打印数组已填充到for循环中

Question

I am coming today with a very basic task that somehow confused me really hard. I have an array that looks like that :

Here is the code:

 double population[][] = {{281.0, 296.0, 325.0, 371.0, 384.5},
    {298.6, 241.2, 301.2, 342.8, 388.7},
    {362.9, 284.1, 276.8, 353.6, 395.1},
    {393.4, 344.8, 295.6, 298.3, 375.0}};
    int year[] = {2011, 2016, 2021, 2026, 2031};
    String ageGroup[] = {"15-19", "20-24", "25-29", "30-34",};
    String output = "Actual and Projected Population in thousands by Age Group (CSO 2016)";
    output += String.format("\n%10s", "");

    for (int i = 0; i < year.length; i++) {
        output += String.format("%10s", year[i]);
    }

    output += String.format("%10s", "%Change");
    double change[] = new double[ageGroup.length];
    for (int i = 0; i < population.length; i++) {
        output += String.format("\n%10s ", ageGroup[i]);
        for (int j = 0; j < population[i].length; j++) {
            output += String.format("%10.1f", population[i][j]);
        }
        change[i] = (((population[i][4] - population[i][0])/
                population[i][0]) * 100);
        output += String.format("%10.1f", change[i]);

    }     
    output += String.format("\n\nTotal (15 - 34): ");           
    System.out.println(output);

}

As you can clearly see I am missing the bottom values - 1335.9, 1166.1, 1198.6, 1543.3 . These values are gained by adding full year eg 2011 - 281 + 298.6 + 362.9 + 393.4

I cannot figure out how to make a for loop in order for this to print out the way I want to.

I tried :

            double total[] = new double[ageGroup.length];
            double hold = 0;
            for(int i = 0; i < population.length; i++){
                    total[i] += hold;       
            for(int j = 0; j < population[i].length; j++){  
                    hold += population[j][i];      

I also tried adding it here

 for (int i = 0; i < population.length; i++) {
            output += String.format("\n%10s ", ageGroup[i]);
            for (int j = 0; j < population[i].length; j++) {
                output += String.format("%10.1f", population[i][j]);
            }
            change[i] = (((population[i][4] - population[i][0])/
                    population[i][0]) * 100);
            output += String.format("%10.1f", change[i]);
            total[i] = (population[0][i] + population[1][i]+ population[2][i]+population[3][i]+population[4][i]);

        }   

Now I am just left confused on how such an easy task made me stuck so hard.

Answer 1

It is a crime that instructors do not teach OO thinking out of the gate, forcing students to muddle through arrays rather than modeling the domain. sigh .

There are two keys here, I think. First, separate the calculation from presentation. Second, realize that rows are age groups and columns are years. Really there should be methods for this stuff rather than just in a single main method. Also, the title and some fluff can be fixed in the final output.

Also, rather than hard coding, eg, [4] , this code uses the .length of the array to make it somewhat easier to deal with adding another year or another age group.

//
// a 2d array, where row is for a given agent group, and column
//  is for a given year
//
static double population[][] = { { 281.0, 296.0, 325.0, 371.0, 384.5 },
        { 298.6, 241.2, 301.2, 342.8, 388.7 },
        { 362.9, 284.1, 276.8, 353.6, 395.1 },
        { 393.4, 344.8, 295.6, 298.3, 375.0 } };
static int year[] = { 2011, 2016, 2021, 2026, 2031 };
static String ageGroup[] = { "15-19", "20-24", "25-29", "30-34", };


public static void main(String[] args)
{
    //
    // hold the totals
    //
    double[] yearTot = new double[year.length]; // total by year
    double[] agTot = new double[ageGroup.length]; //total by ag
    double[] chngAG = new double[ageGroup.length]; //change

    // loop over every age group
    for (int ag = 0; ag < ageGroup.length; ++ag) {
        // get the population for the age group, which is
        // one row in the data
        double[] valsForAG = population[ag];


        // loop over every year, which a column in a given age group
        for (int yr = 0; yr < year.length; ++yr) {
            // get the specific value
            double valForAgInYear = valsForAG[yr];

            // add to the total for the year and to the age group value
            yearTot[yr] += valForAgInYear;
            agTot[ag] += valForAgInYear;

        } // for every year 

        int en = ageGroup.length;
        int st = 0;

        // after processing an age group, calculate the change
        chngAG[ag] = ( ( (valsForAG[en] - valsForAG[st]) /
                valsForAG[st]) * 100);
    } // for every age group

