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Solution Review: Web Application for Statistics

Solution Review: Web Application for Statistics

This lesson discusses the solution to the challenge given in the previous lesson.

We'll cover the following...
package main
import (
	"fmt"
	"log"
	"net/http"
	"sort"
	"strconv"
	"strings"
)

type statistics struct {
	numbers []float64
	mean    float64
	median  float64
}

const form = `<html><body><form action="/" method="POST">
<h1>Statistics</h1>
<h5>Compute base statistics for a given list of numbers</h5>
<label for="numbers">Numbers (comma or space-separated):</label><br>
<input type="text" name="numbers" size="30"><br />
<input type="submit" value="Calculate">
</form></body></html>`

const error = `<p class="error">%s</p>`

var pageTop = ""
var pageBottom = ""

func main() { // Define a root handler for requests to function homePage, and start the webserver combined with error-handling
	http.HandleFunc("/", homePage)
 	if err := http.ListenAndServe(":3000", nil); err != nil {
		log.Fatal("failed to start server", err)
	}
}

func homePage(writer http.ResponseWriter, request *http.Request) { // Write an HTML header, parse the form, write form to writer and make request for numbers 
	writer.Header().Set("Content-Type", "text/html")
	err := request.ParseForm() // Must be called before writing response
	fmt.Fprint(writer, pageTop, form)
	if err != nil {
		fmt.Fprintf(writer, error, err)
	} else {
		if numbers, message, ok := processRequest(request); ok {
			stats := getStats(numbers)
			fmt.Fprint(writer, formatStats(stats))
		} else if message != "" {
			fmt.Fprintf(writer, error, message)
		}
	}
	fmt.Fprint(writer, pageBottom)
}

func processRequest(request *http.Request) ([]float64, string, bool) { // Capture the numbers from the request, and format the data and check for error
	var numbers []float64
	if slice, found := request.Form["numbers"]; found && len(slice) > 0 {
		text := strings.Replace(slice[0], ",", " ", -1)
		for _, field := range strings.Fields(text) {
			if x, err := strconv.ParseFloat(field, 64); err != nil {
				return numbers, "'" + field + "' is invalid", false
			} else {
				numbers = append(numbers, x)
			}
		}
	}
	if len(numbers) == 0 {
		return numbers, "", false // no data first time form is shown
	}
	return numbers, "", true
}

func getStats(numbers []float64) (stats statistics) { // sort the values to get mean and median
	stats.numbers = numbers
	sort.Float64s(stats.numbers)
	stats.mean = sum(numbers) / float64(len(numbers))
	stats.median = median(numbers)
	return
}

func sum(numbers []float64) (total float64) { // seperate function to calculate the sum for mean
	for _, x := range numbers {
		total += x
	}
	return
}

func median(numbers []float64) float64 { // seperate function to calculate the median
	middle := len(numbers) / 2
	result := numbers[middle]
	if len(numbers)%2 == 0 {
		result = (result + numbers[middle-1]) / 2
	}
	return result
}

func formatStats(stats statistics) string {
	return fmt.Sprintf(`<table border="1">
<tr><th colspan="2">Results</th></tr>
<tr><td>Numbers</td><td>%v</td></tr>
<tr><td>Count</td><td>%d</td></tr>
<tr><td>Mean</td><td>%f</td></tr>
<tr><td>Median</td><td>%f</td></tr>
</table>`, stats.numbers, len(stats.numbers), stats.mean, stats.median)
}

In the code above, we define a struct statistics at line 11 to contain all the input data: numbers of type []float64 and the calculation results on them: mean and median.

As we have seen before, the HTML string for the web page is contained in a constant form, defined from line 17 to line 23. We also define a constant for error reporting at line 25. The main() routine is very succinct, defining a root handler for requests to a function homePage, and starting the web server combined with error-handling from line 32 to line 34.

Now, look at the header of the homePage() function at line 37. It starts by writing an HTML ...