R editors  
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Method I: Notepad
-----------------
          We know that we could use Notepad as our text editor 
           to write and edit our R programs. 

Method II: R editor 
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         Step 1: click "File" -> "New Script"

         Step 2: enter your program, such as

                 dd<-function(x)2*x

         Step 3: click "File" -> "Save as"
    
                 Note #1: One of the advantages: 
                 Click "Edit" -> Run line or selection   [ctl-R]
                           or -> Run All 

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Three ways to input values
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 Method 1: input values according to their order

 Method 2: input according to keyword

 Method 3: mixed 

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Two types of comments
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The first type command starts with #

  >pv<-100    # pv for present value

The second type of command are included by a pair of double quotation marks

"
Objective    : estimate present value
                      fv
  formula used: pv= --------
                    (1+r)^n

           pv : present value
           fv : future value
           r  : effective period rate
           n  : number of periods

 Example 1: > .pv_f(0.1,1,100)     # meaning of inputs depend on order
             [1] 90.90909

 Example 2: > .pv_f(0.045,5,225)
             [1] 180.5515

 Example 3: > .pv_f(fv=100,r=0.1,n=1)  # meaning depend on keywords
            > .pv_f(r=0.1,fv=100,n=1)
            > .pv_f(n=1,r=0.1,fv=100)

              Note: The above three give the same result 
"
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Links 
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Bhalla, Deepanshu,2017, Exclusive Tutorial on Data Manipulation with R (50 Examples)
    https://www.datasciencecentral.com/profiles/blogs/exclusive-tutorial-on-data-manipulation-with-r-50-examples

Data Management in R,2017, Best packages for data manipulation in R  
     https://datascienceplus.com/best-packages-for-data-manipulation-in-r/

R document, factor
    https://www.rdocumentation.org/packages/base/versions/3.6.1/topics/factor
SEC index website
    https://www.sec.gov/Archives/edgar/full-index/
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