    //
    // do the output
    //

    // header row
    System.out.printf("%10s", "");
    for (int y = 0; y < year.length; ++y) {
        System.out.printf("\t%7d", year[y]);
    }
    System.out.printf("\t%10s%n", "%Change");

    // data
    for (int ag = 0; ag < ageGroup.length; ++ag) {
        System.out.printf("%10s", ageGroup[ag]);
        for (int yr = 0; yr < year.length; ++yr) {
            System.out.printf("\t%7.1f", population[ag][yr]);
        }
        System.out.printf("%10.1f", chngAG[ag]);
        System.out.println();
    }

    //output the totals
    System.out.println();

    System.out.printf("%10s", "Totals:");
    for (int t = 0; t < yearTot.length; ++t) {
        System.out.printf("\t%7.1f", yearTot[t]);
    }
    System.out.println();
}

Output

  2011 2016 2021 2026 2031 %Change 15-19 281.0 296.0 325.0 371.0 384.5 36.8 20-24 298.6 241.2 301.2 342.8 388.7 30.2 25-29 362.9 284.1 276.8 353.6 395.1 8.9 30-34 393.4 344.8 295.6 298.3 375.0 -4.7 

Totals: 1335.9 1166.1 1198.6 1365.7 1543.3

Answer 2

public static void main(String[] args) {
    double population[][] = {{281.0, 296.0, 325.0, 371.0, 384.5}, {298.6, 241.2, 301.2, 342.8, 388.7}, {362.9, 284.1, 276.8, 353.6, 395.1}, {393.4, 344.8, 295.6, 298.3, 375.0}};

    //1. Step: Find the longest of the arrays
    //You need this if the length of the arrays is different, for example:
    /*
    double population[][] = {
     {281.0, 296.0, 325.0, 371.0, 384.5},
     {298.6, 241.2, 301.2, 342.8, 388.7, 0},
     {362.9, 284.1, 276.8, 353.6, 395.1, 1, 2},
     {393.4, 344.8, 295.6, 298.3, 375.0, 0.5}
    };
    */
    int lengthOfLongestArray = population[0].length;
    for(int i = 0; i < population.length; i++){
        if(population[i].length > lengthOfLongestArray){
            lengthOfLongestArray = population[i].length;
        }
    }

    //2. Step: calculate the sum
    double result[] = new double[lengthOfLongestArray];
    for(int i = 0; i < population.length; i++){
        for(int j = 0; j < population[i].length; j++){
            result[j] += population[i][j];
        }
    }

    System.out.println(Arrays.toString(result));
}

Explanation: To avoid confusing loops and fancy logic I created an array which will hold the result of the calculation called result .

1. Step: By setting the length of this result array to the length longest of the rows in the population 2D array (aka. matrix) we handle the situation when the rows are not exactly the same length (see the example I commented out).

2. Step: Then we just loop through the 2D array and sum the values. As we go through of a row in your 2D array we can take the value from the 'j' position of the row we are checking and add it to the value we have in the 'j' position in the result array.

Cheers, A.

Answer 3

double population[][] = { { 281.0, 296.0, 325.0, 371.0, 384.5 }, { 298.6, 241.2, 301.2, 342.8, 388.7 },
            { 362.9, 284.1, 276.8, 353.6, 395.1 }, { 393.4, 344.8, 295.6, 298.3, 375.0 } };
    int year[] = { 2011, 2016, 2021, 2026, 2031 };
    String ageGroup[] = { "15-19", "20-24", "25-29", "30-34", };
    String output = "Actual and Projected Population in thousands by Age Group (CSO 2016)";
    output += String.format("\n%10s", "");

    for (int i = 0; i < year.length; i++) {
        output += String.format("%10s", year[i]);
    }

    output += String.format("%10s", "%Change");
    double change[] = new double[ageGroup.length];
    for (int i = 0; i < population.length; i++) {
        output += String.format("\n%10s ", ageGroup[i]);
        for (int j = 0; j < population[i].length; j++) {
            output += String.format("%10.1f", population[i][j]);
        }
        change[i] = (((population[i][4] - population[i][0]) / population[i][0]) * 100);
        output += String.format("%10.1f", change[i]);

    }
    output += String.format("\n\nTotal (15 - 34): ");

    // here i changed the code
    // this loop will print the last from (15 to 35)
    // the outer loop iterate like 2011 2016 and so one
    for (int i = 0; i < 5; i++) {
        double temp = 0;
        // the inner loop iterate from up to down and add values
        for (int j = 0; j < 4; j++) {
            temp = temp + population[j][i];
        }
        output += String.format("%10.1f", temp);
    }
    System.out.println(output);