mirror of
https://github.com/jbranchaud/til
synced 2026-01-02 22:58:01 +00:00
59 lines
1.3 KiB
Markdown
59 lines
1.3 KiB
Markdown
# Write A Custom Scan Function For File IO
|
|
|
|
By default a [`bufio.Scanner`](https://pkg.go.dev/bufio#Scanner) will scan
|
|
input line-by-line. In other words, splitting on newlines such that each
|
|
iteration will emit everything up to the next newline character.
|
|
|
|
We can write our own `SplitFunc` and override the default one by calling
|
|
`scanner.Split` with it. Our custom scan function needs to match the type
|
|
signature of [`SplitFunc`](https://pkg.go.dev/bufio#SplitFunc).
|
|
|
|
Here is a custom one that emits each individual character but omits the
|
|
newlines.
|
|
|
|
```go
|
|
func ScanChar(data []byte, atEOF bool) (int, []byte, error) {
|
|
if atEOF || len(data) == 0 {
|
|
return 0, nil, nil
|
|
}
|
|
|
|
start := 0
|
|
for start < len(data) {
|
|
if !utf8.FullRune(data[start:]) {
|
|
return 0, nil, nil
|
|
}
|
|
|
|
r, size := utf8.DecodeRune(data[start:])
|
|
if r == utf8.RuneError {
|
|
return 0, nil, fmt.Errorf("invalid UTF-8 encoding")
|
|
}
|
|
|
|
if r != '\n' {
|
|
return start + size, data[start:start+size], nil
|
|
}
|
|
|
|
// found a \n, advance the start position
|
|
start += size
|
|
}
|
|
|
|
return start, nil, nil
|
|
}
|
|
```
|
|
|
|
We can then use thi `ScanChar` function with a `bufio.Scanner` like so:
|
|
|
|
```go
|
|
func ReadFileByCharacter(file io.Reader) {
|
|
scanner := bufio.NewScanner(file)
|
|
|
|
// override default SplitFunc
|
|
scanner.Split(scanChar)
|
|
|
|
for scanner.Scan() {
|
|
char := scanner.Text()
|
|
|
|
fmt.Printf("- %s\n", char)
|
|
}
|
|
}
|
|
```
|