合并JavaScript中两个数组中的值

Question

I have an array which looks like:

var data = [{"year":[1981],"weight":[3]},
            {"year":[1982],"weight":[4]},
            {"year":[1985],"weight":[7]}]

My data series starts with year 1980 and ends with year 1986. My task is to input all missing values into the array; in my case the final array should be:

var data = [{"year":[1980],"weight":[0]},
            {"year":[1981],"weight":[3]},
            {"year":[1982],"weight":[4]},
            {"year":[1983],"weight":[0]},
            {"year":[1984],"weight":[0]},
            {"year":[1985],"weight":[7]},
            {"year":[1986],"weight":[0]}]

I implemented this task in two steps. First I created an empty array with length of seven elements (for years 1980 - 1986) and initialize each element with value {"year": $CURRENT_YEAR, "weight": 0} . Then I loop through data array, find index of current year in the empty array and replace year and weight fields with current values. My code is pasted below.

I wonder if the code could be rewritten in a more elegant way.

// Create empty array
var my_array = []
var length = 7

// 1st step
year = 1980
for (var i = 0; i < length; i++) {
  my_array.push({"year": year, "weight": 0});
  year++
}

// 2nd step
for (var j = 0; j < data.length; j++) {
  curr_year = data[j]["year"][0];
  curr_weight = data[j]["weight"][0]
  var index = my_array.findIndex(function(item, i) {return item.year === curr_year})
  my_array[index] = {"year": curr_year, "weight": curr_weight}
}

Answer 1

It's best to do this job by .map() Besides if you have a large input array it might be wise to set up a hash (lut) in the first place such as;

 var data = [{"year":[1981],"weight":[3]}, {"year":[1982],"weight":[4]}, {"year":[1985],"weight":[7]}], lut = data.reduce((p,c) => p[c.year[0]] ? p : (p[c.year[0]] = c, p), {}); range = [1980,1986], result = Array(range[1]-range[0] + 1).fill() .map((_,i) => lut[i+range[0]] ? lut[i+range[0]] : {year: [i+range[0]], weight: [0]}); console.log(result); 

Answer 2

You can combine the 2 loops and do both steps in one loop

// Create empty array
var my_array = []
var length = 7


year = 1980
for (var i = 0; i < length; i++) {
    // check if there is data for the year
    var index = data.findIndex(function(item, i) {return item.year === year});
    if(index > -1){ //if there is data, use it
        my_array.push({"year": data[index]["year"][0], "weight": data[index]["weight"][0]});
    }else{ //put in default data
       my_array.push({"year": year, "weight": 0});
    }
    year++;
}

Answer 3

Find index of element in array each time is bad performance for large data. I can suggest the following algorithm:

// Create empty object and fill it with values where keys are years
var years = {};
data.forEach(item => {
    years[item.year[0]] = item.weight[0];
});

// Result array with all years
var result = [];
var startYear = 1980;
var endYear = 1986;

// Generate our result array
for (var i = startYear; i <= endYear; i++) {

    // If property for given year (i) exists in "years" object then add it to "result" array
    // in other case add default object with weight 0
    var o = years[i] ? { year: [i], weight: [years[i]] } : { year: [i], weight: [0] };
    result.push(o);
}

Answer 4

You could do this with just find() and while loop.

 var data = [{"year":[1981],"weight":[3]},{"year":[1982],"weight":[4]},{"year":[1985],"weight":[7]}]; var i = 1980; var result = []; while(i <= 1986) { var find = data.find(e => e.year[0] == i); (find) ? result.push(find) : result.push({year: [i], weight: [0]}); i++; } console.log(result) 

You could also first use map() to get array of years and then use while loop with indexOf() .

 var data = [{"year":[1981],"weight":[3]},{"year":[1982],"weight":[4]},{"year":[1985],"weight":[7]}]; var i = 1980; var result = []; var years = data.map(e => e.year[0]); while(i <= 1986) { var ind = years.indexOf(i); (ind != -1) ? result.push(data[ind]) : result.push({year: [i], weight: [0]}); i++; } console.log(result)