I cannot get the loop to work in the buildDimArray method to store the number combinations "11+11", "11+12", "11+21", "11+22", "12+11", "12+12", "12+21", "12+22", "21+11", "21+12", "21+21", "21+22", "22+11", "22+12", "22+21", and "22+22" into the 2d arraylist with each expression going into one column of the index dimBase-1 row. The loop may work for other people, but for some reason mine isn't functioning correctly. The JVM sees the if dimBase==1 condition, but refuses to check the other conditions. The "WTF" not being printed as a result from the buildDimArray method. If dimBase=1, it prints successfully, but doesn't for the other integers. The dimBase==3 condition needs a loop eventually. The "WTF" is for illustrative purposes. I could get away with a 1d arraylist, but in the future I will likely need the 2d arraylist once the program is completed.
package jordanNumberApp;
import java.util.Scanner;
import java.util.ArrayList;
/*
* Dev Wills
* Purpose: This code contains some methods that aren't developed. This program is supposed to
* store all possible number combinations from numbers 1-dimBase for the math expression
* "##+##" into a 2d arraylist at index row dimBase-1 and the columns storing the
* individual combinations. After storing the values in the arraylist, the print method
* pours the contents in order from the arraylist as string values.
*/
public class JordanNumberSystem {
// a-d are digits, assembled as a math expression, stored in outcomeOutput, outcomeAnswer
public static int dimBase, outcomeAnswer, a, b, c, d;
public static String inputOutcome, outcomeOutput;
public static final int NUM_OF_DIMENSIONS = 9; //Eventually # combinations go up to 9
public static ArrayList<ArrayList<String>> dimBaseArray;
public static Scanner keyboard;
/*
* Constructor for JordanNumber System
* accepts no parameters
*/
public JordanNumberSystem() // Defunct constructor
{
// Declare and Initialize public variables
this.dimBase = dimBase;
this.outcomeOutput = outcomeOutput;
this.outcomeAnswer = outcomeAnswer;
}
// Set all values of variable values
public static void setAllValues()
{
// Initialize
dimBase = 1;
outcomeAnswer = 22; // variables not used for now
outcomeOutput = "1"; // variables not used for now
//a = 1;
//b = 1;
//c = 1;
//d = 1;
dimBaseArray = new ArrayList<ArrayList<String>>();
keyboard = new Scanner(System.in);
}
public static void buildDimArray(int dim)
{
dimBase = dim;
try
{
//create first row
dimBaseArray.add(dimBase-1, new ArrayList<String>());
if( dimBase == 1)
{
a = b = c = d = dimBase ;
dimBaseArray.get(0).add(a+""+b+"+"+c+""+d);
System.out.println("WTF"); // SHOWS
}
else if (dimBase == 2)
{ // dim = 2
a = b = c = d = 1 ;
System.out.println("WTF"); // doesn't show
// dimBaseArray.get(dimBase-1).add(a+""+b+"+"+c+""+d);
for( int i = 1 ; i <= dim ; i++)
a=i;
for( int j = 1 ; j <= dim ; j++)
b=j;
for( int k = 1 ; k <= dim ; k++)
c=k;
for( int l = 1 ; l <= dim ; l++)
{
d=l;
dimBaseArray.get(dim-1).add(a+""+b+"+"+c+""+d);
}
}
else if (dimBase == 3)
{
a = b = c = d = dimBase;
dimBaseArray.get(2).add(a+""+b+"+"+c+""+d);
System.out.println("WTF");
}
}catch (IndexOutOfBoundsException e)
{
System.out.println(e.getMessage());
}
}
public static void printArray(int num) // Prints the contents of the array
{ // Fixing the printing method
try
{
int i = num-1;
for( String string : dimBaseArray.get(i))
{
System.out.println(string);
System.out.println("");
}
} catch (IndexOutOfBoundsException e)
{
System.out.println(e.getMessage());
}
}
public static void main(String[] args) throws java.lang.IndexOutOfBoundsException
{
setAllValues(); // sets the initial a,b,c,d values and dimBase, initializes 2d arraylist
// Get the Dimension Base number
System.out.println("Enter Dimension Base Number. Input an integer: ");
int dimBaseInput = keyboard.nextInt(); // Receives integer
dimBase = dimBaseInput;
if( dimBase != 1 && dimBase != 2 && dimBase != 3)
{// Error checking
System.out.println("invalid Dimension Base Number should be 1 or 2 ");
System.exit(1);
}
// Build the arraylist, print, clear, exit
buildDimArray(dimBase);
printArray(dimBase);
dimBaseArray.clear();
System.exit(1);
}
}// End of class
Beginner here. This piece of code converts number into roman numerals in multiples of 50 if not 10 if not 9 and down to 0. Methods are so intertwined. Is there something (just at a glance) you could suggest I should avoid doing? Thank You.
