Building Blocks

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The Husker Student POWER survey is currently open to first year and new transfer students.

The survey consists of 12 yes/no statements about academic, social, financial, and emotional well-being topics.

Staff and instructors with expertise use the survey responses to provide support and resources to students whose responses raise concerns.

Take a minute and complete the survey if you haven’t already!

Variables

• Declare: set aside a space in memory for a variable

• Assign: Set the value of a variable

In modern languages (R, Python, etc.) these two steps are usually done simultaneously

x <- 3 (R)
y = "hello world" (Python)

Naming Things

• Variable names can’t
  • start with a number
  • start with a symbol (_ is ok in Python)
  • include . (Python)
  • be a reserved word

_x <- 3

Error: <text>:1:2: unexpected symbol
1: _x
     ^

3x <- 3

Error: <text>:1:2: unexpected symbol
1: 3x
     ^

if <- 3

Error: <text>:1:4: unexpected assignment
1: if <-
       ^

Naming Things

• Variable names can’t
  • start with a number
  • start with a symbol (_ is ok in Python)
  • include . (Python)
  • be a reserved word

x.y = 3

Error: NameError: name 'x' is not defined

$y = 3

invalid syntax (<string>, line 1)

3y = 3

invalid decimal literal (<string>, line 1)

if = 3

invalid syntax (<string>, line 1)

Naming Things

When you create a variable:

• Choose descriptive names
  • Match the equation/formula (if math)
  • May want to include units length_mm
• Avoid single character names where possible
  c in particular is a bad name in R!
• Balance length and clarity/information

Basic Variable Types

• Integer - 2, 6, 109243
  • R calls these ‘integer’, Python calls them ‘int’
• Floating point - 3.1415, 2.7, 9.0
  • R calls these ‘double’, Python calls them ‘float’ ¹
• String - “ABCDE”, “ ” , “Eddie is a Jack Russell Terrorist”
  • R calls these ‘character’, Python calls them ‘str’
• Boolean - True, False
  • R calls these ‘logical’, Python calls them ‘bool’

1. The difference between double and float is technical and involves how many decimal places of precision are allocated in memory.

Testing Types

Use a function to test if the type of y is xxx

R: is.xxx(y)

Python: isInstance(y, xxx)

is.logical(3.14)

[1] FALSE

is.character(3.14)

[1] FALSE

is.character("3.14")

[1] TRUE

is.integer(3.14)

[1] FALSE

is.numeric("pi")

[1] FALSE

is.numeric(pi)

[1] TRUE

Testing Types

Use a function to test if the type of y is xxx

R: is.xxx(y)

Python: isinstance(y, xxx)

pi = 3.14159
isinstance(3.14, bool)

False

isinstance(3.14, str)

False

isinstance("3.14", str)

True

isinstance(3.14, int)

False

isinstance("pi", float)

False

isinstance(pi, float)

True

Implicit Type Conversions

R and Python will both try to work with types that don’t match… to a point

v1 <- 1L + 2.0
v1

[1] 3

is.integer(v1)

[1] FALSE

is.double(v1)

[1] TRUE

v2 <- TRUE + 3L
v2

[1] 4

is.logical(v2)

[1] FALSE

is.integer(v2)

[1] TRUE

is.double(v2)

[1] FALSE

v3 <- "3" + 4

Error in "3" + 4: non-numeric argument to binary operator

v4 <- "3" + "4"

Error in "3" + "4": non-numeric argument to binary operator

Variables are converted to the least-complicated/least memory intensive compatible type.

Implicit Type Conversions

R and Python will both try to work with types that don’t match… to a point

v1 = 1 + 2.0
v1

3.0

isinstance(v1, int)

False

isinstance(v1, float)

True

v2 = True + 3
v2

4

isinstance(v2, bool)

False

isinstance(v2, int)

True

isinstance(v2, float)

False

v3 = "3" + 4

Error: TypeError: can only concatenate str (not "int") to str

v4 = "3" + "4"
v4

'34'

Python will concatenate (smush together) strings using +

Variables are converted to the least-complicated/least memory intensive compatible type.

Explicit Type Conversions

You can force things to be converted from one type to the other as well.

Python

int("3")

3

int(3.14)

3

int(3.84)

3

bool(3.0)

True

R

as.integer("3")

[1] 3

as.integer(3.14)

[1] 3

as.integer(3.84)

[1] 3

as.logical(3.0)

[1] TRUE

Functions

• A function is a sequence of commands that performs a task

• Functions (sometimes) take arguments and (sometimes) return values

• We call a function by running code with the function name and values:

Function anatomy

Functions

• A function is a sequence of commands that performs a task

• Functions (sometimes) take arguments and (sometimes) return values

• We call a function by running code with the function name and any necessary arguments

Function arguments

Software Libraries

• We often group commonly used, related functions into packages (R) or modules (Python)

• You install a package with install.packages("pkgname") (R) or pip install pkgname (Python)

• Packages/modules must be loaded before they can be used.
  library(pkgname) (R)
  import pkgname (Python)

• In Python, you access the functions in a package by pkgname.funcname, even after you’ve imported it

• In R, after the library is loaded, you access a function using funcname

Python Package Functions

pkgname.funcname can make for wordy code

import pkgname as pn allows you to use pn.funcname instead
This creates an alias for the package

from pkgname import func1 func2 allows you to use func1() and func2() directly, without the package name
This directly imports ONLY the functions you need

It is considered bad practice to use from pkgname import *, which exposes all functions in the package directly.

• This is bad because multiple packages could have the same function names inside
• This very quickly leads to bugs