Calculating original sale price from total and tax rate

Problem: Given a total price and a tax rate, calculate the original sale price.

Example: Total Price = 27.03 and Tax Rate = 0.08125, or 8.125%

Sale price = Total Price / (1 + Tax Rate)
           = 27.03 / 1.08125
           = 25.00

Tax Amount = Total Price - Sale Price
           = 27.03 - 25.00
           = 2.03
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How to scp and ssh from Ubuntu to a Raspberry Pi without a password

From your local machine, be able to log in or rcp or rsync files to the rpi without having to enter a password.  This enables automated scripts to push code to the rpi as a server, or to automate backups.
  1. Create a local .ssh directory with cd ~; mkdir .ssh
  2. Create a public/private key pair with ssh-keygen -t rsa (take all the defaults when prompted)
    • The private key is ~/.ssh/id_rsa
    • The public key is ~/.ssh/id_rsa.pub
  3. Copy the public key to the remote rpi with scp id_rsa.pub pi@192.168.1.100:./ (your ip address may be different)
  4. ssh to the rpi with ssh pi@192.168.1.100
  5. Create a .ssh directory with mkdir .ssh
  6. Move id_rsa.pub to the .ssh directory and rename it with mv id_rsa.pub .ssh/authorized_keys
  7. Test with ssh pi@192.168.1.100 - you should be logged in without entering a password.
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Opening the gnome terminal maximized in Ubuntu 16.04

Using the Ctrl-Alt-T keyboard shortcut:
  1. Search for "Keyboard" in the Unity launcher
  2. Click the shortcuts tab
  3. Click "Custom Shortcuts"
  4. Click the "+" at the bottom of the window
  5. Enter a name, e.g., "Terminal"
  6. Enter the command: gnome-terminal --maximize
  7. Click "Apply"
  8. Set the keyboard shortcut by highlighting the row and typing the Ctrl-Alt-T key combination

Using the Unity Launcher
  1. Edit the file ~/.local/share/applications/gnome-terminal.desktop
  2. Paste these lines into the file:
    [Desktop Entry]
    Type=Application
    Name=Terminal
    Icon=utilities-terminal
    Exec=gnome-terminal --maximize
  3. Save the file
  4. Log out and back in again
The gnome terminal should launch maximized from either the Unity launcher or the Ctrl-Alt-T keyboard shortcut.  This does not solve the problem opening the terminal from Nautilus maximized.

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Restore the Default Wallpaper on an Android Device

Works on Android 4.4.2

  1. Settings→Display→Wallpaper→Home Screen
  2. Choose a Live Wallpaper
  3. Check the home screen to confirm the Live Wallpaper is working

Then:
  1. Settings→Application Manager
  2. Force stop the live wallpaper
  3. Return to the home screen: your device is set to the default wallpaper

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Calculating percentages

26 is 3/4 (75%) of what number?
Two ways to calculate:

  1. =26/(3/4)
  2. =4*26/3

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Programming the KLIK1U to work with a Genie garage door opener


Step Result
Slide open the battery compartment Battery compartment is open and “program” button is exposed
Press and hold the “program” button until the led on the remote lights up LED on remote is lit
Press the Code/Learn button on the garage door opener LED on garage door opener begins flashing
Press the “I” button 6 times, ensuring the LED on the remote stops flashing after each press LED on garage door opener will be on steady after the 6th press
Press and hold the program button on the remote LED on garage door opener turns off
Press the “I” button on the remote to test Garage door opens and closes with the remote

https://www.chamberlain.com/CatalogResourcesV3/en-us/shared/files/tucmanuals/114A3485.pdf

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Getting rid of a deleted email in iPhone

An email that was deleted from Microsoft Exchange server would not disappear from iPhone.

Steps to remove:
  1. Settings→Accounts & Passwords→Exchange (Mail, Contacts, Reminders, Notes)
  2. Turn off Mail for a few seconds, then back on.
  3. The account will re-sync everything and the email should disappear.


