Simplify your programs with LINQ - Language Integrate Query (LINQ) study series IV

July 29, 2016

 LINQ - blogs to read:

http://igoro.com/archive/7-tricks-to-simplify-your-programs-with-linq/

http://igoro.com/archive/extended-linq-additional-operators-for-linq-to-objects/

Notes:

1. Initialize an array

int[] a = Enumerable.Repeat(-1,10).ToArray(); 

int[] b = Enumerable.Range(0,10).ToArray(); 

int[] c = Enumberable.Range(0,10).Select(i=>100+10*i).ToArray(); 

Enumerable.Repeat, Range, 

no for loops necessary, using LINQ 

2. Iterate over multiple arrays in a single loop

foreach(var x in array1)

  DoSomthing(x); 

foreach(var x in array2)

  DoSomething(x); 

LINQ

foreach(var x in array1.Concat(array2))

  DoSomething(x); 

LINQ operates at the enumerator level (?), it will not allocate a new array to hold elements of array1 and array2. So, space-efficient. 

3. Generate a random sequence

Random rand = new Random(); 

var randomSeq = Enumerable.Repeat(0,N).Select(i=>rand.Next()); 

lazy nature of LINQ, the sequence is not pre-computed and stored in an array, but instead random numbers are generated on-demand, as you 

iterate over randomSeq. 

4. Generate a string 

Generate a strign with the repeating pattern "ABCABCABC..." of length N. 

string str = new string(Enumerable.Range(0,N).Select(i => (char)('A' + i%3)).ToArray()); 

string values = string.Join(string.Empty, Enumerable.Repeat(pattern, N).ToArray()); 

5. Convert sequences or collections 

IEnumerable<string> strEnumerable = ...;

IEnumerable<object> objEnumerable = strEnumerable.Cast<object>(); 

List<string> strList = ...;

List<object> objList =  new List<object>(strLsit.Cast<object>()); 

var objList = strList.Cast<object>().ToList(); 

6. Convert a value to a sequence of length 1

IEnumerable<int> seq = Enumerable.Repeat(myValue, 1); 

7. Iterate over all subsets of a sequence

For small input, three problems:

subset sum

boolean satisfiability

knapsack problem

NP-complete problems - 

solve easily by iterating over all subsets of some sequence: