Create Training JS #30: methods of arrayObject---reduce() and reduceRight().js

Author: Automedon

Training JS #30: methods of arrayObject---reduce() and reduceRight().js

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+/*
+Description:
+Training JS #30:
+methods of arrayObject---reduce() and reduceRight()
+
+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.
+
+Some example:
+```
+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
+```
+*/
+function tailAndHead(arr){
+  let m=[];
+  for(let i=0;i<arr.length-1;i++)
+    m.push(arr[i]%10+Number((arr[i+1]+'')[0]))
+  return m.reduce((a,b)=>a*b)
+}