##################################################################
### using R as a calculator
1 + 1
log(2)
log(1000, 10)
25/3
25%/%3
25%%3
# creating an R object and assigning it the value 
houda <- 42;
houda
# manipulating objects
houda + 2
houda <- 4 + 6
houda

##################################################################
### loading datasets
## a) using RStudio/Rcmdr import feature (preferred)
# aka using readxl::read_excel (load the package readxl and use the function read_excel)
library(readxl)
scores <- read_excel(path = "sample-data.xlsx", sheet = "scores")
# shorter way (assuning a right order of the function parameters)
scores <- read_excel("sample-data.xlsx", "scores")
# if you don't know where the file resides
scores <- read_excel(file.choose(), "scores")

## b) using read.csv/read.csv2 function (backsafe, always working)
# first export the desired Excel sheet to CSV format 
# read.csv for CSV comma delimited and decimal point (English format)
# read.csv2 for CSV semicolon delimited and decimal comma (German format)
scores <- read.csv2("scores.csv")

## d) by the direct input (backsafe, always working but unhandy)
scores <- data.frame (student = c(1:8), gpa = c(2.8, 3.4, 3, 3.5, 3.6, 3, 2.7, 3.7), ACT = c(21, 24, 26, 27, 29, 25, 25, 30))

##################################################################
### working with datasets
# view the whole dataset (don't try for large datasets!)
scores
# or use the View feature
View(scores)
# list only several first rows
head(scores)
# list the names of the variables 
names(scores)
# access a particular variable
scores$gpa
# and a particular value
scores$gpa[2]


# subset of a dataset / a variable 
scores[1:5,]                       # only observation number from 1 to 5 selected
scores[scores$student %% 2 == 0,]  # only observation with even student number selected
scores[scores$ACT > 25,]           # only students with ACT > 25 selected
scores[scores$ACT > 25,]$gpa       # and the corresponding gpa values

# let work directly with variables
attach(scores)
gpa
gpa[2]
# don't need the access anymore
detach(scores)
# (due to masking, try to avoid naming the dataset with the same name as one of the variables)