---
title: "Lecture 2: Exercises"
date: October 2nd, 2018
output: 
  html_notebook:
    toc: true
    toc_float: true
---


# Execise 1: Control Flow 

## Part 1.1

Use a for loop to:

a. Print all the letters of the Latin alphabet:`

b. Print the numbers 10 to 100 that are divisible by 7

c. Print the numbers 1 to 100 that are divisible by 5 but not by 3.



## Part 1.2

a. Find all numbers not greater than 10,000 that are divisible by 
5, 7 and 11 and print them.

b. Print for each of the numbers x = 2, . . . 20, all numbers that divide x
(all factors) excluding 1 and x. Hence, for 18, it should print 2 3 6 9.


# Execise 2: Functions

## Part 2.1

a.Create a function what will return the number
of times a given integer is contained a given vector of integers.
The function should have two arguments one for a vector
and the other for a scalar. 

b. Then, generate a random vector of 100 
integers (in a range 1-20) use the function to count the number
of times the number 12 is in that vector. 

## Part 2.2

Write a function that takes in a data.frame as an input, 
prints out the column names, and returns its dimensions.

# Execise 3: Apply family functions

## Part 1
Below we print six first rows of the built-in dataset, `mtcars`, 
from the 1974 Motor Trend US magazine, which comprises information
on the fuel consumption and 10 aspects of automobile design 
and performance for 32 selected car models. 

```{r}
head(mtcars)
```

Use `apply()` function to find the standard deviation and the 0.8-quantile  
of each of the automobile characteristic.

## Part 2

Below is a vector of dates in year 2018.

```{r}
set.seed(1234)
y2018 <- seq(as.Date("2018-01-01", format = "%Y-%m-%d"), 
             as.Date("2018-12-31", format = "%Y-%m-%d"), 
             "days")
length(y2018)
# A random sample of 10 dates from 2018
y2018_sample <- sample(y2018, size = 10)
y2018_sample
```

Use an `apply` family function to return the number of weeks
left from each day in `y2018_sample` to the New Year, 2019/01/01.