function groupBy(array, test) {
var groups = {};
array.forEach(
function(element){
var groupKey = test(element);
if(!groups[groupKey]) {
groups[groupKey] = [];
}
groups[groupKey].push(element);
}
);
return groups;
}
var centGroups = groupBy(ancestry,
function(person){return Math.ceil(person.died / 100);});
for(var cent in centGroups) {
console.log(cent+":", average(centGroups[cent].
map(function(person){return person.died-person.born;})));
}
Chap 5 Historical life expectancy
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