Expand io chapter

Signed-off-by: Baiju Muthukadan <baiju.m.mail@gmail.com>

code/io/uppercase/uppercase.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+func main() {
+	stdin, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		panic(err)
+	}
+	str := string(stdin)
+	newStr := strings.TrimSuffix(str, "\n")
+	upper := strings.ToUpper(newStr)
+	fmt.Println(upper)
+}

functions.tex

@@ -490,7 +490,8 @@ \section{Methods}
 
 \section{Exercises}
 
-\textbf{Exercise 1:} Write a method to calculate the area of a rectangle.
+\textbf{Exercise 1:} Write a method to calculate the area of a rectangle for a
+given struct with width and height.
 
 \textbf{Solution:}
 

io.tex

@@ -145,8 +145,31 @@ \section{Standard Streams}
 
 As you can see from this program, the \textit{Printf} function writes to
 standard output and the \textit{Scanf} reads the standard input. Go can also
-writes to standard error output stream. This is explained in the section about
-the \textit{io} package later in this chapter.
+writes to standard error output stream.
+
+The \textit{io} package provides a set of interfaces and functions that allow
+developers to work with different types of input and output streams.
+
+Consider a use case to convert everything that comes to standard input to
+convert to upper case. This can be achieved by reading all standard input
+using \texttt{io.ReadAll} and converting to upper case.  Here is code:
+
+\lstinputlisting[caption=Convert standard input to upper case]{code/io/uppercase/uppercase.go}
+
+You can run this program similar to how you did with the previous program.
+
+You can use \textit{fmt.Fprintf} with \textit{os.Stderr} as the first argument
+to write to standard error.
+
+\begin{lstlisting}[numbers=none]
+fmt.Fprintf(os.Stderr, "This goes to standard error: %s", "OK")
+\end{lstlisting}
+
+Alternatively, you can call \textit{WriteString} method of \textit{os.Stderr}:
+
+\begin{lstlisting}[numbers=none]
+os.Stderr.WriteString("This goes to standard error")
+\end{lstlisting}
 
 \section{Using flag Package}
 
@@ -279,7 +302,7 @@ \section{String Formatting}
 printed will be whatever returned by that function.  Here is an
 example:
 
-\lstinputlisting[caption=Representation format]{code/io/custom/custom.go}
+\lstinputlisting[caption=Custom representation using Stringer]{code/io/custom/custom.go}
 
 The output of the above program will be like this:
 
@@ -290,13 +313,6 @@ \section{String Formatting}
 main.Temperature{Value:30.456, Unit:"Celsius"}
 \end{lstlisting}
 
-\section{Using io Package}
-
-The \textit{io} package provides a set of interfaces and functions that allow
-developers to work with different types of input and output streams.
-
-TODO
-
 \section{Exercises}
 
 {\bfseries Exercise 1:} Write a program to read length and width of a