public static void main(String[] args) {
System.out.println(fiftyAndAbove(37));
}
public static String nineAndDown(int number) {
String one = "I", five = "V", ten = "X", sum = "";
if(number == 5) {
return five;
} else if(number == 9) {
return one + ten;
}
else if(number > 5) {
for(int i=1; i<=number-5; i++) {
sum += one;
}
return five + sum;
} else {
if(number == 4 ) {
return one + five;
} else
for(int i=1; i <=number; i++) {
sum += one;
}
} return sum;
}
public static String tenAndAbove(int number) {
int remainder = number % 10, numberOftens = number/10;
String ten = "X", sum = "";
if(numberOftens > 0) {
while(numberOftens > 0) {
sum += ten;
numberOftens -= 1;
}
}
return sum + nineAndDown(remainder);
}
public static String fiftyAndAbove(int number) {
int remainder = number % 50, numberOfFifty = number/50;
String fifty = "L", sum = "";
if(numberOfFifty > 0) {
while(numberOfFifty > 0) {
sum += fifty;
numberOfFifty -= 1;
}
}
return sum + tenAndAbove(remainder);
}
Is there something (just at a glance) you could suggest I should avoid doing?
I'd not unnecessarily complicate the logic as with
if(numberOfFifty > 0) {
while(numberOfFifty > 0) {
…
}
}
which is equivalent to
while (numberOfFifty > 0)
{
…
}
You could also have a look at this implementation and see what you prefer:
import java.util.Arrays;
…
public static String fiftyAndAbove(int number)
{
int remainder = number%50, numberOfFifty = number/50;
char [] Ls = new char [numberOfFifty];
Arrays.fill(Ls, 'L');
return new String(Ls) + tenAndAbove(remainder);
}
You have four places like this in your program where you need a string of a character repeated. If you're willing to require a certain Java version or above, you can also use one of the methods described at Java: String - add character n-times; otherwise I'd suggest to use a function to do it.
You could also think about whether you find
String one = "I", five = "V", ten = "X", sum = "";
if(number == 5) {
return five;
} else if(number == 9) {
return one + ten;
}
really better than
if (number == 5) return "V";
if (number == 9) return "IX";
NOTE: A cumulative histogram is a histogram in which each value is added to the sum of the values that have gone before, for example if there are 3 apples, 2 bananas and 6 oranges, then then numbers in the normal histogram will [3,2,6] but in a cumulative histogram they will be [3,5,11].
Question: Create a subclass of Histogram called CumulativeHistogram. I only need to override the toString() method to solve the problem.
class Histogram
{
public char symbol = '*';
protected String[] categories;
protected int[] frequencies;
public Histogram(int numCategories)
{
categories = new String [ numCategories ];
frequencies = new int [ numCategories ];
for (int index = 0; index < numCategories; index++) {
categories[index] = "unlabeled";
frequencies[index] = 0;
}
}
public void setCategory(int index, String name, int frequency)
{
categories[ index ] = name;
frequencies[ index ] = frequency;
}
public String toString()
{
String result = "";
for (int index = 0; index<categories.length; index++){
result+=categories[index] +": ";
result+=repeatSymbol(frequencies[index]);
result+="\n";
}
return result;
}
protected String repeatSymbol(int numTimes)
{
String result = "";
for (int index = 0; index <numTimes; index++)
result += symbol;
return result;
}
}
Here is a template for the class:
class CumulativeHistogram extends Histogram {
public CumulativeHistogram(int numCategories) {
super(numCategories);
}
public String toString() {
//*****fill in this method
}
}
Some test code
Histogram hist = new Histogram(3);
hist.setCategory(0, " Apples", 4);
hist.setCategory(1, "Bananas", 2);
hist.setCategory(2, "Oranges", 5);
println( hist );
CumulativeHistogram hist2 = new CumulativeHistogram(3);
hist2.setCategory(0, " Apples", 4);
hist2.setCategory(1, "Bananas", 2);
hist2.setCategory(2, "Oranges", 5);
println( hist2 );
And its output
Apples: ****
Bananas: **
Oranges: *****
Apples: ****
Bananas: ******
Oranges: ***********
Declare another counter outside the loop, incrementing with the loop, and use that with the index as such:
result+=repeatSymbol(frequencies[index]+counter);
counter+=frequencies[index];
I am not sure how to write comparator for Collectionos.binarySearch(). Can anyone help ? sample code:
List<Object> list1 = new ArrayList<>();
List<List<Object>> list2 = new ArrayList<>();
//loop starts
// adds elements into list1
list1.add(values);//values is an object containing elements like [3, John, Smith]
if (list2.size() == 0) {
list2.add(list1);//first element
} else {
if (index >= 0) {
int index = Collections.binarySearch(list2, list1, comparator);
list2.add(index, list1);//I want to add these elements in ascending order ?
