Question

How do I create a loop to generate min, max, avg for 2 array lists, i have only generated the min, max and avg with sum for single array lists so far.

These are the 2 arrays User[] & Withdrawals[]:

User, Withdrawals
1 , 90.00
2 , 85.00
4 , 75.00
5 , 65.00
2 , 40.00
1 , 80.00
3 , 50.00
5 , 85.00
4 , 80.00
1 , 70.00

size = 10

This is what i have tried, as i have no clue about 2 arrays interdependent:

double min = 0.0;
double max = 0.0;
double sum = 0.0;
double avg = 0.0;

for(int i = 0; i <size; i++){
.
.
for(int j = 0; j < Withdrawals.length; j++){
   if(Withdrawals[User[i]] > max){  
      max = Withdrawals[j];  
   }  
   if(Withdrawals[User[i]] < min){  
      min = Withdrawals[j];  
   }
}  
sum += Withdrawals[j];
avg = sum/size;
}

how do i print the min, max, avg from the no of withdrawals per user ? :S

I have already counted the number of withdrawals per user.

Conditions are: create everything from scratch instead of using available library features of Java.

Answer 1

Divide and conquer 🙂
Yes, I know that is a term used for an algorithm technique, in this case what I mean is… work with small parts.

First having the min, max, avg for a simple array:

double[] values = {2,3,4,5,6,7};

double min = values[0];
double max = values[0];
double sum = 0;

for (double value : values) {
     min = Math.min(value, min);
     max = Math.max(value, max);
     sum += value;
}

double avg = sum / values.length;

System.out.println("Min: " + min);
System.out.println("Max: " + max);
System.out.println("Avg: " + avg);

Note: Since you can’t use Java libraries for your assignment, is easy to do your own versions of the min/max functions (read the Math JavaDoc)

Now you can encapsulate this code in a function, you can start by returning another array:

static double[] minMaxAvg(double[] values) {
    double min = values[0];
    double max = values[0];
    double sum = 0;

    for (double value : values) {
        min = Math.min(value, min);
        max = Math.max(value, max);
        sum += value;
    }

    double avg = sum / values.length;

    return new double[] {min, max, avg};
}

public static void main(String[] args) {
    double[] values = {2,3,4,5,6,7};
    double[] info = minMaxAvg(values);
    System.out.println("Min: " + info[0]);
    System.out.println("Max: " + info[1]);
    System.out.println("Avg: " + info[2]);
}

Using an array is a little bit ugly to read, so is better if you create a class to hold the min, max, avg. So lets refactor the code a little bit:

class ValueSummary {
    final double min;
    final double max;
    final double avg;

    static ValueSummary createFor(double[] values) {
        double min = values[0];
        double max = values[0];
        double sum = 0;

        for (double value : values) {
            min = Math.min(value, min);
            max = Math.max(value, max);
            sum += value;
        }

        double avg = sum / values.length;

        return new ValueSummary(min, max, avg);
    }

    ValueSummary(double min, double max, double avg) {
        this.min = min;
        this.max = max;
        this.avg = avg;
    }

    public String toString() {
        return "Min: " + min + "\nMax: " + max +"\nAvg: " + avg;
    }
}


public static void main(String[] args) {
    double[] values = {2,3,4,5,6,7};
    ValueSummary info = ValueSummary.createFor(values);
    System.out.println(info);
}

You don’t specify it in your question, but I assume that you have an array for each user (maybe each withdrawals is another array).
Now that you have the bottom parts, we can switch to a top-down thinking.

So your code could be something like this:

for (User aUser : users) {
     System.out.println("User: " + aUser);
     System.out.println(ValueSummary.createFor(withdrawalsOf(aUser)));
}

Ok, but this is just the idea, you still have the problem to relate aUser with its withdrawals. You have several options here:

1. Make a “table” User-> Withdrawals, that is what you are trying to do with the two arrays. The User index in the array acts like a “user id”. When you learn about Map you will see that you can use a better representation for the index.
2. Having a Map or array is just an optimization, of the relationship User->Withdrawls, but you can represent that relationship with an object (ie UserWithdrawls)

Option 1:

static class User {
    final String name;
    public User(String s) { name = s; }
}
public static void main(String[] args) {
    User[] users = { new User("John"), new User("Doe")};
    double[][] withdrawals = {
         new double[] { 1, 2, 3}, new double[] { 10,22, 30} 
    };
    for (int i = 0; i < users.length; i++) {
        System.out.println("User: " + users[i].name);
        System.out.println(ValueSummary.createFor(withdrawals[i]));
    }
}

Option 2:

static class User {
    final String name;
    public User(String s) { name = s; }
}
static class UserWithdrawls {
    final User user;
    final double[] withdrawals;
    final ValueSummary summary;
    UserWithdrawls(User user, double[] withdrawals) {
        this.user = user;
        this.withdrawals = withdrawals;
        this.summary = ValueSummary.createFor(withdrawals);
    }
}
public static void main(String[] args) {
    UserWithdrawls[] userWithdrawls = {
            new UserWithdrawls(new User("John"), new double[] { 1, 2, 3}),
            new UserWithdrawls(new User("Doe"), new double[] { 10, 22, 30})
    };
    for (UserWithdrawls uw : userWithdrawls) {
        System.out.println("User: " + uw.user.name);
        System.out.println(uw.summary);
    }
}

Additional notes: If you are studying Computer Science, you’ll learn in the future that the loop to calculate max, min, avg has a complexity of O(n). If the values array is fully loaded in memory, doing the max/min/avg in three different functions (thus reading the array 3 times) is still an algorithm of O(n) order with a bigger constant. With the power of today’s computers the constant is so small, that most of the time you’ll not get any gain from calculating min/max/avg in the same loop. In contrast you can gain code readability, for example in Groovy the minMaxAvg code could be written like this:

 def values = [2,3,4,5,6,7];
 println values.min()
 println values.max()
 println values.sum() / values.size()