wish to withdraw in quarter number 1?
will appear on the screen. Respond with the amount being withdrawn for that quarter in the form 10.23. That is, give the number in decimal form and without a dollar sign. The program will finally respond with a message like: 
The final balance in your account is 103.47

Code:

MODULE BankInterest;

(* Name: Nellie Hacker
 * Student Number: 922001
 * CMPT 140 Fall 2005
 * Lab 00
 * Bank Interest Computation
 *)

FROM STextIO IMPORT
  WriteString, WriteLn, SkipLine;

FROM SWholeIO IMPORT
  WriteCard;

FROM SRealIO IMPORT
  ReadReal, WriteFixed;

CONST
  time = 0.25; (* i.e., one quarter *)

VAR
  principal, rate, interest, withdrawl: REAL;
  quarter: CARDINAL;

BEGIN
  WriteString ("This program computes interest on an account");
  WriteLn;
  WriteString ("quarterly, and then allows for withdrawals.");
  WriteLn;
  WriteLn;
  WriteString ("It was written by");
  WriteLn;
  WriteString ("Nellie Hacker");
  WriteLn;
  WriteString ("for CMPT 140");
  WriteLn;
  WriteString ("Lab 00 Due 14 Sep 2005");
  WriteLn;
  WriteLn;
  WriteString ("What is the opening balance? ");
  ReadReal (principal);
  SkipLine;
  WriteLn;
  WriteString ("What is the annual interest rate? ");
  WriteLn;
  WriteString ("Enter as a decimal; E.g.: 6.5% is 0.065. == ");
  ReadReal (rate);
  SkipLine;
  WriteLn;

  quarter := 1;
  WHILE quarter <= 4
    DO
      interest := principal * rate * time;
      principal := principal + interest;	(* compute new principal *)
      WriteString ("What is the withdrawal in quarter number ");
      WriteCard (quarter, 0);
      WriteString ("? == ");			(* complete a readable prompt *)
      ReadReal (withdrawal);
      SkipLine;
      WriteLn;
      principal := principal - withdrawal;	(* recompute principal *)
      quarter := quarter + 1;
    END;

  WriteString ("The final balance in your account is $ ");
  WriteFixed (principal, 2, 0);
  WriteLn;
END BankInterest.

Actual Output

This program computes interest on an account
quarterly, and then allows for withdrawals.

It was written by
Nellie Hacker
for CMPT 140
Lab 00 Due 14 Sep 2005

What is the opening balance? 1000.00
What is the annual interest rate?
Enter as a decimal; E.g.: 6.5% is 0.065. == 0.08
What is the withdrawal in quarter number 1? == 11.00
What is the withdrawal in quarter number 2? == 9.00
What is the withdrawal in quarter number 3? == 5.00
What is the withdrawal in quarter number 4? == 6.00
The final balance in your account is $1050.30
NOTE: An actual lab assignment submission should include several sample runs with different input.