Salesforce Add value in 2d array (Index out of bound 0) - salesforce

Question

I am trying to add array in an array i.e 2d array, while I am trying it throws this error:
List index out of bound 0
I am trying at this line:
df_depts_obs[df_mk_index] = departmentobs_columns;
private void initializeInconsistentRecommendations() {
recmn_qty = new List<Integer>();
String mk_unique_Data;
Integer bookmark;
bookmark=0;
mk_unique_Data='';
totalCount = 0;
inconsistentRecommendations = new List<AA_Recommendations__c>();
inconsistentAccessorys = new List<AA_Accessory__c>();
df_depts = new List<AA_DepartmentObservation__c>();
df_depts_obs = new List<List<AA_DepartmentObservation__c>>();
df_depts_recmds = new List<List<AA_DepartmentRecommendation__c>>();
df_depts_prds = new List<List<AA_Recommendations__c>>();
String df_mk_unique_dept='';
Integer df_mk_index=-1;
departmentRecommendations_data = new List<AA_DepartmentRecommendation__c>();
List<AA_DepartmentObservation__c>departmentObservations = [SELECT Id, Name, Observation_ID__c,Department_ID__r.Name,Observation_ID__r.Observation__c FROM AA_DepartmentObservation__c
WHERE Department_ID__r.Assessment__c = :assessment.Id LIMIT 10];
if (departmentObservations.size() > 0) {
for (integer index = 0; index < departmentObservations.size(); index++ ) {
AA_DepartmentObservation__c departmentObservation = departmentObservations[index];
//df_depts.add(departmentObservation); // FOR DEPARTMNET HEADING
if(!df_mk_unique_dept.contains((String)departmentObservations[index].Department_ID__r.Name)){
df_mk_unique_dept += '<li>'+departmentObservations[index].Department_ID__r.Name+'</li>';
df_depts.add(departmentObservation); // FOR DEPARTMNET HEADING
df_mk_index = df_mk_index + 1;
}
List<AA_DepartmentObservation__c> departmentobs_columns = new List<AA_DepartmentObservation__c>();
departmentobs_columns.add(departmentObservation);
df_depts_obs[df_mk_index] = departmentobs_columns;

Answer 1

In Apex, you cannot use array notation [ ] to add objects, only set values for indexes in the array already exists.
You can either declare the size of the array so that it is initialized, or grow the array dynamically using the add method.
This example shows both options:
List<List<String>> stringList = new List<List<String>>(1);
List<String> innerListOne = new List<String> {'Red','Blue'};
List<String> innerListTwo = new List<String> {'One','Two'};
stringList[0] = innerListOne;
stringList.add(innerListTwo);