Declarative vs Imperative programming

June 11, 2018

Imperative programming focuses on ‘how’ and Declarative programming focuses on ‘what’.

Examples

//Collection of integer objects
List<int> collection = new List<int> { 1, 2, 3, 4, 5 };

//imperatively checking and creating a list of odd numbers
List<int> results = new List<int>();
foreach(var num in collection)
{
    if (num % 2 != 0)
          results.Add(num);
}

//doing the same 'declaratively' by using LINQ
var results = collection.Where( num => num % 2 != 0);

Java

List<Integer> numbers = new ArrayList<Integer>(Arrays.asList(1,2,3,5,8,13,21));

//declarative approach
List<Integer> oddList = new ArrayList<>();
for (int each: numbers){
    if (numbers%2!=0) oddList.add(each);
}

//imperative approach using java-8 stream
oddList = numbers.stream().filter(each -> each % 2 != 0).collect(Collectors.toList());

SQL

//finding top 5 scorers from a table
//sql is declarative language
select * from students order by marks desc limit 5;
//how does it happen internally? Consult your sql vendor (mysql, postgresql)

Imperative programming style demands us to write step by step process detailing what to do and how to do.
Declarative programming abstracts away as much detail as possible within underlying non-mutable functions and programmer just has to follow the internal API and state what they want
It’s all about abstraction. LINQ or java stream or SQL engine internally do everything in an optimized way so the programmer can rest assured and go about doing the important stuff.