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Downgrade Libreoffice to the current Ubuntu version

Upgraded to the latest LibreOffice and had performance problems.
Here are the terminal commands to revert to the current Ubuntu version:
  1. sudo apt-get remove --purge libreoffice*
  2. software-properties-gtk to open Software Sources
    • Click "Other Sources" tab and remove the libreoffice PPA
  3. sudo apt-get update
  4. sudo apt-get clean && sudo apt-get autoremove
  5. sudo apt-get install libreoffice


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Calculating Percent Change

  • Method 1: (new - old) / old
  • Method 2: (new / old) - 1

Old value 100
New value 50
Method 1 =(B2-B1)/B1 -50.00%
Method 2 B2/B1-1 -50.00%

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Density = weight / volume

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Calculating Inflation in Excel

Future value of 100 dollars at 3% inflation in 10 years = $74.41:

=100/(1+.03)^10

Formula to copy in cells to calculate inflated value:
Col A        Col B
Year     Inflated Value
0        =100*(1+.03)^A1
=A1+1    =100*(1+.03)^A2
=A2+1    =100*(1+.03)^A3
=A3+1    =100*(1+.03)^A4
=A4+1    =100*(1+.03)^A5
=A5+1    =100*(1+.03)^A6
=A6+1    =100*(1+.03)^A7
=A7+1    =100*(1+.03)^A8
=A8+1    =100*(1+.03)^A9
=A9+1    =100*(1+.03)^A10
=A10+1   =100*(1+.03)^A11
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Microsoft VBA Get Existing Outlook Instance

Sub Get_Existing_Outlook_Instance()
  Dim objOutlook As Outlook.Application
  ' Gets the existing instance (if Outlook is already open) regardless of early or late binding
  On Error Resume Next
  Set objOutlook = GetObject(, "Outlook.Application")
  If objOutlook Is Nothing Then
      Debug.Print "Outlook is not open, getting a new instance"
      Set objOutlook = New Outlook.Application
  End If
  Set objOutlook = Nothing
End Sub
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Microsift VBA Early vs. Late Binding Example


Private Sub Early_vs_Late_Binding_Example()
#Const LATEBINDING = False

#If LATEBINDING Then
    Dim objOutlook As Object
    Set objOutlook = CreateObject("Outlook.Application")
#Else
    ' NOTE! Early binding requires a typelib reference.  In the VBE click Tools -> References
    ' and make sure that "Microsoft Outlook xx.x Object Library" is selected
    Dim objOutlook As Outlook.Application
    Set objOutlook = New Outlook.Application
#End If
    
    Set objOutlook = Nothing
End Sub
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Move Ubuntu Unity Launcher to the Bottom of the Screen

gsettings set com.canonical.Unity.Launcher launcher-position Bottom

 To reset to the left side:
gsettings set com.canonical.Unity.Launcher launcher-position Left
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Changing the window placement in Ubuntu

To see the current setting:
dconf read /org/compiz/profiles/unity/plugins/place/mode

To update the setting:
dconf write /org/compiz/profiles/unity/plugins/place/mode 1

(Try different values to understand the behavior.)
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Restoring the default login screen background in Ubuntu Xenial

Changing the desktop wallpaper also changed the login screen. Follow these steps to override that behavior and reset the login screen back to the default.
cd /usr/share/glib-2.0/schemas/
sudo vi com.canonical.unity-greeter.gschema.xml
Look for this key node:
<key name="draw-user-backgrounds" type="b">
<default>true</default>
<summary>Whether to draw user backgrounds</summary>
</key>
Change the value true to false.

You can also remove the white dots by changing this key node:
<key name="draw-grid" type="b">
<default>true</default>
<summary>Whether to draw an overlay grid</summary>
</key>
Now recompile the schema with this command:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
Log out and back in to see the change.
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Biweekly vs. semi-weekly

  • Biweekly means once every two weeks.
  • Semi-weekly means twice per week.
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Ubuntu Unity Configurations

To position the launcher at the bottom of the screen:
gsettings set com.canonical.Unity.Launcher launcher-position Bottom

To return to the default position on the left:
gsettings set com.canonical.Unity.Launcher launcher-position Left

To open new windows centered on the screen without installing compiz-conf:
dconf write /org/compiz/profiles/unity/plugins/place/mode 1
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Getting a list of stock symbols with Python

Lists of stock symbols can be found at either of these links:


You can download a list of symbols from the nasdaq.com site for the Nasdaq, NYSE, and AMEX exchanges.

The FTP directory at ftp.nasdaqtrader.com gets updated every night at 3:00AM (symbols can change). Here is a quick Python script to download the nasdaqlisted.txt and otherlisted.txt files from the ftp site:


#!/usr/bin/python

 # typical use: python get_symbols.py > symbols.txt

 import urllib

 def get_data( url ):
    data = urllib.urlopen( url )
    lines = []
    for line in data:
        lines.append( line.rstrip().split('|') )
    return lines
url = 'ftp://ftp.nasdaqtrader.com/SymbolDirectory/'
files = ['nasdaqlisted.txt', 'otherlisted.txt']

 for f in files:
    print get_data( url + f )


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List Comprehensions in Python

# create a new list
>>> a = [1,2,3,4,5]
>>> a [1, 2, 3, 4, 5]

 # sample list comprehension expression; equivalent to b = a
>>> b = [a[i] for i in range(len(a))]
>>> b [1, 2, 3, 4, 5]

 # only even indices
>>> b = [a[i] for i in range(len(a)) if i % 2 == 0]
>>> b [1, 3, 5]

 # only even indices and conditionally modifies the value of a[i] 
>>> b = [a[i]*2 if i > 1 else a[i] for i in range(len(a)) if i % 2 == 0]
>>> b [1, 6, 10]
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Deck of Cards Exercises

The Deck of Cards exercises.

