Calculating average of an array list

Question

I m trying to use the below code to calculate the average of a set of values that a user enters and display it in a jTextArea but it does not work properly  Say  a user enters 7  4  and 5  the program displays 1 as the average when it should display 5 3      ArrayList  lt Integer gt  marks   new ArrayList      Collections addAll marks   Integer parseInt markInput getText          private void analyzeButtonActionPerformed java awt event ActionEvent evt          analyzeTextArea setText  Class average     calculateAverage marks           private int calculateAverage List  lt Integer gt  marks          int sum   0        for  int i 0  i lt  marks size    i                  sum    i                return sum   marks size          What is wrong with the code

User · Answer

sum    i    You re adding the index  you should be adding the actual item in the ArrayList   sum    marks get i     Also  to ensure the return value isn t truncated  force one operand to double and change your method signature to double   return  double sum   marks size

User · Answer

List stream   mapToDouble a- gt a  average

User · Answer

Here a version which uses BigDecimal instead of double   public static BigDecimal calculateAverage final List lt Integer gt  values        int sum   0      if   values isEmpty              for  final Integer v   values                sum    v                    return new BigDecimal sum  divide new BigDecimal values size     2  RoundingMode HALF UP             return BigDecimal ZERO

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You can use standard looping constructs or iterator listiterator for the same    List lt Integer gt  list   Arrays asList 1  2  3  4  5  6  7  8   double sum   0  Iterator lt Integer gt  iter1   list iterator    while  iter1 hasNext          sum    iter1 next      double average   sum   list size    System out println  Average       average     If using Java 8  you could use Stream or IntSream operations for the same    OptionalDouble avg   list stream   mapToInt Integer  intValue  average    System out println  Average       avg getAsDouble       Reference   Calculating average of arraylist

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Using Guava  it gets syntactically simplified   Stats meanOf numericList

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With Java 8 it is a bit easier   OptionalDouble average   marks              stream                mapToDouble a - gt  a               average      Thus your average value is average getAsDouble    return average isPresent     average getAsDouble     0

User · Answer

If using Java8 you can get the average of the values from a List as follows       List lt Integer gt  intList   Arrays asList 1 2 2 3 1 5        Double average   intList stream   mapToInt val - gt  val  average   orElse 0 0     This has the advantage of having no moving parts   It can be easily adapted to work with a List of other types of object by changing the map method call     For example with Doubles        List lt Double gt  dblList   Arrays asList 1 1 2 1 2 2 3 1 1 5 5 3       Double average   dblList stream   mapToDouble val - gt  val  average   orElse 0 0     NB  mapToDouble is required because it returns a DoubleStream which has an average method  while using map does not    or BigDecimals    Test public void bigDecimalListAveragedCorrectly         List lt BigDecimal gt  bdList   Arrays asList valueOf 1 1  valueOf 2 1  valueOf 2 2  valueOf 3 1  valueOf 1 5  valueOf 5 3        Double average   bdList stream   mapToDouble BigDecimal  doubleValue  average   orElse 0 0       assertEquals 2 55  average  0 000001       using orElse 0 0  removes problems with the Optional object returned from the average being  not present

User · Answer

Correct and fast way compute average for List lt Integer gt     private double calculateAverage List lt Integer gt  marks        long sum   0      for  Integer mark   marks            sum    mark            return marks isEmpty    0  1 0 sum marks size        This solution take into account    Handle overflow Do not allocate memory like Java8 stream  Do not use slow BigDecimal   It works coorectly for List  because any list contains less that 2 31 int  and it is possible to use long as accumulator     PS  Actually foreach allocate memory - you should use old style for   cycle in mission critical parts

User · Answer

Why use a clumsy for loop with an index when you have the enhanced for loop   private double calculateAverage List  lt Integer gt  marks      Integer sum   0    if  marks isEmpty          for  Integer mark   marks            sum    mark            return sum doubleValue     marks size          return sum

User · Answer

When the number is not big  everything seems just right  But if it isn t  great caution is required to achieve correctness    Take double as an example   If it is not big  as others mentioned you can just try this simply   doubles stream   mapToDouble d - gt  d  average   orElse 0 0     However  if it s out of your control and quite big  you have to turn to BigDecimal as follows  methods in the old answers using BigDecimal actually are wrong    doubles stream   map BigDecimal  valueOf  reduce BigDecimal ZERO  BigDecimal  add          divide BigDecimal valueOf doubles size     doubleValue      Enclose the tests I carried out to demonstrate my point        Test     public void testAvgDouble             assertEquals 5 0  getAvgBasic Stream of 2 0  4 0  6 0  8 0    1E-5           List lt Double gt  doubleList   new ArrayList lt  gt  Arrays asList Math pow 10  308   Math pow 10  308   Math pow 10  308   Math pow 10  308                Double MAX VALUE   1 7976931348623157e 308         BigDecimal doubleSum   BigDecimal ZERO          for  Double d   doubleList                doubleSum    doubleSum add new BigDecimal d toString                        out println doubleSum divide valueOf doubleList size     doubleValue             out println getAvgUsingRealBigDecimal doubleList stream              out println getAvgBasic doubleList stream              out println getAvgUsingFakeBigDecimal doubleList stream                 private double getAvgBasic Stream lt Double gt  doubleStream            return doubleStream mapToDouble d - gt  d  average   orElse 0 0              private double getAvgUsingFakeBigDecimal Stream lt Double gt  doubleStream            return doubleStream map BigDecimal  valueOf                   collect Collectors averagingDouble BigDecimal  doubleValue               private double getAvgUsingRealBigDecimal Stream lt Double gt  doubleStream            List lt Double gt  doubles   doubleStream collect Collectors toList             return doubles stream   map BigDecimal  valueOf  reduce BigDecimal ZERO  BigDecimal  add                   divide valueOf doubles size     BigDecimal ROUND DOWN  doubleValue            As for Integer or Long  correspondingly you can use BigInteger similarly

User · Answer

Use a double for the sum  otherwise you are doing an integer division and you won t get any decimals   private double calculateAverage List  lt Integer gt  marks        if  marks    null    marks isEmpty              return 0             double sum   0      for  Integer mark   marks            sum    mark             return sum   marks size        or using the Java 8 stream API       return marks stream   mapToInt i - gt  i  average   orElse 0

[java] Calculating average of an array list?

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