Quick review of recursive functions (2018-06-04)

experiments.js

// Playing with recursion a bit for review
/*
  what is there to know about it? lol. hmm
  - need a base case, when to STOP recursing!
  - function calling itself
  - good for recursive structures like trees
  - question: what does the call stack look like? how to visualize it?
    -- the tricky part is seeing how each call resolves;
    -- ....can't show that with console.log :(
*/

/////////////////////////////////////////////////////////////////
//    Useless example: recursion to increment a number up to 10
/////////////////////////////////////////////////////////////////

function addToSelf(num) {
    console.log("Called addToSelf(" + num + ")");
     
    if (num >= 10) {
      return num; 
    }
     
    return addToSelf(num+1); 
    }
    
    let result = addToSelf(5);
    console.log(result);


// // Alternate version...
// // Exploring: how is this different from recursion? How's it the same?
// function addToSelf(num) {
//   console.log("Called addToSelf(" + num + ")");
  
//   return num + 1; 
// }

// let result = addToSelf( addToSelf( addToSelf(1) ) );
// console.log(result);


/////////////////////////////////////////////////////////////////
//    Factorials  (5! === 5 + 4 + 3 + 2 + 1 === 15)
/////////////////////////////////////////////////////////////////

// // Iterative way:
// function loopFactorial(num) {
// let sum = 0;
// for (let i = num; i > 0; i--) { 
//   sum += i;
//   console.log("i: " + i + ", sum: " + sum);
// }
// return sum;
// }

// let sum = loopFactorial(5);
// console.log(sum);
    

// Recursive way:
function recurseFactorial(factorialNum, partialSum) { 
    console.log("called recurseFactorial(" + factorialNum + ", " + partialSum + ")");
   
    if (factorialNum == undefined || factorialNum < 0) {
      //throw new Error("One positive number is required to call recurseFactorial.");
      console.log("One positive number is required to call recurseFactorial.");
      return;
    // if function is first called with 1 or 0, return 1 or 0
    // * assuming user will only ever call the function wiht one argument!
    } else if (partialSum == undefined && factorialNum <= 1) {  // 0! = 0, and 1! = 1
      return factorialNum;
    }
   
  // Default value for partial sum: start at 0;
  if (partialSum == undefined) {
    partialSum = 0;
  }
   
  if (factorialNum === 1) { 
    // NOTE: always decrement 1 at a time, guaranteed for the num to equal 1 at some point, right?
    return partialSum + 1;
  }
  
    //partialSum += factorialNum;
                        
    return recurseFactorial( factorialNum - 1, partialSum + factorialNum);
  }
  
  console.log( recurseFactorial(5) );