}
}
//loop ends
How do I write comparator, so that elements in list 2 are added in ascending or descending order.
You can use an anonymous class which implements a Comparator<List<Object>>:
int index = Collections.binarySearch(list2, list1, new Comparator<List<Object>>() {
#Override
public int compare(List<Object> o1, List<Object> o2) {
// Your implementation here
return 0;
}
});
You could implement an IComparer<List<Object>> class, or use a lambda expression.
You just need to create a class that implements the Comparator interface.
For example, you can do this inline with an anonymous class:
Comparator<List<Object>> comparator = new Comparator<List<Object>>() {
#Override
public int compare(List<Object> x, List<Object> y) {
// custom logic to compare x and y here. Return a negative number
// if x < y, a positive number if x > y, and 0 otherwise
}
};
Collections.binarySearch(list, comparator);
I don't like using an indexed array for no reason other than I think it looks ugly. Is there a clean way to sum with an anonymous function? Is it possible to do it without using any outside variables?
Dart iterables now have a reduce function (https://code.google.com/p/dart/issues/detail?id=1649), so you can do a sum pithily without defining your own fold function:
var sum = [1, 2, 3].reduce((a, b) => a + b);
int sum = [1, 2, 3].fold(0, (previous, current) => previous + current);
or with shorter variable names to make it take up less room:
int sum = [1, 2, 3].fold(0, (p, c) => p + c);
This is a very old question but
In 2022 there is actually a built-in package.
Just import
import 'package:collection/collection.dart';
and call the .sum extension method on the Iterable.
FULL EXAMPLE
import 'package:collection/collection.dart';
void main() {
final list = [1, 2, 3, 4];
final sum = list.sum;
print(sum); // prints 10
}
If the list is empty, .sum returns 0.
You might also be interested in list.average...
I still think this is cleaner and easier to understand for this particular problem.
num sum = 0;
[1, 2, 3].forEach((num e){sum += e;});
print(sum);
or
num sum = 0;
for (num e in [1,2,3]) {
sum += e;
}
There is not a clean way to do it using the core libraries as they are now, but if you roll your own foldLeft then there is
main() {
var sum = foldLeft([1,2,3], 0, (val, entry) => val + entry);
print(sum);
}
Dynamic foldLeft(Collection collection, Dynamic val, func) {
collection.forEach((entry) => val = func(val, entry));
return val;
}
I talked to the Dart team about adding foldLeft to the core collections and I hope it will be there soon.
Starting with Dart 2.6 you can use extensions to define a utility method on the List. This works for numbers (example 1) but also for generic objects (example 2).
extension ListUtils<T> on List<T> {
num sumBy(num f(T element)) {
num sum = 0;
for(var item in this) {
sum += f(item);
}
return sum;
}
}
Example 1 (sum all the numbers in the list):
var numbers = [1, 2, 3];
var sum = numbers.sumBy((number) => number);
Example 2 (sum all the Point.x fields):
var points = [Point(1, 2), Point(3, 4)];
var sum = points.sumBy((point) => point.x);
I'd just like to add some small detail to #tmaihoff's answer (about using the collection.dart package):
The sum getter he talks about only works for iterables of num values, like List<int> or Set<double>.
