initializing different values of map in different locations in golang

Question

I'm creating a map of structs to hold different information. A sample struct I am using is:

type Test struct{
  Value1 string
  Value2 string
  Value3 string
  Value4 string
}

func main() {
  testMap := make(map[string]*Test) //using a pointer to map
  func2(testMap)
  //pass map to another function for more additions.
}

func func2 (testMap map[string]*Test) {
  a, b := getVal(); //get some random values
  res := concat(a,b) //basically create a string key based on values a and b
  testMap[res].value1 = a   //****
  testMap[res].value2 = b
  //do something else
  testMap[res].value3 = "hello"

}

I'm basically trying to create a map and add values to it as I get them, but im getting a invalid memory address or nil pointer dereference error on **** line (see code for ***).

That means that testMap[res] is nil. As the error says, you can't dereference (eg access fields of) a nil pointer. — Adrian
– Adrian, Commented Feb 28, 2019 at 18:45
that makes sense.. so how do I create an entry for map.. without knowing all the fields? (apart from getting all the fields and creating it in the end with all the values) — user2883071
– user2883071, Commented Feb 28, 2019 at 18:46
testMap[res] = &Test{} would insert a new empty instance, is that what you mean? Have you taken the Tour of Go? — Adrian
– Adrian, Commented Feb 28, 2019 at 18:53
That was it. Thank you. For some reason I missed that in the tour. I did recall it doing it by value and not through pointers, which is what confused me. Thanks! — user2883071
– user2883071, Commented Feb 28, 2019 at 19:03

Ed Chan · Accepted Answer · 2019-02-28 19:01:33Z

1

Try:

func func2 (testMap map[string]*Test) {
  a, b := getVal(); //get some random values
  res := concat(a,b) //basically create a string key based on values a and b
  testMap[res] = &Test{
     Value1: a,
     Value2: b,
     Value3: "string",
  }

}

Or if you want to create the object first, then populate the value, try

func func2 (testMap map[string]*Test) {
  a, b := getVal(); //get some random values
  res := concat(a,b) //basically create a string key based on values a and b
  testMap[res] = &Test{}
  testMap[res].value1 = a   //****
  testMap[res].value2 = b
  //do something else
  testMap[res].value3 = "hello"

}

answered Feb 28, 2019 at 19:01

Ed Chan

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