I have the following struct:

struct Group {
    string name;
    address[] members;

and the following array:

Group[] public groups;

I don't need values, I only want to add one address after the next to check easier if a user is in a group or not (without creating another array and pointing that array to the struct).

Now I know, that for example groups[0].members[0] could be msg.sender.

Is there a way to create like only one way arrays, for example, that it is members[msg.sender] instead of members[0,1,273,37382...]?


I edited the whole answer after your explanation.

I think what you are looking for is a mapping. I understand that your concern is that the previous answer does not organize by groups, but you can have that order as follows:

mapping(string => mapping(address => bool)) public Groups;

If you want to know if a particular address lets say 0x12345 is in the group called "GroupA" just need to do as follows:

if (mapping['GroupA'][0x12345]){
  //it exists!!
 //do not exists 

The whole thing can be implemented as follows as follows:

contract CheckAddress{
 mapping(string => mapping(address => bool)) public Groups;

 function CheckAddress(){

 function setAddresToGroup(address addr, string group) public {
   Groups[group][addr] = true; // set an addres in a group

 function checkExistenceInGroup(address addr,string group) public returns (bool){
   return Groups[group][addr]; // will return true if the address is in group
  • Thank you and I know how to do it like you said, but tha's not how I would like to have it. That would create a huge array mapping. Instead I want for each group an own array like written in the main post. – dkb Mar 29 '18 at 21:38
  • can you explain what do you mean by this: "Is there a way to create like only one way arrays, for example, that it is members[msg.sender] instead of members[0,1,273,37382...]?" – Jaime Mar 29 '18 at 22:00
  • 1
    If I make an array like "address[] arrayName" I get values with calling arrayName[0], arrayName[1], arrayName[2],... etc. Those numbers than have the values, for example arrayName[0] could be "0x001...", arrayName[1] could be "0x123..." etc. But I want an array or other that has the address in the name itself. For example arrayName[0x001] or arrayName[0x123] etc. Because this would be easier to check. When I would check the first method I need to check each value if the address is the address I look for. In the second method I need only one check, if arrayName[msg.sender] exists. – dkb Mar 30 '18 at 16:27
  • I modified the answer for a complete description of how to do this. I hope this will help you. Good luck! – Jaime Mar 30 '18 at 18:18
  • Thank you! I have added an answer like exactly how I wanted it. :) – dkb Apr 2 '18 at 15:55

@Jaime thanks for your answer!

With your code I have found the solution I was looking for.
I wanted the members array right into the struct itself.
I edited the struct to the following:

struct Group {
    string name;
    mapping(address => bool) members; // <- ADDED THIS

Now I can do

groups[0].members[msg.sender] = true;

and get

if (groups[0].members[msg.sender] == true)

Thank you!

  • 1
    I'm glad I could help, and that you solve it. Good luck – Jaime Apr 5 '18 at 14:56

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