session_state
////////////////////////////////
 The objective is to save time without rerun the whole code
  assume that we have a global varible
     number_of_obs=5

if 'number_of_obs' not in session_state:
    session_state['number_of_obs']=5
     # or
    # session_state.number_of_obs=5

increment=st.button('show more rows ')
if increment:
   st.session.number_of_obs +=1

decrement=st.button('show less rows ')
if decrement:
   st.session.number_of_obs -=1
////////////////////////////////

2nd example 
////////////////////////////////
Assume that the Git Bash is installed. 

 Step 1: go to the subdirectory
 Step 2: righ-click your mouse
         -> choose 'Git Bash here'
 Step 3: git init
         ls -al

 Step 4: git status

 Step 5: add 
////////////////////////////////

videos
////////////////////////////////
 Simplilearn,2020,  Git Push Command | Git Bash Tutorial | Git Commands | Git Tutorial For Beginners | 
   https://www.youtube.com/watch?v=QJ0iUNe27c8

 Becoming a Data Scientist, 2020,How to Install Git Bash on Windows 10
   https://www.youtube.com/watch?v=qdwWe9COT9k

 Steele,Colt, 2020,Learn Git In 15 Minutes
   https://www.youtube.com/watch?v=USjZcfj8yxE

   https://www.youtube.com/watch?v=iGutIN5t9Mo&t=88s
////////////////////////////////

From where to download R
////////////////////////////////
To install R, we have the following steps:

  Step 1: go to http://r-project.org

  Step 2: click "CRAN" on the left
  
  Step 3: choose a server nearby

  Step 4: choose appropriate type such as Windows, Mac

  Step 5: download "Base"

///////////////////////////////

launch/quit R, one line for this course
////////////////////////////////
There are several ways to quit R
-------------------------------
  1) type q()
  2) click "File" -> "Quit"
  3) click the red x on top right

  Note: after type q(), you will be asked "Keep the objectives".
     If you want to keep your variables or functions, answer "yes".
     Otherwise, no. 

One-line R code for this course 
---------------------------------
 source("http://datayyy.com/fmr.R")


///////////////////////////////