Back to basics - Operator Precedence

Why do we need to test forensic tools why the programmers compiled the code without any errors?  The concept of logical errors and algorithm implementations can not be detected by compiling code, they can be found by continuous testing with the right input and output needs to be monitored for the correct values.  We need to avoid garbage in, garbage out conditions for reliable tool testings.  One of the implementation issues that can be detected by testing is the operator precedence.

In this presentation, I wanted to talk about the order of operations that are ignored in many cases.  Order of operations are used by systems to evaluate the value of an expression by parsing the expression by operator precedence as defined for the given system.

Analyzing code requires not just pattern recognition to specific code, but also the recognition of logical errors that might have been exploited.

In this chart, I give an example of the flow of operator evaluation, but the accompanying video will give a more in-depth explanation.  http://youtu.be/7EQ5YZOU7tw

             You can practice operator precedence on the command line by setting variables 
             by arithmetic operations.                                                                                         
             C:\>set /a test=(9*9)*4/(9*(5*5*5)-(14-6))                                                               
             0                                                                                                                              

This operation can also represented in postfix notation and used with DC command line utility.  The above expression in postfix notation is 9 9 * 4 * 9 5 5 * 5 * 14 * 6 - / 

 Download UnxUtils.zip from
https://sourceforge.net/projects/unxutils/
Extract files from UnxUtils.zip to c:\temp
Change directory to
cd c:\temp\UnxUtils\usr\local\wbin
Type dc to start
dc

You will only see a blinking cursor, but that is your prompt and you can just type values.
Type
34
2
/
p

p is to print the result to the screen.  If you are done using it, type q to exit

 c:\Users\<UID>\Downloads\UnxUtils\usr\local\wbin>dc
9
9
*
4
*
9
5
5
*
5
*
14
*
6
-
/
p
0
q

c:\Users\<UID>\Downloads\UnxUtils\usr\local\wbin>

 As you can see, DC only works as integer operations, so the result will be screwed, but is should still give you a good idea how protfix notation works.

Here is an online converter to make the conversion easier, but only use it to verify your convernsions otherwise you will never learn how to do it on your own.  It is very important for you to learn this in order to understand queue and stack operations.

http://www.mathblog.dk/tools/infix-postfix-converter/