Log File Analyzer

Learn log file analyzer through an executable example, then connect the small demonstration to production code.

A log analyzer scans lines, extracts severity levels or structured fields, and produces a useful summary. Keep parsing and presentation separate so malformed lines can be handled explicitly.

This example analyzes a small in-memory log fixture; replace the reader with an actual file for a CLI tool.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

Extensions: accept a filename via flags, stream instead of loading an entire large file, support JSON log entries, allow time-range filtering and test malformed records.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"bufio"
	"fmt"
	"strings"
)

func main() {
	input := "INFO booted\nWARN retrying\nERROR disk full\nERROR failed write\n"
	counts := map[string]int{}
	scanner := bufio.NewScanner(strings.NewReader(input))
	for scanner.Scan() {
		fields := strings.Fields(scanner.Text())
		if len(fields) > 0 {
			counts[fields[0]]++
		}
	}
	if err := scanner.Err(); err != nil {
		panic(err)
	}
	for _, level := range []string{"INFO", "WARN", "ERROR"} {
		fmt.Println(level, counts[level])
	}
}

Output

INFO 1
WARN 1
ERROR 2

The scanner reads four lines and counts the first token on each. Printing levels in a fixed list avoids depending on map order.

• Scanner has a default token-size limit; raise its buffer size for large log lines when required.
• Real logs may be JSON or use formats in which the first token is not the severity level.

Parse newline-delimited JSON records, count errors by component, and report malformed lines separately.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Log File Analyzer in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.