如何计算数组中以最高编号开头的项目数？

Question

I've got an array that is filled with dates. They each start with the number of the year. How do I cound the number of items present in the array that start with the highest number?

My array in question:

 ["12-A*januari", "12-B*februari", "12-C*maart", "12-D*april", "12-E*mei", "12-F*juni", "12-G*juli", "12-H*augustus", "12-I*september", "12-J*oktober", "12-K*november", "12-L*december", "13-A*januari", "13-B*februari", "13-C*maart", "13-D*april", "13-E*mei", "13-F*juni", "13-G*juli", "13-H*augustus", "13-I*september", "13-J*oktober", "13-K*november", "13-L*december", "14-A*januari", "14-B*februari", "14-C*maart", "14-D*april", "14-E*mei", "14-F*juni", "14-G*juli"]

So in this example I want to count the number of items in the array that start with 14, which in this example case would be 7. The numbers are dynamic so in the future the highest number will be different.

How do i return the count for the items that start with the highest number?

Answer 1

If your data is stored in arr :

var numbers = arr.map(function(x){ return parseInt(x); })
var max = Math.max.apply(Math,numbers)
var n = numbers.filter(function(x){ return x==max; }).length

Then your count will be in n .

PS in your example array there are 7 elements starting with 14.

Answer 2

you can sort the array, reverse it, then just filter to get the count

var count = ar.sort().reverse().filter(function(v){    
    return v.split('-')[0] == ar[0].split('-')[0];
}).length;

FIDDLE

Answer 3

First loop through your array, splitting your string and finding the largest element, by looking at the first element of the new array, like this:

var splitString = iteratingString.split("-");
if(Number(splitString[0]) > max){
    max = Number(splitString[0]);
}

Then you iterate again, split each string again into a temporary array and increase the counter if the first element of the temp array is equal to max.

Answer 4

Combinations of map/max/filter is elegant, but unless these functions were lazy, it's quite inefficient in theory since you have to loop multiple times over the input for each functionnal operation.

It really doesn't matter in practice, unless your input is very large, but why not something standard like the following?

highestNumberOccurenceFrom(['13a', '12b', '13c']); //2

function highestNumberOccurenceFrom(arr) {
    var count = 0, max = 0, i = arr.length, num;

    while (i) {
        num = parseInt(arr[--i]);

        if (num > max) max = num, count = 1;
        else if (num === max) ++count;
    }

    return count;
}

Answer 5

You could try something like this:

var maxCount = 0;
var maxVal = null;
var curVal = null;

for (var ctr=0; ctr< array.length; ctr++){
    //TODO: parse the number from the beginning of the string 
    //      You will need to write the function 'parseValFromString'
    curVal = parseValFromString(array[ctr]);
    if (curVal > maxVal || maxVal === null){
         maxVal = curVal;
         maxCount = 1;
    }
    else if (curVal ===  maxVal){
         maxCount++;
    }
}
console.log("count=" + maxCount;

Answer 6

Sort the Array first. Let the name of array be list

list.sort();

If you are sure that the array is already sorted, you can omit this step.

Get the max number

var max = list[list.length-1].substr(0,2);

Now filter the Array like this.

var list2 = list.filter(function(a){
    return max == a.substr(0,2);
})

Now get number of elements in list2.

var answer = list2.length;