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+\section{Functions}\label{functions}
+
+Like a traditional mathematical functions, python functions can take an
+input, process it, and give an output. In python, the input variables
+are referred to as \emph{arguments}. Functions are blocks of code that
+is run every time it's called. This allows us to re-use code.
+
+Functions are defined by using the def keyword. Reminder: it is
+important to keep track of indentations as it signifies the end of the
+function when the indentation returns back to the same level.
+
+\subsection{Defining Functions}\label{defining-functions}
+
+\subsubsection{Simple function}\label{simple-function}
+
+A simple function with no input variable can be useful if you need to
+re-use code multiple times without having to re-write it.
+
+\begin{Shaded}
+\begin{Highlighting}[]
+    \KeywordTok{def}\NormalTok{ function\_name():}
+        \BuiltInTok{print}\NormalTok{(}\StringTok{"This is from a function"}\NormalTok{)}
+\end{Highlighting}
+\end{Shaded}
+
+\subsubsection{Defining a function with one
+input}\label{defining-a-function-with-one-input}
+
+We can pass variables through to the function to be processed as
+follows:
+
+\begin{Shaded}
+\begin{Highlighting}[]
+    \KeywordTok{def}\NormalTok{ function(x):}
+        \BuiltInTok{print}\NormalTok{(x }\OperatorTok{+} \StringTok{" is best"}\NormalTok{)}
+\end{Highlighting}
+\end{Shaded}
+
+Note input variables can be of any data type (integer, float, string,
+etc.). \#\#\# Returning values from a function If we want to calculate a
+value and pass it back to the script for further use, we can use the
+\texttt{return} keyword. Let's define a linear function that takes two
+inputs, \texttt{x} and \texttt{b}, computes the corresponding \texttt{y}
+value, and returns it so it can be used elsewhere in the code.
+
+\begin{Shaded}
+\begin{Highlighting}[]
+    \KeywordTok{def}\NormalTok{ function(x, b):}
+\NormalTok{        y }\OperatorTok{=} \DecValTok{3}\OperatorTok{*}\NormalTok{x}\OperatorTok{+}\NormalTok{b}
+        \ControlFlowTok{return}\NormalTok{ y}
+\end{Highlighting}
+\end{Shaded}
+
+For multiple output variables we can add \#\# Calling functions Now that
+we've covered defining functions we want to call the function in order
+to execute the block inside the function. To do this, we simply re-call
+the function name as follows.
+
+\begin{Shaded}
+\begin{Highlighting}[]
+\NormalTok{function(}\DecValTok{2}\NormalTok{,}\OperatorTok{{-}}\DecValTok{1}\NormalTok{)}
+\end{Highlighting}
+\end{Shaded}
+
+Note that when running this code, nothing happens. This is because we
+haven't told the computer what to do with the output. Hence, if we wish
+to store the output then we need to use the assign operator \texttt{=}.
+
+\begin{Shaded}
+\begin{Highlighting}[]
+\NormalTok{output }\OperatorTok{=}\NormalTok{ function(}\DecValTok{2}\NormalTok{,}\OperatorTok{{-}}\DecValTok{1}\NormalTok{)}
+
+\BuiltInTok{print}\NormalTok{(output)}
+\end{Highlighting}
+\end{Shaded}
+
+In case you want to return multiple output variable from a single
+function we will have\ldots{}
+
+\subsection{Summary}\label{summary}
+
+\begin{Shaded}
+\begin{Highlighting}[]
+\KeywordTok{def}\NormalTok{ function\_name(argument1, argument2, argument3)}
+\NormalTok{    output1 }\OperatorTok{=}\NormalTok{ argument1 }\OperatorTok{*}\NormalTok{ argument2 }\OperatorTok{{-}}\NormalTok{ argument3}
+\NormalTok{    output2 }\OperatorTok{=}\NormalTok{ argument2 }\OperatorTok{+}\NormalTok{ argument3}
+    \ControlFlowTok{return}\NormalTok{ output1, output2}
+
+\NormalTok{[solution1, solution2] }\OperatorTok{=}\NormalTok{ function\_name(}\DecValTok{1}\NormalTok{,}\DecValTok{2}\NormalTok{,}\DecValTok{3}\NormalTok{)}
+\end{Highlighting}
+\end{Shaded}
+
+\begin{itemize}
+\tightlist
+\item
+  \texttt{def} - defines a function. All the code that is indented
+  underneath is considered inside the function block.
+\item
+  \texttt{function\_name} - this is used to call the function block.
+\item
+  \texttt{argument1} (optional) - input variable. This is data that can
+  be pass to the function. It is possible to have multiple variables
+  separated by a comma. As well as can be omitted if the function should
+  just give you an output such as.
+\item
+  \texttt{return} (optional) - if you wish to return something to your
+  script, the return keyword is used. The keyword can be followed by an
+  output variable or a constant. For multiple output variables, separate
+  them by a comma.
+\end{itemize}