Updated 2017-11-10 19:16:45 by gold

## Monthly Car Loan Payments and eTCL demo example calculator, numerical analysis edit

gold Here is some eTCL starter code for Monthly Car Loan Payments in calculator shell.

In planning any software, it is advisable to gather a number of testcases to check the results of the program. The results of the testcases are estimated using the hand calculator and then checked in the eTCL slot calculator. Pseudocode and equations are developed from the hand calculations and theory. Small console programs are written to check or proof the alternate subroutines or procedures, rather than keeping the unblessed code and comment lines in the main slot calculator. Finally the improved or alternate subroutines are loaded into the slot calculator. The eTCL slot calculator is effectively a shell program to input entries, host calculation routines, and display results. Additional significant figures are used to check the eTCL calculator, not to infer the accuracy of inputs and product reports.

For the first testcase of was hand calculated in formula.

For the second testcase of =

For the third testcase of

## Pseudocode Section edit

```    # using  pseudocode for  procedure algorithm.
3 quantities needed
total loan amount dollars
number of months
interest_rate in months
check function in TCL
ref. number format procedure
check_answer   new area =? desired goal , desired goal reached (yes/no)
set answers and printout with resulting values```

### Testcases Section

In planning any software, it is advisable to gather a number of testcases to check the results of the program. The math for the testcases can be checked by pasting statements in the TCL console. Aside from the TCL calculator display, when one presses the report button on the calculator, one will have console show access to the capacity functions (subroutines).

#### Testcase 1

table 1printed in tcl wiki format
quantity value comment, if any
1:testcase_number
2600.0 :total loan amount dollars
24.0 :number of months
0.10000000000000001 :function factor
289.37941851178823 :answers: intermediate term in formula
0.0 :optional
0.0 :optional
2600.0 :total loan
289.38 :monthly interest rate

#### Testcase 2

table 3 printed in tcl wiki format 2: testcase_number 1000.0 : total loan amount dollars 24.0 : number of months 0.20000000000000001 : function factor 202.54787303089748 : answers: intermediate term in formula 0.0 : optional 0.0 : optional 1000.0 : total loan 202.55 : monthly interest rate quantity value comment, if any

#### Testcase 3

table 3printed in tcl wiki format
quantity value comment, if any
3:testcase_number
10000.0 :total loan amount dollars
24.0 :number of months
0.20000000000000001 :function factor
2025.4787303089749 :answers: intermediate term in formula
0.0 :optional
0.0 :optional
10000.0 :total loan
2025.5 :monthly interest rate

### References:

• Calculating a Car Payment,
• Grade Level: 9 - 12 by Barbara Christopher
• How Does Car Loan Interest Work?, www.ifsautoloans.com
• Estimate your monthly car loan payment, www.calculator.net

## Appendix Code edit

### appendix TCL programs and scripts

