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NotesOn_HandsOnGeospatialAnalysisWithRAndQgis.md

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Notes on Hands-On Geospatial Analysis with R and QGIS

By Shammunul Islam, Packt Publishing, Nov. 2018

ISBN 9781788991674

Setting up R and QGIS Environments for Geospatial Tasks

Basic Data Types and Data Structures in R

Basic Data Types in R

  • Three main types:
    • Numerics - any numbers with decimal values, and integers
    • Logical / Boolean - TRUE and FALSE
    • Character - text values, inside double quotes

Variable

  • Variable is a container that holds values of different types or the same type
  • Variable assignment can be x <- 2 or x = 2
  • R community prefers the assignment arrow

Data Structures in R

Vectors

  • Store single or multiple values of similar data types in a variable
  • One dimensional arrays
  • To assign multiple values into a vector, use c(): x = c(1,2,3,4,5)
  • You can mix data types in a vector, but they all get cast to the same thing
  • R attempts to cast all data types in a vector to the same type
  • You can check the class of a variable with class(varname)
  • You can give labels to different values in a vector: temp = c(morning = 20, before_noon = 23, after_noon = 25, evening = 20)
Basic operations with vector
jan_price = c(10, 20, 30)
increase = c(1, 2, 3)
mar_price = jan_price + increase
  • Vector ops that do element-wise computation: +, -, *, /
  • Boolean ops will test a value against each element, like mar_price > 15
  • Indexing into a vector is 1-based
  • Negative indexing doesn't work like you'd think--it gives you all but that element

Matrix

  • Matrix is a 2D array with a certain number of rows and columns

  • Matrices can also only contain one type of element

  • When creating a matrix the default behavior is to turn vectors into columns

  • To have a row-wise matrix made, use byrow=TRUE

  • Creating a matrix:

    alpha = c(1,2,3)
bravo = c(4,5,6)
charlie = c(7,8,9)
A = matrix(c(alpha,bravo,charlie), nrow=3)
B = matrix(c(alpha,bravo,charlie), nrow=3, byrow=TRUE)

Array

  • Arrays are like matrices but with more dimensions
  • The following produces an array, combined, which has two matrices of 3x3
> jan_18 = c(10,11,20)
> mar_18 = c(20,22,25)
> jun_18 = c(30,33,33)
> jan_17 = c(10,10,17)
> mar_17 = c(18,23,21)
> jun_17 = c(25,31,35)
> combined = array(c(jan_18, mar_18, jun_18, jan_17, mar_17, jun_17), dim=c(3,3,2))
> combined
, , 1

     [,1] [,2] [,3]
[1,]   10   20   30
[2,]   11   22   33
[3,]   20   25   33

, , 2

     [,1] [,2] [,3]
[1,]   10   18   25
[2,]   10   23   31
[3,]   17   21   35

Data frames

  • Like matrices, but allows a mix of different element types
  • Access to items in a data frame is via either [[]] or $
  • Add a row using rbind()
> alpha = c("one","two","three")
> bravo = c(1,2,3)
> charlie = c(TRUE,FALSE,TRUE)
> my_frame = data.frame(alpha, bravo, charlie)
> my_frame
  alpha bravo charlie
1   one     1    TRUE
2   two     2   FALSE
3 three     3    TRUE
> my_frame$bravo
[1] 1 2 3
> my_frame[["alpha"]]
[1] one   two   three
Levels: one three two
> my_frame[2,3]
[1] FALSE

Lists

Factor

Looping, functions, and apply family in R

Looping in R

Functions in R

Apply family - lapply, sapply, apply, tapply

apply

lapply

sapply

tapply

Plotting in R