For each suit, perform a certain exercise:
  • Clubs = burpees
  • Hearts = pushups
  • Spades = jumping jacks
  • Diamonds = sit ups

Use the value of the card as the number of repetitions:
  • 2-10 = number of reps on the card
  • Jack = 11 reps
  • Queen = 12 reps
  • King = 13 reps
  • Ace = 15 reps
  • Joker = 30/60/90 second rest -OR- 10/15/20 reps of all exercises
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Implementing ATAN2

PHP implements ATAN2 but here is an implementation of ATAN2 in PHP just because:

class Math {
  public static function atan2( $dy, $dx ) {
    if( $dy > 0 ) {
      if( $dx > 0  ) $tcl = atan($dy/$dx);
      if( $dx < 0  ) $tcl = M_PI - atan(-$dy/$dx);
      if( $dx == 0 ) $tcl = M_PI/2;
    }

    if( $dy < 0 ) {
      if( $dx > 0  ) $tcl = -atan(-$dy/$dx);
      if( $dx < 0  ) $tcl = atan($dy/$dx) - M_PI;
      if( $dx == 0 ) $tcl = -M_PI/2;
    }

    if( $dy == 0 ) {
      if( $dx > 0  ) $tcl = 0.0;
      if( $dx < 0  ) $tcl = M_PI;
      if( $dx == 0 ) $tcl = 0.0; // the 2 points are the same, default to zero
    }

    return $tcl;
  }
}

And here is an example of how to use it:
 

$heading = 270.0;  // degrees
$Ax =  0.00;
$Ay =  0.00;
$Bx = -7.89;
$By = -0.86;

// get target distance and bearing in degrees
$dx = $Bx-$Ax;
$dy = $By-$Ay;
$dist = sqrt( pow($dx,2) + pow($dy,2) );
$dir = Math::atan2( $dy, $dx ) * 180 / M_PI;
printf( "Target spotted at %0.3f degrees, distance %0.3f !!\n", $dir, $dist );

// get new heading
$diff = $heading - $dir;
if( $diff < -180 ) $diff += 360;
if( $diff > 180 ) $diff -= 360;
printf( "Turn %s %0.3f degrees\n", ($diff < 0 ? 'left' : 'right'), $diff );
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Implementing Logic Gates in a Spreadsheet

Here are some of the basic logic gates, implemented as spreadsheet formulas.

GateSpreadsheet Formula
ANDAND(A,B)
OROR(A,B)
NOTNOT(B)
NANDNOT(AND(A,B))
NORNOT(OR(A,B))
XOROR(AND(A,NOT(B)),AND(NOT(A),B))
XNORNOT(OR(NOT(OR(A,NOT(OR(A,B)))),NOT(OR(B,NOT(OR(A,B))))))

It is possible to implement any gate using only NOT and OR (NOR), or with NOT and AND (NAND) gates. For example, here is an AND gate implemented using only NOT and OR gates:

AND using only NOR gatesNOT(OR(NOT(OR(A,A)),NOT(OR(B,B)))

With AND, OR and NOT, we can implement a half adder, like this:

Column AColumn B
A0
B1
Carry (AND)=AND(B1,B2)
Sum (XOR)=OR(AND(B1,NOT(B2)),AND(NOT(B1),B2))

By stringing together two half adders and OR-ing the carry bit, we can create a full adder:

BitSpreadsheet Formula
A0
B0
Cin0
Cout=AND(B2,B3)
Sum=OR(AND(B2,NOT(B3)),AND(NOT(B2),B3))
Cout=AND(B6,B4)
Sum=OR(AND(B4,NOT(B6)),AND(NOT(B4),B6))
Cout=OR(B7,B5)
Sum=B8

Implementing a 4-bit adder is just a matter of simply replicating the full adder logic four times:

