publish reading-files

examples.txt

@@ -53,8 +53,9 @@ Number Parsing
 URL Parsing
 SHA1 Hashes
 Base64 Encoding
-# Reading Files
+Reading Files
 # Writing Files
+# File Operations
 Line Filters
 Command-Line Arguments
 Command-Line Flags

examples/reading-files/reading-files.go

@@ -1,14 +1,83 @@
+// Reading and writing files are basic tasks needed for
+// many Go programs. First we'll look at some examples of
+// reading files.
+
 package main
 
-import "io/ioutil"
-import "fmt"
+import (
+    "bufio"
+    "fmt"
+    "io"
+    "io/ioutil"
+    "os"
+)
 
-func main() {
-    contents, err := ioutil.ReadFile("xx-file-read.go")
-    if err != nil {
-        return
+// Reading files requires checking most calls for errors.
+// This helper will streamline our error checks below.
+func check(e error) {
+    if e != nil {
+        panic(e)
     }
-    fmt.Print(string(contents))
 }
 
-// todo: streaming reads
+func main() {
+
+    // Perhaps the most basic file reading task is
+    // slurping a file's entire contents into memory.
+    dat, err := ioutil.ReadFile("/tmp/dat")
+    check(err)
+    fmt.Print(string(dat))
+
+    // You'll often want more control over how and what
+    // parts of a file are read. For these tasks, start
+    // by `Open`ing a file to obtain an `os.File` value.
+    f, err := os.Open("/tmp/dat")
+
+    // Read some bytes from the beginning of the file.
+    // Allow up to 5 to be read but also not how many
+    // actually were read.
+    b1 := make([]byte, 5)
+    n1, err := f.Read(b1)
+    check(err)
+    fmt.Printf("%d bytes: %s\n", n1, string(b1))
+
+    // You can also `Seek` to a known location in the file
+    // and `Read` from there.
+    o2, err := f.Seek(6, 0)
+    check(err)
+    b2 := make([]byte, 2)
+    n2, err := f.Read(b2)
+    check(err)
+    fmt.Printf("%d bytes @ %d: %s\n", n2, o2, string(b2))
+
+    // The `io` package provides some functions that may
+    // be helpful for file reading. For example, reads
+    // like the ones above can be more robustly
+    // implemented with `ReadAtLeast`.
+    o3, err := f.Seek(6, 0)
+    check(err)
+    b3 := make([]byte, 2)
+    n3, err := io.ReadAtLeast(f, b3, 2)
+    check(err)
+    fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))
+
+    // There is no built-in rewind, but `Seek(0, 0)`
+    // accomplishes this.
+    _, err = f.Seek(0, 0)
+    check(err)
+
+    // The `bufio` package implements a buffered
+    // reader that may be useful both for it's efficiency
+    // with many small reads and because of the additional
+    // reading methods it provides. 
+    r4 := bufio.NewReader(f)
+    b4, err := r4.Peek(5)
+    check(err)
+    fmt.Printf("5 bytes: %s\n", string(b4))
+
+    // Close the file when you're done (usually this would
+    // be scheduled immediately after `Open`ing with
+    // `defer`).
+    f.Close()
+
+}

examples/reading-files/reading-files.sh

@@ -0,0 +1,9 @@
+$ echo "hello" > /tmp/dat
+$ echo "go" >>   /tmp/dat
+$ go run reading-files.go 
+hello
+go
+5 bytes: hello
+2 bytes @ 6: go
+2 bytes @ 6: go
+5 bytes: hello