Exercise: Interest Calculator

For this exercise, you're going to perform a compound interest calculation. This program will calculate the interest on a savings account, given some information about interest rates, deposits, and time. The formula you're going to use is as follows :

Using your text editor, type the program from Listing 2.2 and save it as Interest . Do not type in the line numbers . Make the program executable according to the instructions you learned in Hour 1.

When you're done, try running the program by typing the following at a command line:

 perl Interest 

Listing 2.3 shows a sample of the Interest program's output.

Listing 2.2. The Complete Source of the Interest Program

 1:   #!/usr/bin/perl -w 2: 3:   print "Monthly deposit amount? "; 4:   $pmt=<STDIN>; 5:   chomp $pmt; 6: 7:   print "Annual Interest rate?  (ex. 7% is .07) "; 8:   $interest=<STDIN>; 9:   chomp $interest; 10: 11:  print "Number of months to deposit? "; 12:  $mons=<STDIN>; 13:  chomp $mons; 14: 15:  # Formula requires a monthly interest 16:  $interest/=12; 17: 18:  $total=$pmt * ( ( ( 1 + $interest) ** $mons ) -1 )/ $interest; 19: 20:  print "After $mons months, at $interest monthly you\n"; 21:  print "will have $total.\n"; 

Line 1 : This line contains the path to the interpreter (you can change it so that it's appropriate to your system) and the -w switch. Always have warnings enabled!

Line 3 : The user is prompted for an amount.

Line 4 : $pmt is read from the standard input device (the keyboard).

Line 5 : The newline character is removed from the end of $pmt .

Lines 7 “9 : $interest is read in from the keyboard, and the newline is removed.

Lines 11 “13 : $mons is read in from the keyboard, and the newline is removed.

Line 16 : $interest is divided by 12 and stored back in $interest .

Line 18 : The interest calculation is performed, and the result is stored in $total .

Lines 20 “21 : The results are printed.

Listing 2.3. Output from the Interest Program

 1: Monthly deposit amount?  180  2: Annual Interest rate?  (ex. 6% is .06)  .06  3: Number of months to deposit?  120  4: After 120 months, at 0.005 monthly you 5: will have 29498.2824251624.