$scope.headings data value undefined outside of $http.get

Goal: Populate a table that has dynamic table headings (retrieved via GET). As such, the values (retrieved via another GET) to be used to populate the table have a header_id that can connect the two:

i.e.

headers = [{name:header1, id:1}, {name:header2, id2}]
list_entries = [{value:'pop', header_id:1},{value:'bop', header_id:3}]

I can't directly use ng-repeat on list_entries to populate the table since I must respect the fact that some table cells will be empty (no value matched to a header).

I want to access $scope.headings so that I can iterate through it and then use logic to match a value to it (compare header_id).

This may seem like a dumb question to others, but I really have tried looking all over for a good approach. I'd really appreciate just being pointed in the right direction with respect to this.

<script>
    var list = angular.module('listApps', []);

    list.config(function($httpProvider) {
             ... set defaults ...
    });

    list.controller('ListCtrl', function ($scope, $http, $timeout){

        var table_headings = xxxxxx; //root + ending

        /* Pull Table Headings*/    
        $http.get(table_headings.toString()).success(function(data,status){
            $scope.headings = data.response.fields_names;
            $scope.status = status;
            console.log(data.response.fields_names);
        }).error(function(result,status){
            $scope.data = result.data || "Request failed";
            $scope.status = status;
        });

        // match table headings to the respective values in order to populate table correctly
        $scope.mapping = [];

        // At this point, not even focusing on the function; just need to access $scope.headings
        angular.forEach($scope.headings, function(value, key){
            this.push(key+':'+value);
        }, $scope.mapping);
    });
</script>