Convert interface to int

Question

I m trying to get a value from a JSON and cast it to int but it doesn t work  and I don t know how to do it properly   Here is the error message      cannot convert val  type interface     to type int  need type assertion   And the code       var f interface       err   json Unmarshal   byte jsonStr    amp f      if err    nil           utility CreateErrorResponse w   Error  failed to parse JSON data            return            m    f  map string interface         val  ok    m  area id       if  ok           utility CreateErrorResponse w   Error  Area ID is missing from submitted data            return            fmt Fprintf w   Type    v   val        lt --- Type   float64     iAreaId    int val                     lt --- Error on this line      testName     Area     iAreaId         not reaching here

User · Answer

I wrote a library that can help with type convertions https   github com KromDaniel jonson  js    jonson New   interface   55 6  70 8  10 4  1   48    -90     js SliceMap func jsn  jonson JSON  index int   jonson JSON       jsn MutateToInt       return jsn    SliceMap func jsn  jonson JSON  index int   jonson JSON       if jsn GetUnsafeInt    gt  50          jsn MutateToString             return jsn         55   70  10 1 48 -90

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To better understand the type conversion  look at the code below   package main import  fmt  func foo a interface          fmt Println a  int       conversion of interface into int   func main         var a int   10     foo a      This code executes perfectly and converts interface type to int type     For an expression x of interface type and a type T  the primary expression   x  T  asserts that x is not nil and that the value stored in x is of type T  The notation x  T  is called a type assertion    More precisely  if T is not an interface type  x  T  asserts that the dynamic type of x is identical to the type T  In this case  T must implement the  interface  type of x  otherwise the type assertion is invalid since it is not possible for x to store a value of type T  If T is an interface type  x  T  asserts that the dynamic type of x implements the interface T    Going back to your code  this  iAreaId    val  int   should work good  If you want to check error occured while conversion  you can also re-write above line as  iAreaId  ok    val  int

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I whole-heartedly agree with zzzz s type assertion answer and I strongly prefer that way over others   That said  here s what I ve had to do when the preferred method has not worked     long story related to cross-serialization of data    You can even chain this into a switch statement with case errInt    nil and similar expressions   package main  import  fmt  import  strconv   func main         var v interface       v    4       i  errInt    strconv ParseInt v  string   10  64       if errInt    nil           fmt Printf   d is a int   i             do what you wish with  i  here              Like I said above  try type assertion first before trying this way

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maybe you need func TransToString data interface     res string        switch v    data  type        case float64          res   strconv FormatFloat data  float64    f   6  64      case float32          res   strconv FormatFloat float64 data  float32     f   6  32      case int          res   strconv FormatInt int64 data  int    10      case int64          res   strconv FormatInt data  int64   10      case uint          res   strconv FormatUint uint64 data  uint    10      case uint64          res   strconv FormatUint data  uint64   10      case uint32          res   strconv FormatUint uint64 data  uint32    10      case json Number          res   data  json Number  String       case string          res   data  string      case   byte          res   string v      default          res    quot  quot            return

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Simplest way I did this  Not the best way but simplest way I know how   import  fmt   func main         fmt Print addTwoNumbers 5  6      func addTwoNumbers val1 interface    val2 interface    int       op1       val1  int      op2       val2  int       return op1   op2

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You need to do type assertion for converting your interface   to int value   iAreaId    val  int  iAreaId  ok    val  int    More information is available

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Instead of iAreaId    int val   you want a type assertion  iAreaId    val  int  iAreaId  ok    val  int     Alt  non panicking version   The reason why you cannot convert an interface typed value are these rules in the referenced specs parts   Conversions are expressions of the form T x  where T is a type and x is an expression that can be converted to type T        A non-constant value x can be converted to type T in any of these cases   x is assignable to T  x s type and T have identical underlying types  x s type and T are unnamed pointer types and their pointer base types have identical underlying types  x s type and T are both integer or floating point types  x s type and T are both complex types  x is an integer or a slice of bytes or runes and T is a string type  x is a string and T is a slice of bytes or runes    But iAreaId    int val   is not any of the cases 1 -7

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Adding another answer that uses switch    There are more comprehensive examples out there  but this will give you the idea    In example  t becomes the specified data type within each case scope  Note  you have to provide a case for only one type at a type  otherwise t remains an interface   package main  import  fmt   func main         var val interface      your starting value     val   4      var i int    your final value      switch t    val  type        case int          fmt Printf   d     T n   t  t          i   t     case int8          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case int16          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case int32          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case int64          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case bool          fmt Printf   t     T n   t  t                not covertible unless               if t               i   1              else               i   0                  case float32          fmt Printf   g     T n   t  t          i   int t     standardizes across systems     case float64          fmt Printf   f     T n   t  t          i   int t     standardizes across systems     case uint8          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case uint16          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case uint32          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case uint64          fmt Printf   d     T n   t  t          i   int t     standardizes across systems     case string          fmt Printf   s     T n   t  t             gets a little messy        default             what is it then          fmt Printf   v     T n   t  t             fmt Printf  i     d n   i

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Best avoid casting by declaring f to be f the correct type to correspond to the JSON

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I am assuming  If you sent the JSON value through browser then any number you sent that will be the type float64 so you cant get the value directly int in golang   So do the conversion like     As that says   fmt Fprintf w   Type    v   val      lt --- Type   float64  var iAreaId int   int val  float64     This way you can get exact value what you wanted

[go] Convert interface{} to int

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