How to check if struct value exists within a struct

Question

I am retrieving data from an API. The struct output is :

 {
    StreamSpecification: {
      StreamEnabled: true,
      StreamViewType: "NEW_AND_OLD_IMAGES"
     },
    TableStatus: "ACTIVE"
  }

But if the API output does not have StreamSpecification in it, I am receiving the following error when trying to print the struct.

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=xxxxxxxx]

How to check if the struct StreamSpecification exists in the value? Or how to fix the issue in any other way?

Akavall · Accepted Answer · 2018-12-10 04:12:11Z

If I understand the question correctly, I would convert the struct into a map, and then check if the field you are interested in is in the map.

For example:

package main                                                                                                                       

import (                                                                                                                           
    "encoding/json"                                                                                                                
    "fmt"                                                                                                                          
)                                                                                                                                  

type MyStruct struct {                                                                                                             
    Name  string                                                                                                                   
    Score int                                                                                                                      
}                                                                                                                                  

func main() {                                                                                                                      

    ms := MyStruct{Name: "Amy", Score: 34}                                                                                         

    var myMap map[string]interface{}                                                                                               
    data, _ := json.Marshal(ms)                                                                                                    
    fmt.Println(data)                                                                                                              

    json.Unmarshal(data, &myMap)                                                                                                   

    fmt.Println(myMap)                                                                                                             

    _, ok := myMap["Name"]                                                                                                         
    fmt.Printf("name is in myMap: %t\n", ok)                                                                                       

    _, ok = myMap["Location"]                                                                                                      
    fmt.Printf("Location is in myMap: %t\n", ok)                                                                                   

}

Go Playground

Luke Joshua Park · Accepted Answer · 2018-12-09 19:57:17Z

-1

It sounds like you're trying to access StreamEnabled or StreamViewType when you haven't first confirmed if StreamSpecification was provided in the JSON object.

Assuming you have the internal StreamSpecification as a reference to a struct, you need to ensure that StreamSpecification isn't nil:

if (instance.StreamSpecification == nil) {
    // StreamSpecification was not passed in the JSON.
}

answered Dec 9, 2018 at 19:57

Luke Joshua Park

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2 Comments

Arpan Solanki Over a year ago

I had tried that but i am getting a compilation error "cannot convert nil to type dynamodb.StreamSpecification" Here is the struct docs.aws.amazon.com/sdk-for-go/api/service/dynamodb/…

Luke Joshua Park Over a year ago

Can you just include a minimal reproducible example in your question - hard to diagnose if we can't see your actual code.

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