8421
Input A0000
Input B0000
A=B2=C2=D2=E2
B=B3=C3=D3=E3
Cin=C12=D12=E120
Cout=AND(B5,B6)=AND(C5,C6)=AND(D5,D6)=AND(E5,E6)
Sum=OR(AND(B5,NOT(B6)),AND(NOT(B5),B6))=OR(AND(C5,NOT(C6)),AND(NOT(C5),C6))=OR(AND(D5,NOT(D6)),AND(NOT(D5),D6))=OR(AND(E5,NOT(E6)),AND(NOT(E5),E6))
Cout=AND(B7,B9)=AND(C7,C9)=AND(D7,D9)=AND(E7,E9)
Sum=OR(AND(B7,NOT(B9)),AND(NOT(B7),B9))=OR(AND(C7,NOT(C9)),AND(NOT(C7),C9))=OR(AND(D7,NOT(D9)),AND(NOT(D7),D9))=OR(AND(E7,NOT(E9)),AND(NOT(E7),E9))
Cout=OR(B8,B10)=OR(C8,C10)=OR(D8,D10)=OR(E8,E10)
Sum=B11=C11=D11=E11
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Radians made easy

If you are in danger of losing your Geek cred because you don't understand radians, fear not! Radians aren't bad, they're just drawn that way.


What is a radian?

Diagram A
You already know some things about degrees, like there are 360 degrees in a circle and that you can measure an angle with two points on the circle's edge (see Diagram A).

Radians are just another way to measure angles. Here's how:

Start with a circle. The radius of the circle is a line from the center of the circle to a point on the edge. In Diagram A, that's the line CA. Start point A moving counterclockwise around the circle. It will trace out an arc as it travels. When the arc is as long as the radius of the circle, stop. Call that point B. You now have an arc AB, and an angle ACB. That angle at C is equivalent to one radian. So, the length of arc AB is the same as the length of the radius of the circle, which is line CA. Here's the rule:
  • The lengths of the arc and the radius are equal.
Next, you already know that the ratio of the circumference of a circle to its diameter is pi, or π, which is 3.14. The formula is C = πd. Since the radius of a circle is half the diameter, then d = 2r, so C = πd = π2r, more commonly written C = 2πr.

Since C = 2πr, and since we already said the radius of the circle equals one radian, setting r = 1 means that C = 2π radians. In other words, there are 2π radians in the circumference of the circle, which is 360 degrees. So, 360 degrees = 2π radians. Knowing that, we can now convert between radians and degrees.

Since there are 2π radians in 360 degrees, we get: 2π rad = 360 deg. Diving both sides by 2π, rad = 360/2π = 180/π. And, because 360 deg = 2π rad: dividing both sides by 360, deg = 2π/360 = π/180. To summarize:
  • Given degrees, you get radians with rad = deg × 180/π.
  • Given radians, you get degrees = rad × π/180.
Radians for common degrees

You'll see charts that tell you 90 degrees equals π/2 radians, or 315 degrees equals 7π/4 radians. How did they get that?

Remember that C = 2π, meaning that there are 2π radians in 360 degrees. To find out how many radians are in, for example, 90 degrees, we just multiply 2π by the ratio 90/360, like this:

radians = 90/360 × 2π.

Reducing the fraction, we get ¼ × 2π, which simplifies to 2π/4, or π/2. So 90 degrees = π/2 radians.

Let's do a few more:

180 degrees = 180/360 × 2π = ½ × 2π = 2π/2 = π radians.
270 degrees = 270/360 × 2π = ¾ × 2π = 6π/4 = 3π/2 radians.
360 degrees = 360/360 × 2π = 2π radians.

Try the calculations for yourself! 45, 135, 225, and 315 degrees are all common angles.
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Cheaper Gas Calculator (spreadsheet)

Is is worth driving to a more distant gas station to buy gas that's slightly cheaper? Here's a spreadsheet to find out. Just copy and paste into cell A1 in your favorite spreadsheet program, then enter your own data in cells A1 through A6. Your savings in cells A14 and A15 should be a positive number to make the trip worth while.

In cells A5 and A6 just put the one-way distance to the away station. The spreadsheet automatically calculates the round-trip cost.

Tested in Excel, OpenOffice Calc and Gnumeric.

15tank size (gallons)
29.55miles per gallon
3.619local price per gallon
3.599away price per gallon
2.00miles to local station (one way)
5.00miles to away station (one way)
  
=A1*A3cost to fill up at local station
=A1*A4cost to fill up at away station
=A8-A9savings from cheaper gas
  
=(A6-A5)*2extra round trip distance to away station
=A12/A2extra gallons to drive to cheaper station and back
=A13/A3extra travel cost to away station (based on local gas price)
  
=A10-A14savings (loss)
=A16/A1savings (loss) per gallon
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