---date: "2021-01-01"title: "16. Functions by Chapter"---## Useful information...incomplete...### OperatorsThere are several operators you can use to help build expressions as shown in Table \ref{tab:operators}.\begin{table}[htbp]\begin{small}\begin{center}\caption{Logical operators for use in R.}\label{tab:operators}\newcolumntype{R}{>{\raggedright\arraybackslash}X}%\begin{tabularx}{\textwidth}{l X l}\topruleOperator & Meaning & Example \\\midrule\texttt{\textless{}} & less than & \texttt{x\ \textless{}\ 3} \\\texttt{\textgreater{}} & greater than & \texttt{x\ \textgreater{}\ 5} \\\texttt{\textgreater{}=} & greater than or equal to & \texttt{x\ \textgreater{}=\ 24} \\\texttt{\textless{}=} & less than or equal to & \texttt{x\ \textless{}=\ 19} \\\texttt{==} & exactly equal to & \texttt{x\ ==\ 666} \\\texttt{!=} & not equal to & \texttt{x\ !=\ 777} \\\texttt{\&} & AND; returns TRUE if statements on both sides of the \texttt{\&} are TRUE & \texttt{(x\ \textgreater{}\ 3)\ \&\ (y\ !=\ 5)} \\\texttt{\textbar{}} & OR; the pipe symbol \texttt{\textbar{}}; TRUE if a statement on either side of the \texttt{\textbar{}} is TRUE & \texttt{(x\ \textless{}\ 10)\ \textbar{}\ (x\ \textgreater{}\ 20)} \\\bottomrule\end{tabularx}\end{center}\end{small}\end{table}\begin{mybox}{}{Question}The logical AND: If we had said \texttt{subset(dat, site == "Kommetjie" \& site == "Baboon")}, what would have happened? Why? Try it and see...\end{mybox}### FunctionsSome example functions covered so far are presented in Table \ref{tab:functions}.\begin{table}[htbp]\begin{small}\begin{center}\caption{Common functions used for daily work in R.}\label{tab:functions}\newcolumntype{R}{>{\raggedright\arraybackslash}X}%\begin{tabularx}{\textwidth}{R l R R }\toprule\textbf{Type} & \textbf{Function} & \textbf{What it does} & \textbf{Syntax} \\\midruleChecking a dataframe & \texttt{class()} & Gives the class (type of data structure) of the object & \texttt{class(dat)} \\Numerical calculations & \texttt{mean()}, \texttt{sd()} & Calculations on a numerical R object (variables, vectors, arrays) & \texttt{mean(dat\$var)} \\Gets working directory & \texttt{getwd()} & Gets current working directory & \texttt{getwd()} \\Sets working directory & \texttt{setwd()} & Sets working directory to specified path & \texttt{setwd("yourpath")} \\Getting your data into R & \texttt{read.table()} & Reads file in table format (with specified separator sep) and creates a dataframe from it & \texttt{read.table("filename.csv", header= TRUE, sep = ",")} \\Getting .csv file into R & \texttt{read.csv()} & Reads in .csv file to a dataframe; as above, but `sep' predefined & \texttt{read.csv("filename.csv", header= TRUE)} \\List variables & \texttt{ls()} & Lists objects in memory & \texttt{ls()} \\Variable names in dataframe & \texttt{names()} & Gives variable names & \texttt{names(dat)} \\Subsetting a dataframe & \texttt{subset()} & Subset a dataframe & \texttt{subset(dat, Var1 == "Low" | Var2 <= 5)} \\Cleaning up & \texttt{rm()} & Removes object(s) from the Environment & \texttt{rm(object\_name)}; \texttt{rm(list = ls())} \\\bottomrule\end{tabularx}\end{center}\end{small}\end{table}Some summary functions are presented in Table \ref{tab:summaries}.\begin{table}[htbp]\begin{small}\begin{center}\caption{Commonly used and useful summary functions.}\label{tab:summaries}\newcolumntype{R}{>{\raggedright\arraybackslash}X}%\begin{tabularx}{\textwidth}{R R R X }\toprule\textbf{Type} & \textbf{Function} & \textbf{What it does} & \textbf{Syntax} \\\midruleSpecifying variable in dataframe & \texttt{\$} & Gives the class (type of data structure) of the object & \texttt{dat\$blade\_length} \\ & \texttt{str()} & Lists variables in your object by name, their data type (continuous, factor) and an indication of the actual data & \texttt{str(dat)} \\& \texttt{head()} & Gives first 6 rows of a dataframe, or number of rows (\texttt{n}) specified & \texttt{head(dat, 20)} \\& \texttt{tail()} & Gives last 6 (or \texttt{n}) rows of a dataframe & \texttt{tail(dat, 10)} \\& \texttt{names()} & Gives variable names & \texttt{names(dat)} \\& \texttt{summary()} & Tabulates variables in dataframe and provides summary statistics & \texttt{summary(dat)} \\<!-- & \texttt{attach()} & Loads dataframe into memory & \texttt{attach(dat)} \\ --><!-- & \texttt{detach()} & Removes dataframe from memory & \texttt{detach(dat)} \\ -->& \texttt{with()} & Specify a dataframe & \texttt{with(dat, mean(blade\_length))} \\Summary statistics & \texttt{mean()}, \texttt{sd()}, \texttt{range()}, \texttt{var()} & Calculates particular statistic for a variable. Use \texttt{na.rm = TRUE} if your data contain \texttt{NA}s & \emph{e.g.} \texttt{mean(dat\$blade\_length, na.rm = TRUE)} \\& \texttt{length()} & Number of elements in a variable. Use \texttt{na.omit()} if your data contain \texttt{NA}s & \texttt{length(dat\$blade\_length)}; \texttt{length(na.omit(dat\$blade\_length))} \\Data manipulation & \texttt{table()} & Frequency table by variables specified & \texttt{table(dat\$site, dat\$blade\_length)} \\& \texttt{ftable()} & Flat frequency table useful for displaying multidimensional tables & \texttt{ftable(dat\$catvar1, dat\$catvar2, dat\$catvar3)} \\Saving an object file & \texttt{write.csv()} & Save an object to a .csv file & \texttt{write.csv(sp\_by\_site, file = "Laminaria.csv", row.names = TRUE)} \\\bottomrule\end{tabularx}\end{center}\end{small}\end{table}
