Selective file copy: Difference between revisions
Go solution
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<br>The ''by name'' assignment is a little extra available in PL/I.
<br>Data conversions may be necessary (as shown here for data element ''c''.
=={{header|Go}}==
JSON is popular these days for structured data and Go has support for this kind of selective copy in the JSON reader. The reader also supports custom conversions on fields. The common idiom for record construction and field initialization is shown.
<lang go>package main
import (
"encoding/json"
"log"
"os"
"bytes"
"errors"
"strings"
)
// structure of test file, generated and then used as input.
type s1 struct {
A string
B string
C int
D string
}
// structure of output file
type s2 struct {
A string
C intString // it's a string, but unmarshals from a JSON int.
X string
}
type intString string
func (i *intString) UnmarshalJSON(b []byte) error {
if len(b) == 0 || bytes.IndexByte([]byte("0123456789-"), b[0]) < 0 {
return errors.New("Unmarshal intString expected JSON number")
}
*i = intString(b)
return nil
}
// "constructor" initializes X
func NewS2() *s2 {
return &s2{X: "XXXXX"}
}
func main() {
// generate test file
o1, err := os.Create("o1.json")
if err != nil {
log.Fatal(err)
}
e := json.NewEncoder(o1)
for i := 1; i <= 5; i++ {
err := e.Encode(s1{
strings.Repeat("A", i),
strings.Repeat("B", i),
i,
strings.Repeat("D", i),
})
if err != nil {
log.Fatal(err)
}
}
o1.Close()
// reopen the test file, also open output file
in, err := os.Open("o1.json")
if err != nil {
log.Fatal(err)
}
out, err := os.Create("out.json")
if err != nil {
log.Fatal(err)
}
// copy input to output, streaming
d := json.NewDecoder(in)
e = json.NewEncoder(out)
for d.More() {
// a little different than the PL/I example. PL/I reads into s1, then
// does the selective copy in memory. The Go JSON reader can read the
// s1 formated JSON directly into the s2 Go struct without needing any
// intermediate s1 struct.
s := NewS2()
if err = d.Decode(s); err != nil {
log.Fatal(err)
}
if err = e.Encode(s); err != nil {
log.Fatal(err)
}
}
}</lang>
{{out|o1.json}}
<pre>
{"A":"A","B":"B","C":1,"D":"D"}
{"A":"AA","B":"BB","C":2,"D":"DD"}
{"A":"AAA","B":"BBB","C":3,"D":"DDD"}
{"A":"AAAA","B":"BBBB","C":4,"D":"DDDD"}
{"A":"AAAAA","B":"BBBBB","C":5,"D":"DDDDD"}
</pre>
{{out|out.json}}
<pre>
{"A":"A","C":"1","X":"XXXXX"}
{"A":"AA","C":"2","X":"XXXXX"}
{"A":"AAA","C":"3","X":"XXXXX"}
{"A":"AAAA","C":"4","X":"XXXXX"}
{"A":"AAAAA","C":"5","X":"XXXXX"}
</pre>
=={{header|Java}}==
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