package mainimport (    "io"    "strings"    "fmt"    "os")func ReadFrom(reader io.Reader, num int) ([]byte, error) {    p := make([]byte, num)    n,err := reader.Read(p)    if n > 0 {        return p[:n], nil    }    return p, err}//从字符串读func sampleReadFromString() {    data, _ := ReadFrom(strings.NewReader("from string"), 12)    fmt.Println(string(data))}//从终端读func sampleReadFromStdin() {    fmt.Println("please input from std:")    data, _ := ReadFrom(os.Stdin, 11)    fmt.Println(string(data))}//从文件读func sampleReadFromFile() {    file, _ := os.Open("io操作.go")    defer file.Close()    data, _ := ReadFrom(file, 9)    fmt.Println(string(data))}func main() {    sampleReadFromString()    sampleReadFromStdin()    sampleReadFromFile()}

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