If you have a list of other object types that represent values (like Money, Decimal, Rational, or any others) you must map it to numbers. For example, to count the number of chars in a list of strings you can do:
// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].map((e) => e.length).sum;
As of 2022, another way of doing it, is using the sumBy() method of the fast_immutable_collections package:
// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].sumBy((e) => e.length), 15);
Note: I'm the package author.
I suggest you to create this function in any common utility file.
T sum<T extends num>(T lhs, T rhs) => lhs + rhs;
int, double, float extends num class so you can use that function to sum any numbers.
e.g.,
List<int> a = [1,2,3];
int result = a.reduce(sum);
print(result); // result will be 6
Herewith sharing my Approach:
void main() {
int value = sumTwo([1, 4, 3, 43]);
print(value);
}
int sumTwo(List < int > numbers) {
int sum = 0;
for (var i in numbers) {
sum = sum + i;
}
return sum;
}
If when using fold gives a double TypeError, you can use reduce:
var sum = [0.0, 4.5, 6.9].reduce((a, b) => a + b);
If you are planning on doing a number of mathematical operations on your list, it may be helpful to create another list type that includes .sum() and other operations by extending ListBase. Parts of this are inspired by this response with performance tweaks from this response.
import 'dart:collection';
import 'dart:core';
class Vector<num> extends ListBase<num> {
List<num> _list;
Vector() : _list = new List<num>();
Vector.fromList(List<num> lst): _list = lst;
void set length(int l) {
this._list.length=l;
}
int get length => _list.length;
num operator [](int index) => _list[index];
void operator []=(int index, num value) {
_list[index]=value;
}
// Though not strictly necessary, for performance reasons
// you should implement add and addAll.
void add(num value) => _list.add(value);
void addAll(Iterable<num> all) => _list.addAll(all);
num sum() => _list.fold(0.0, (a, b) => a + b) as num;
/// add additional vector functions here like min, max, mean, factorial, normalize etc
}
And use it like so:
Vector vec1 = Vector();
vec1.add(1);
print(vec1); // => [1]
vec1.addAll([2,3,4,5]);
print(vec1); // => [1,2,3,4,5]
print(vec1.sum().toString()); // => 15
Vector vec = Vector.fromList([1.0,2.0,3.0,4.0,5.0]); // works for double too.
print(vec.sum().toString()); // => 15
A solution that has worked cleanly for me is:
var total => [1,2,3,4].fold(0, (e, t) => e + t); // result 10
Different ways to find the sum of all dart list elements,
Method 1: Using a loop :
This is the most commonly used method. Iterate through the list using a loop and add all elements of the list to a final sum variable. We are using one for loop here :
main(List<String> args) {
var sum = 0;
var given_list = [1, 2, 3, 4, 5];
for (var i = 0; i < given_list.length; i++) {
sum += given_list[i];
}
print("Sum : ${sum}");
}
Method 2: Using forEach :
forEach is another way to iterate through a list. We can also use this method to find out the total sum of all values in a dart list. It is similar to the above method. The only difference is that we don’t have to initialize another variable i and list.length is not required.
main(List<String> args) {
var sum = 0;
var given_list = [1, 2, 3, 4, 5];
given_list.forEach((e) => sum += e);
print("Sum : ${sum}");
}
Method 3: Using reduce :
reduce method combines all elements of a list iteratively to one single value using a given function. We can use this method to find out the sum of all elements as like below :
main(List<String> args) {
var given_list = [1, 2, 3, 4, 5];
var sum = given_list.reduce((value, element) => value + element);
print("Sum : ${sum}");
}
Method 4: Using fold :
fold() is similar to reduce. It combines all elements of a list iteratively to one single value using a function. It takes one initial value and calculates the final value based on the previous value.
main(List<String> args) {
var sum = 0;
var given_list = [1,2,3,4,5];
sum = given_list.fold(0, (previous, current) => previous + current);
print("Sum : ${sum}");
}
for more details:https://www.codevscolor.com/dart-find-sum-list-elements
extension DoubleArithmeticExtensions on Iterable<double> {
double get sum => length == 0 ? 0 : reduce((a, b) => a + b);
}
extension IntArithmeticExtensions on Iterable<int> {
int get sum => length == 0 ? 0 : reduce((a, b) => a + b);
}
Usage:
final actual = lineChart.data.lineBarsData[0].spots.map((s) => s.x).sum;