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\end{lstlisting}    Variable names must begin with lowercase letters. You can use numbers, underscores, and uppercase letters within the name, just not as the first character. All characters must be contiguous (no spaces or punctuation). In short, stick to letters and numbers and the occasional underscore, and avoid all other characters when naming your variables. \texttt{$\sim$myFreqs}, \texttt{$\sim$theBestSineWave}, $\sim$myFreqs, $\sim$theBestSineWave,  and \texttt{$\sim$banana\_3} $\sim$banana\_3  are valid names. \texttt{$\sim$MyFreqs}, \texttt{$\sim$theBest\&*\#\@SineWave}, $\sim$MyFreqs, $\sim$theBest\&*\#\@SineWave,  and \texttt{$\sim$banana!!!} $\sim$banana!!!  are bad names. There are two types of variables that you can create: \emph{environment} variables and \emph{local} variables.