basic

R code chunks in R Markdown

Sections where R code can run inside the document ({r} ...).

Purpose of R Markdown

Combine text + code to create reproducible reports (PDF/HTML/Word).

echo=FALSE

Hides the code, but shows the results.

eval=FALSE

Shows code but does not execute it.

message=FALSE and warning=FALSE

Hides messages & warnings in the output.

cache=TRUE

Reuses previously stored results to speed up knit time.

How to plot a graph with hidden code

```{r, echo=FALSE, message=FALSE, warning=FALSE}

Importance of version control

Tracks changes, rollback versions, supports collaboration & branching.

Initial Git configuration commands

git config --global user.name "Name"
git config --global user.email "email"

git fetch

Downloads changes from remote — does not merge.

git pull

Fetch and merge remote changes into local branch.

git clone URL

Makes a full local copy of a remote repository.

File states in Git

• Untracked — Git doesn't know it
• Unmodified — No recent changes
• Modified — Changed locally
• Staged — Ready for commit

After committing a file, its status

Unmodified.

Why use branching?

Develop features/experiments without affecting main working code.

Command to check file status

git status.

How to ignore files in Git

Add patterns to .gitignore.

Difference between redirect and append

> — overwrite
>> — append

Pipe operator in Linux

| — send output of one command to another.

chmod 700 file

Only owner can read, write, execute.

Main atomic data types in R

numeric, character, logical, integer, complex.

Difference: NA vs NULL vs NaN

• NA = missing value
• NULL = no value / empty
• NaN = "not a number" (e.g., 0/0).

What is a pure function?

Output depends only on inputs, no side effects.

Anonymous function

A function with no name, usually inline:

function(x) x^2

.

Create a sequence from 0 to 100 by 2

seq(0, 100, by=2).

Repeat 1 three times

rep(1, 3).

Main indexing start in R

1 (not 0!).

What does %>% do?

Pipes data into the next function.

Functions of filter(), select(), mutate(), summarise(), arrange()

• filter — subset rows
• select — choose columns
• mutate — new columns
• summarise — collapse to summary row(s)
• arrange — reorder rows.

Extract airline, model, tailnum & manufacturer for dest="IAH" (concept)

Join flights + planes + airlines, then filter dest == "IAH".

What does anti-join do?

Returns rows in left table with no match in right table.

How does inner join work?

Only retains rows with matching key values in both tables.

apply vs sapply vs tapply vs mapply

• apply — matrix dims
• sapply — simplify results
• tapply — apply by group
• mapply — multiple vectors.

Difference between ggplot() and plot()

ggplot uses layered grammar of graphics; plot is single-call base graphics.

Basic scatter in ggplot

ggplot(df, aes(x,y)) + geom_point()

.

Good plotting practices

Clear labels, avoid clutter, focus on one message.

What is facetting used for?

Split into multiple small plots by a grouping variable.

for loop vs while loop

for = iteration over fixed sequence
while = runs until condition false.

What is an error message?

Code cannot run until the issue is corrected.

Steps to load, examine, arrange, analyse

read.csv() → head()/summary() → tidyverse verbs → plots/models.