Is there possibility of sum of ArrayList without looping

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无人共我
无人共我 2020-11-27 04:24

Is there possibility of sum of ArrayList without looping?

PHP provides sum(array) which will give the sum of array.

The PHP code is

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  • 2020-11-27 04:50

    If you know about the map function, then you know that a map is also can be recursive loop or recursive loop. But obviously you have to reach each element for that. so, I could not work out the Java 8, because some syntax mismatch but wanted a very short so this is what I got.

    int sum = 0
    for (Integer e : myList) sum += e;
    
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  • 2020-11-27 04:52

    You can use GNU Trove library:

    TIntList tt = new TIntArrayList();
    tt.add(1);
    tt.add(2);
    tt.add(3);
    int sum = tt.sum();
    
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  • 2020-11-27 04:55

    Once java-8 is out (March 2014) you'll be able to use streams:

    If you have a List<Integer>

    int sum = list.stream().mapToInt(Integer::intValue).sum();
    

    If it's an int[]

    int sum = IntStream.of(a).sum();
    
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  • 2020-11-27 04:56

    Then write it yourself:

    public int sum(List<Integer> list) {
         int sum = 0; 
    
         for (int i : list)
             sum = sum + i;
    
         return sum;
    }
    
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  • 2020-11-27 04:57

    You can use apache commons-collections API.

    class AggregateClosure implements org.apache.commons.collections.Closure {
            int total = 0;
    
            @Override
            public void execute(Object input) {
                if (input != null) {
                    total += (Integer) input;
                }
            }
    
            public int getTotal() {
                return total;
            }
        }
    

    Then use this closure as shown below:

    public int aggregate(List<Integer> aList) {
            AggregateClosure closure = new AggregateClosure();
            org.apache.commons.collections.CollectionUtils.forAllDo(aList, closure);
            return closure.getTotal();
    }
    
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  • 2020-11-27 04:58

    The only alternative to using a loop is to use recursion.

    You can define a method like

    public static int sum(List<Integer> ints) {
       return ints.isEmpty() ? 0 : ints.get(0) + ints.subList(1, ints.length());
    }
    

    This is very inefficient compared to using a plain loop and can blow up if you have many elements in the list.

    An alternative which avoid a stack overflow is to use.

    public static int sum(List<Integer> ints) {
        int len = ints.size();
        if (len == 0) return 0;
        if (len == 1) return ints.get(0);
        return sum(ints.subList(0, len/2)) + sum(ints.subList(len/2, len));
    }
    

    This is just as inefficient, but will avoid a stack overflow.


    The shortest way to write the same thing is

    int sum = 0, a[] = {2, 4, 6, 8};
    
    for(int i: a) {
        sum += i;
    }
    
    System.out.println("sum(a) = " + sum);
    

    prints

    sum(a) = 20
    
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