```        # pretty print from autoindent and ased editor
# Monthly Car Loan Payments calculator (amortized over loan)
# written on Windows XP on eTCL
# working under TCL version 8.5.6 and eTCL 1.0.1
# gold on TCL WIKI, 5nov2017
package require Tk
package require math::numtheory
namespace path {::tcl::mathop ::tcl::mathfunc math::numtheory }
set tcl_precision 17
frame .frame -relief flat -bg aquamarine4
pack .frame -side top -fill y -anchor center
set names {{} {total loan amount dollars :} }
lappend names {number of months:}
lappend names {monthly interest rate: }
lappend names {answers: intermediate term in formula }
lappend names {optional  :}
lappend names {optional : }
lappend names {total loan amount dollars: }
lappend names {monthly payments :}
foreach i {1 2 3 4 5 6 7 8} {
label .frame.label\$i -text [lindex \$names \$i] -anchor e
entry .frame.entry\$i -width 35 -textvariable side\$i
set msg "Calculator for Monthly Car Loan Payments
from TCL WIKI,
written on eTCL "
tk_messageBox -title "About" -message \$msg }
proc ::tcl::mathfunc::precision {precision float}  {
#  tcl:wiki:Floating-point formatting, [AM]
set x [ format "%#.5g" \$float ]
return \$x
}
#proc errorx always returns a positive error.
#Normally assume \$aa is human estimate,
#assume \$bb is divinely exact.
proc errorx  {aa bb} {expr { \$aa > \$bb ?   ((\$aa*1.)/\$bb -1.)*100. : ((\$bb*1.)/\$aa -1.)*100.}}
proc calculate {     } {
global side1 side2 side3 side4 side5
global side6 side7 side8
global testcase_number
incr testcase_number
set side1 [* \$side1 1. ]
set side2 [* \$side2 1. ]
set side3 [* \$side3 1. ]
set side4 [* \$side4 1. ]
set side5 [* \$side5 1. ]
set side6 [* \$side6 1. ]
set side7 [* \$side7 1. ]
set side8 [* \$side8 1. ]
set loan_amount \$side1
set number_months \$side2
set monthly_interest_rate \$side3
set term1 [+ 1. \$monthly_interest_rate ]
set monthly_payments [/  [* \$loan_amount \$monthly_interest_rate ] [- 1. [** \$term1 [* -1.  \$number_months ]]]]
set side4 \$monthly_payments
set side7 \$loan_amount
set side8 [::tcl::mathfunc::precision 4 \$monthly_payments ]
}
proc fillup {aa bb cc dd ee ff gg hh} {
.frame.entry1 insert 0 "\$aa"
.frame.entry2 insert 0 "\$bb"
.frame.entry3 insert 0 "\$cc"
.frame.entry4 insert 0 "\$dd"
.frame.entry5 insert 0 "\$ee"
.frame.entry6 insert 0 "\$ff"
.frame.entry7 insert 0 "\$gg"
.frame.entry8 insert 0 "\$hh"
}
proc clearx {} {
foreach i {1 2 3 4 5 6 7 8 } {
.frame.entry\$i delete 0 end } }
proc reportx {} {
global side1 side2 side3 side4 side5
global side6 side7 side8
global testcase_number
console show;
puts "%|table \$testcase_number|printed in| tcl wiki format|% "
puts "&| quantity| value| comment, if any|& "
puts "&| \$testcase_number:|testcase_number | |&"
puts "&| \$side1 :|total loan amount dollars |   |&"
puts "&| \$side2 :|number of months | |& "
puts "&| \$side3 :|function factor | |& "
puts "&| \$side4 :|answers: intermediate term in formula| |&"
puts "&| \$side5 :|optional  | |&"
puts "&| \$side6 :|optional |  |&"
puts "&| \$side7 :|total loan |  |&"
puts "&| \$side8 :|monthly interest rate |  |&"
}
frame .buttons -bg aquamarine4
::ttk::button .calculator -text "Solve" -command { set side8 0 ; calculate   }
::ttk::button .test2 -text "Testcase1" -command {clearx;fillup 2600.  24.0  0.1 289.0  0.0   0.0    2600.00  289.38}
::ttk::button .test3 -text "Testcase2" -command {clearx;fillup 1000.0 24.0 0.2 202.0   0.0   0.0  1000.00     202.55}
::ttk::button .test4 -text "Testcase3" -command {clearx;fillup 10000.0 24.0 0.2  2025.0  0.0    0.0  10000.00  2025.50}
::ttk::button .clearallx -text clear -command {clearx }
::ttk::button .cons -text report -command { reportx }
::ttk::button .exit -text exit -command {exit}
pack  .clearallx .cons .about .exit .test4 .test3 .test2   -side bottom -in .buttons
grid .frame .buttons -sticky ns -pady {0 10}
. configure -background aquamarine4 -highlightcolor brown -relief raised -border 30
wm title . "Monthly Car Loan Payments Calculator"
```

### Pushbutton Operation

For the push buttons, the recommended procedure is push testcase and fill frame, change first three entries etc, push solve, and then push report. Report allows copy and paste from console.

For testcases in a computer session, the eTCL calculator increments a new testcase number internally, eg. TC(1), TC(2) , TC(3) , TC(N). The testcase number is internal to the calculator and will not be printed until the report button is pushed for the current result numbers. The current result numbers will be cleared on the next solve button. The command { calculate; reportx } or { calculate ; reportx; clearx } can be added or changed to report automatically. Another wrinkle would be to print out the current text, delimiters, and numbers in a TCL wiki style table as
```  puts " %| testcase \$testcase_number | value| units |comment |%"
puts " &| volume| \$volume| cubic meters |based on length \$side1 and width \$side2   |&"  ```