Typecast a string - trigger to pass value to lookup from picklist

Question

I'm trying nothing more than to update a lookup field when I create a new record. The field it should pull the value from is a lookup.

I have this code:

List <Job__c> opListInsert = new List<Job__c>();
  List <Job__c> opListUpdate = new List<Job__c>();
   if(trigger.isInsert){
    for(Job__c op:trigger.New){
    if(op.Acct__c != Null){
        op.Account__c = op.Acct__c;
        opListInsert.add(op); }
     }
    }  
  else if(trigger.isUpdate){
    for(Job__c op:trigger.New){
     if(op.Acct__c != Null && op.Acct__c !=trigger.oldMap.get(op.id).Acct__c){
         op.Account__c = op.Acct__c;
          opListUpdate.add(op);
    }    
 }

}

But it's throwing the following error:

Error:Apex trigger UpdateAccounts caused an unexpected exception, contact your administrator: UpdateAccounts: execution of BeforeUpdate caused by: System.StringException: Invalid id: accountname : Trigger.UpdateAccounts: line 20, column 1

Anyone know how I can convert the account to string or vice versa?

Update

Thanks for the help, don't know why I figured it would add the string not the ID.

Here is my updated trigger that works (might not be the cleanest option):

trigger UpdateAcct on Job__c (before insert, before update) {

List<String> Accounts = new List<String>(); 
for (Job__c obj: trigger.new){
   Accounts.add(obj.Acct__c);
}
list<Account> acctlist = [select Name from account where Name in :Accounts];
  if (acctlist.size() > 0 ){
    for (Integer i = 0; i < Trigger.new.size(); i++){
      if (Trigger.new[i].Acct__c != null){
        Trigger.new[i].Account__c = acctlist[i].ID; 
      }   
      else{
        Trigger.new[i].Account__c = null;
      }
    }
  }
}

Answer 1

Id to String conversion (and the reverse) is handled automatically when you do an assignment. If you get an invalid ID error it means that the String value is not a valid 15 or 18 character Salesforce identifier.

What are the values in the picklists? If they're account names then you can't assign the name to a lookup field. If the picklist is a list of IDs it should work ok in code (in formulae you need to call the TEXT() funciton).

I suspect you're either not using IDs in the picklist, or an ID is not being selected, in which case you need to assign null, or do no assignment, rather than assigning an empty string.