Kata

This lesson we learn two methods of array: reduce() and reduceRight(). The two method is approximate. It applies a function against an accumulator and each value of the array to reduce it to a single value. The only difference is: the order of reduce() is from left to right, but reduceRight() is from right to left.

For more information, please refer to:

• Array.prototype.reduce()
• Array.prototype.reduceRight()

Before looking at some examples, we need to understand that the callback function used for the reduce() can use up to 4 parameters: previousValue, currentValue, index and array. In general, we use only two parameters: previousValue and currentValue. index and array are optional, and we use it when necessary.

Here are some examples to help us understand the use of reduce() and reduceRight():

The first is a classic example, we can use reduce() method to get the sum of the number array:

var arr=[1,2,3,4,5];
console.log(arr.reduce((prev,curr)=>prev+curr)); //output: 15
//My habit is to use a and b
console.log(arr.reduce((a,b)=>a+b));             //output: 15

In addition to the callback function, reduce() also has an optional parameter: initialvalue. In the example above, the initialvalue is omitted. Then the initialvalue is the first element, and then start the cumulative operation from second element.

The following example uses the initialvalue:

var arr=[1,2,3,4,5];
console.log(arr.reduce((a,b)=>a+b,""));             //output: 12345

This example uses an empty string as the initialvalue, followed by these operation:

""+1-->"1","1"+2-->"12",......,"1234"+5-->"12345"

As long as you have enough imagination, reduce() can do a lot of work.

Calculated average:

function average(arr){
  return arr.reduce((a,b)=>a+b)/arr.length
}
console.log(average([1,2,3,4,5])); //output: 3

Count something

var arr=[1,2,3,4,5];
//count odd number:
console.log(arr.reduce((a,b)=>a+(b%2?1:0),0)); //output: 3
//count even number:
console.log(arr.reduce((a,b)=>a+(b%2?0:1),0)); //output: 2

The following is a complex example, which uses a callback function of the four parameters and the initialvalue:

var arr1=[1,2,3,4,5];
var arr2=[6,7,8,9,10];
var result=arr1.map(x=>x*x).reduce((a,b,i,ar)=>a.concat(ar[i],arr2[i]),[])
console.log(result); //output: [ 1, 6, 4, 7, 9, 8, 16, 9, 25, 10 ]

In the example above, Arr1 uses the map method to turn each element into its square.

[1,2,3,4,5]--->[1,4,9,16,25]

Then, using the reduce method. the two elements (ar[i] and arr2[i]) of the same index in turn are put into the initial value (an empty array).

[1,4,9,16,25]
                ---> [ 1, 6, 4, 7, 9, 8, 16, 9, 25, 10 ]
[6,7,8,9,10]

It should be noted that when reading the value of the first array, we used the ar[i] (fourth parameters of the callback function). This value is changed by the map() method([1,4,9,16,25]). If we used arr1[i] reading the value, will be the value of the original array([1,2,3,4,5])

Ok, lesson is over. let's us do some task.

Task

Coding in function tailAndHead. function accept 1 parameter arr(a number array).

We use an example to explain this task:

You got an array [123,456,789,12,34,56,78].

First, from left to right, the tail of an element and the head of the next element are added together. The results are put into an array. like this:

[123,456,789,1 2,3 4,5 6,78]
   3+4 6+7 9+1 2+3 4+5 6+7  
    |   |   |   |   |   |
   [7 ,13 ,10 , 5 , 9 ,13]

And then, Calculate their product and return it(7x13x10x5x9x13=532350)

So, tailAndHead([123,456,789,12,34,56,78]) should return 532350

All elements of arr are positive integer. If a number is less than 10, its head and tail are the same. Please try to use reduce() solve this kata.

Examples

tailAndHead([1,2,3,4,5]) should return 945
tailAndHead([111,2345,66,78,900]) should return 7293
tailAndHead([35456,782,569,2454,875]) should return 12012

Series

( ↑↑↑ Click the link above can get my newest kata list, Please add it to your favorites)

• #1: create your first JS function helloWorld
• #2: Basic data types--Number
• #3: Basic data types--String
• #4: Basic data types--Array
• #5: Basic data types--Object
• #6: Basic data types--Boolean and conditional statements if..else
• #7: if..else and ternary operator
• #8: Conditional statement--switch
• #9: loop statement --while and do..while
• #10: loop statement --for
• #11: loop statement --break,continue
• #12: loop statement --for..in and for..of
• #13: Number object and its properties
• #14: Methods of Number object--toString() and toLocaleString()
• #15: Methods of Number object--toFixed(), toExponential() and toPrecision()
• #16: Methods of String object--slice(), substring() and substr()
• #17: Methods of String object--indexOf(), lastIndexOf() and search()
• #18: Methods of String object--concat() split() and its good friend join()
• #19: Methods of String object--toUpperCase() toLowerCase() and replace()
• #20: Methods of String object--charAt() charCodeAt() and fromCharCode()
• #21: Methods of String object--trim() and the string template
• #22: Unlock new skills--Arrow function,spread operator and deconstruction
• #23: methods of arrayObject---push(), pop(), shift() and unshift()
• #24: methods of arrayObject---splice() and slice()
• #25: methods of arrayObject---reverse() and sort()
• #26: methods of arrayObject---map()
• #27: methods of arrayObject---filter()
• #28: methods of arrayObject---every() and some()
• #29: methods of arrayObject---concat() and join()
• #30: methods of arrayObject---reduce() and reduceRight()
• #31: methods of arrayObject---isArray() indexOf() and toString()
• #32: methods of Math---round() ceil() and floor()
• #33: methods of Math---max() min() and abs()
• #34: methods of Math---pow() sqrt() and cbrt()
• #35: methods of Math---log() and its family
• #36: methods of Math---kata author's lover:random()
• #37: Unlock new weapon---RegExp Object
• #38: Regular Expression--"^","$", "." and test()
• #39: Regular Expression--"?", "*", "+" and "{}"
• #40: Regular Expression--"|", "[]" and "()"
• #41: Regular Expression--""
• #42: Regular Expression--(?:), (?=) and (?!)