DATA MINING Desktop Survival Guide by Graham Williams
Desktop Survival Project Home List of Figures List of Tables Data Mining with Rattle Introduction Data Mining with Rattle Data Exploring Data Transforming Data Cluster Analysis Association Analysis Classification and Regression Models Evaluation and Deployment Issues Moving into R Troubleshooting R for the Data Miner R Data Graphics in R Understanding Data Preparing Data Descriptive and Predictive Analytics Issues Evaluating Models Reporting Cluster Analysis Text Mining Text Mining Algorithms Bagging Bayes Classifier Cluster Analysis Conditional Trees Hierarchical Clustering K-Nearest Neighbours Linear Models Neural Networks Support Vector Machines Open Products AlphaMiner Borgelt Data Mining Suite KNime R Rattle Weka Closed Products C4.5 Clementine Equbits Foresight GhostMiner InductionEngine ODM Enterprise Miner Statistica Data Miner TreeNet Virtual Predict Appendicies Installing GTK, R, and Rattle Glossary Bibliography Index

Assignment

Storing data into variables is a fundamental operation in any language, and R provides the arrow notation for the assignment operator:

> x <- 10

The assignment can operate in the opposite direction but it is not so common, and is probably best avoided:

> 10 -> x

Occasionally you might also see the double headed arrow notation:

> x <<- 10

It is not generally used, should be avoided, and is included here only for completeness. It is used, for example, to maintain information in a function across different function calls to the same function, or to share some information across a number of functions. In effect it assigns a value to a variable that exists outside the function (in fact, it searches through the list of environments up to the Global Environment to find where the variable is defined, and assigns the value to the first instance it finds, or creates a new one in the Global Environment if none found). This breaks some of the principles of functional programming--in that user variables might be changed unexpectedly, possibly resulting in erroneous computations.

The single headed assignment will only assign to an object in the current environment.

An alternative to <- is =, and in most situations they are equivalent. There are subtle differences as in:

  if (x=0) 1 else 2

This gives a syntax error, whereas replacing = with <- is quite legitimate (though unusual). This helps to avoid a common coding bug where this test is not expected to be an assignment, but in fact a comparison (which should really be using ==).

Also, the = assignment is actually used within function calls to pass an object value through to the function body (a child environment).

These three quite different assignment operations should be distinguished syntactically, so although the S language allows the <- to be replaced with = it is advised against. use the syntax to highlight the different semantics!

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