How to add auto increment id to a HashMap?

Developers
1

Hello there,

I just started to code a blog with ICP and I’m following this tutorial.

The tutorial is a bit old and the recommendation is to create a HashMap instead of an Array of blog posts. I found the following code that works but I can’t figure it out how to add an auto increment in the HashMap instead of the Id as Text. Any pointers would be much appreciated.

    type Id = Text;

    type Entry = {
        author : Text;
        content : Text;
    };

    let entries = Map.HashMap<Id, Entry>(0, Text.equal, Text.hash);

    public func insert(id : Id, entry : Entry): async () {
        entries.put(id, entry);

    };
    public query func lookup(id : Id) : async ?Entry {
        entries.get(id)
    };
2 
import Nat32 "mo:base/Nat32";
type Id = Nat32;

type Entry = {
    author : Text;
    content : Text;
};

let entries = Map.HashMap<Id, Entry>(0, Nat32.equal, func (a : Nat32) : Nat32 {a});
let nextEntryId : Id = 0;

public func insert(entry : Entry): async () {
    entries.put(nextEntryId, entry);
    nextEntryId++;
};
public query func lookup(id : Id) : async ?Entry {
    entries.get(id)
};
3

@stephenandrews thanks for the quick response.

It looks like this is not working ==> nextEntryId++;

unexpected token ';', expected one of token or <phrase> sequence:
  <exp_un(ob)>

I also added the following ( nextEntryId += 1; ) as I have seen in other examples but then I get this error:

type error [M0073], expected mutable assignment target
4

Sorry yeah +=1 is what you need to do.

Whopps my bad, switch it to a var then - defining it with let is immutable.

5

This works…thank you so much!!

6

Chur chur, welcome to the IC my friend!

7

Al the following functions are working except showEntries.

Do you know what am I missing? Sorry I’ve never worked with HashMaps

type Id = Nat32;

    type Entry = {
        author : Text;
        content : Text;
    };

    let entries = Map.HashMap<Id, Entry>(0, Nat32.equal, func (a : Nat32) : Nat32 {a});
    var nextEntryId : Id = 0;

    public func insert(entry : Entry): async () {
        entries.put(nextEntryId, entry);
        nextEntryId += 1;
    };
    public query func showSize () : async Nat {
        entries.size()
    };
    public query func lookup(id : Nat32) : async ?[Entry] {
        entries.get(id)
    };
    public query func showEntries () : async ?[Entry] {
        entries.entries()
    };

This is the error:

type error [M0096], expression of type
  ?Entry
cannot produce expected type
  ?[Entry]
/Users/fer/Projects/ic-projects/ic-playground/src/backend/blog/main.mo:33.9-33.26: type error [M0096], expression of type
  Iter/1<(Id, Entry)> = {next : () -> ?(Id, Entry)}
cannot produce expected type
  ?[Entry]
8

Do this instead:

public query func showEntries () : async [(Id, Entry)] {
    Iter.toArray(entries.entries());
};

Gives you both the ID and the Entry in a set of arrays. e.g.:

[
  [0, {...entry}], 
  ...
]
1 Like
10

I had a typo but I figure it out.

Thanks a lot again…It works!!!