Programming language: Go
License: MIT License
Latest version: v0.7.0

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Package fixedwidth provides encoding and decoding for fixed-width formatted Data.

go get github.com/ianlopshire/go-fixedwidth


Struct Tags

The struct tag schema schema used by fixedwidth is: fixed:"{startPos},{endPos},[{alignment},[{padChar}]]"1.

The startPos and endPos arguments control the position within a line. startPos and endPos must both be positive integers greater than 0. Positions start at 1. The interval is inclusive.

The alignment argument controls the alignment of the value within it's interval. The valid options are default2, right, and left. The alignment is optional and can be omitted.

The padChar argument controls the character that will be used to pad any empty characters in the interval after writing the value. The default padding character is a space. The padChar is optional and can be omitted.

Fields without tags are ignored.


// define some data to encode
people := []struct {
    ID        int     `fixed:"1,5"`
    FirstName string  `fixed:"6,15"`
    LastName  string  `fixed:"16,25"`
    Grade     float64 `fixed:"26,30"`
    Age       uint    `fixed:"31,33"`
    {1, "Ian", "Lopshire", 99.5, 20},

data, err := fixedwidth.Marshal(people)
if err != nil {
fmt.Printf("%s", data)
// Output:
// 1    Ian       Lopshire  99.5020


// define the format
var people []struct {
    ID        int     `fixed:"1,5"`
    FirstName string  `fixed:"6,15"`
    LastName  string  `fixed:"16,25"`
    Grade     float64 `fixed:"26,30"`
    Age       uint    `fixed:"31,33"`

// define some fixed-with data to parse
data := []byte("" +
    "1    Ian       Lopshire  99.50 20" + "\n" +
    "2    John      Doe       89.50 21" + "\n" +
    "3    Jane      Doe       79.50 22" + "\n")

err := fixedwidth.Unmarshal(data, &people)
if err != nil {

fmt.Printf("%+v\n", people[0])
fmt.Printf("%+v\n", people[1])
fmt.Printf("%+v\n", people[2])
// Output:
//{ID:1 FirstName:Ian LastName:Lopshire Grade:99.5 Age:20}
//{ID:2 FirstName:John LastName:Doe Grade:89.5 Age:21}
//{ID:3 FirstName:Jane LastName:Doe Grade:79.5 Age:22}

It is also possible to read data incrementally

decoder := fixedwidth.NewDecoder(bytes.NewReader(data))
for {
    var element myStruct
    err := decoder.Decode(&element)
    if err == io.EOF {
    if err != nil {

If you have an input where the indices are expressed in unicode codepoints, and not raw bytes fixedwidth supports this. Your data must be UTF-8 encoded:

decoder := fixedwidth.NewDecoder(strings.NewReader(data))
// Decode as usual now

Alignment Behavior

Alignment Encoding Decoding
default Field is left aligned The padding character is trimmed from both right and left of value
left Field is left aligned The padding character is trimmed from right of value
right Field is right aligned The padding character is trimmed from left of value


  1. {} indicates an argument. [] indicates and optional segment [](#a1)
  2. The default alignment is similar to left but has slightly different behavior required to maintain backwards compatibility [](#a2)