Notebook for chapter 1_part 2.Rmd

---
title: "R Notebook"
output: html_notebook
---

This is an [R Markdown](http://rmarkdown.rstudio.com) Notebook. When you execute code within the notebook, the results appear beneath the code. 

Try executing this chunk by clicking the *Run* button within the chunk or by placing your cursor inside it and pressing *Ctrl+Shift+Enter*. 


# Special values: 

1. Inf: stands for Infinity.  

```{r}
v <- 1/0
v
```

2. NaN: Not a Number. A reserved keyword to represent a quantity that 
cannot be mathematically defined.

```{r}
v <- 0/0
v
```
3. NA: not available. Used when the data is missing or unavailable in a dataset. 

4. NULL: No value. 

For NaN we actually know what the value is, because it’s something insane like 0/0. 

For NA, we believe that there is supposed to be a value “out there”, but we don’t quite know what it is. 

But for NULL we strongly believe that there is no value at all.


# Factor: 

Say 5 students just got their marks cards and the scores were as follows: 

```{r}
marks <- c(24, 30, 35, 19, 20)
```

None of them were happy and so they opted for a re-exam. Their marks doubled.
```{r}
reexam_marks <- marks*2
```

Now let's say there are three groups that got three different treatments
I'll label them
```{r}
treatmentGroup <- c(1,1,2,2,3,3)
class(treatmentGroup)
```
Now if I attempt to do some operations like
```{r}
treatmentGroup + 1
```
And we know this makes no sense given treatment group is a nominal variable
But how do we make R make sense of this? 
```{r}
treatmentGroup <- as.factor(treatmentGroup)
treatmentGroup
```
```{r}
class(treatmentGroup)
```
```{r}
treatmentGroup + 1
```



# Lists
Lists are like dataframes but are even more flexible. 

Data frames are nice “rectangular” table of data. But Lists have no such constraints. 

```{r}
Dan <- list( age = 34,
            nerd = TRUE,
            parents = c("Joe","Liz") 
)
print(Dan)
```

# Loops

Sometimes you want to repeat a line of code multiple times

A) A WHILE loop:

Say, you want to find the smallest multiple of 17 that is greater than or equal to 1000. 
Here’s how you could do this:

```{r}
x <- 0
while ( x < 1000 ) {
  x <- x + 17
}
print( x )
```
 
B) A FOR loop:
```{r}
for ( i in 1:3 ) {
  print( "hello" )
}
```


# Conditional: 

Say if a condition holds (student belongs to IITK), then you ask something (their feedback related to IITK) , else you dont ask anything. How can you do that? 

```{r}
today <- Sys.Date()       # pull the date from the system clock
day <- weekdays( today )  # what day of the week is today

# now make a choice depending on the day...
if ( day == "Monday" ) {
  print( "Yes I have class today" )
} else {
  print( "No class today! Yay!" )
}
```

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