How to check if a map contains a key in Go

Question

I know I can iterate over a map m by   for k  v    range m           and look for a key but is there a more efficient way of testing a key s existence in a map    I couldn t find the answer in the language spec

User · Answer

In addition to The Go Programming Language Specification, you should read Effective Go. In the section on maps, they say, amongst other things:

An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. A set can be implemented as a map with value type bool. Set the map entry to true to put the value in the set, and then test it by simple indexing.
attended := map[string]bool{
    "Ann": true,
    "Joe": true,
    ...
}

if attended[person] { // will be false if person is not in the map
    fmt.Println(person, "was at the meeting")
}
Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that 0 because it's not in the map at all? You can discriminate with a form of multiple assignment.
var seconds int
var ok bool
seconds, ok = timeZone[tz]
For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report:
func offset(tz string) int {
    if seconds, ok := timeZone[tz]; ok {
        return seconds
    }
    log.Println("unknown time zone:", tz)
    return 0
}
To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value.
_, present := timeZone[tz]

User · Answer

As noted by other answers  the general solution is to use an index expression in an assignment of the special form    v  ok   a x  v  ok    a x  var v  ok   a x  var v  ok T   a x    This is nice and clean  It has some restrictions though  it must be an assignment of special form  Right-hand side expression must be the map index expression only  and the left-hand expression list must contain exactly 2 operands  first to which the value type is assignable  and a second to which a bool value is assignable  The first value of the result of this special form will be the value associated with the key  and the second value will tell if there is actually an entry in the map with the given key  if the key exists in the map   The left-hand side expression list may also contain the blank identifier if one of the results is not needed   It s important to know that if the indexed map value is nil or does not contain the key  the index expression evaluates to the zero value of the value type of the map  So for example   m    map int string   s    m 1     s will be the empty string    var m2 map int float64    m2 is nil  f    m2 2     f will be 0 0  fmt Printf   q  f   s  f     Prints     0 000000   Try it on the Go Playground   So if we know that we don t use the zero value in our map  we can take advantage of this   For example if the value type is string  and we know we never store entries in the map where the value is the empty string  zero value for the string type   we can also test if the key is in the map by comparing the non-special form of the  result of the  index expression to the zero value   m    map int string      0   zero       1   one      fmt Printf  Key 0 exists   t nKey 1 exists   t nKey 2 exists   t       m 0         m 1         m 2           Output  try it on the Go Playground    Key 0 exists  true Key 1 exists  true Key 2 exists  false   In practice there are many cases where we don t store the zero-value value in the map  so this can be used quite often  For example interfaces and function types have a zero value nil  which we often don t store in maps  So testing if a key is in the map can be achieved by comparing it to nil   Using this  technique  has another advantage too  you can check existence of multiple keys in a compact way  you can t do that with the special  comma ok  form   More about this  Check if key exists in multiple maps in one condition  Getting the zero value of the value type when indexing with a non-existing key also allows us to use maps with bool values conveniently as sets  For example   set    map string bool       one   true       two   true     fmt Println  Contains  one     set  one     if set  two         fmt Println   two  is in the set     if  set  three         fmt Println   three  is not in the set       It outputs  try it on the Go Playground    Contains  one   true  two  is in the set  three  is not in the set   See related  How can I create an array that contains unique strings

User · Answer

Short Answer    exists    timeZone tz        Just checks for key existence val  exists    timeZone tz      Checks for key existence and retrieves the value  Example Here s an example at the Go Playground  Longer Answer Per the Maps section of Effective Go   An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map  For instance  if the map contains integers  looking up a non-existent key will return 0  Sometimes you need to distinguish a missing entry from a zero value  Is there an entry for  quot UTC quot  or is that the empty string because it s not in the map at all  You can discriminate with a form of multiple assignment  var seconds int var ok bool seconds  ok   timeZone tz   For obvious reasons this is called the    comma ok    idiom  In this example  if tz is present  seconds will be set appropriately and ok will be true  if not  seconds will be set to zero and ok will be false  Here s a function that puts it together with a nice error report  func offset tz string  int       if seconds  ok    timeZone tz   ok           return seconds           log Println  quot unknown time zone  quot   tz      return 0    To test for presence in the map without worrying about the actual value  you can use the blank identifier     in place of the usual variable for the value     present    timeZone tz

User · Answer

Searched on the go-nuts email list and found a solution posted by Peter Froehlich on 11 15 2009   package main  import  fmt   func main             dict    map string int   foo    1   bar    2          value  ok    dict  baz           if ok                   fmt Println  value     value            else                   fmt Println  key not found                 Or  more compactly   if value  ok    dict  baz    ok       fmt Println  value     value    else       fmt Println  key not found       Note  using this form of the if statement  the value and ok variables are only visible inside the if conditions

User · Answer

better way here  if    ok    dict  foo    ok         do something here

User · Answer

var d map string string     value  ok    d  key       if ok           fmt Println  Key Present    value        else           fmt Println   Key Not Present

User · Answer

It is mentioned under  Index expressions        An index expression on a map a of type map K V used in an assignment   or initialization of the special form  v  ok   a x   v  ok    a x   var v  ok   a x        yields an additional untyped boolean value  The value of ok is true if   the key x is present in the map  and false otherwise

User · Answer

var empty struct       var ok bool     var m map string struct       m   make map string struct        m  somestring     empty          ok   m  somestring       fmt Println  somestring exists    ok          ok   m  not       fmt Println  not exists    ok    Then  go run maps go  somestring exists  true not exists  false

User · Answer

One line answer   if val  ok    dict  foo    ok         do something here     Explanation   if statements in Go can include both a condition and an initialization statement  The example above uses both    initializes two variables - val will receive either the value of  foo  from the map or a  zero value   in this case the empty string  and ok will receive a bool that will be set to true if  foo  was actually present in the map evaluates ok  which will be true if  foo  was in the map   If  foo  is indeed present in the map  the body of the if statement will be executed and val will be local to that scope

User · Answer

A two value assignment can be used for this purpose  Please check my sample program below  package main  import        fmt     func main           creating a map with 3 key-value pairs     sampleMap    map string int  key1   100   key2   500   key3   999        A two value assignment can be used to check existence of a key      value  isKeyPresent    sampleMap  key2         isKeyPresent will be true if key present in sampleMap     if isKeyPresent             key exist         fmt Println  key present  value       value        else             key does not exist         fmt Println  key does not exist

User · Answer

have a look at this snippet of code     nameMap    make map string int      nameMap  quot river quot     33     v  exist    nameMap  quot river quot       if exist           fmt Println  quot exist  quot  v

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