How to perform some mathematical operation on some specific elements of a list using java 8?

user_3350632 0
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Based on some conditions I want to perform some operation on a specific element of a list only.

I have a list of integers like this:

List<Integer> list = new ArrayList(Arrays.asList(30,33,29,0,34,0,45));

and I want to subtract 1 from each element EXCEPT 0.

I have tried some approaches like by applying the filter of Java 8 but it removed the zero values from the list. I tried to apply other methods provided for streams API like foreach() or .findFirst(),.findAny() but it didn't work. 

List<Integer> list2 = list.stream().filter(x -> x > 0).map(x -> x - 1).collect(Collectors.toList());
//list.stream().findFirst().ifPresent(x -> x - 1).collect(Collectors.toList()); //This is giving error
list.stream().forEach(x ->x.); //How to use this in this case

Actual Result : [29,32,28,-1,33,-1,44]

Expected Result : [29,32,28,0,33,0,44]

java-8 java
4 Answers
list.stream()
    .map(x -> x == 0 ? x : x - 1)
    .collect(Collectors.toList());
user_10593720

In the example, you can use Math.max method:

list.stream()
    .map(x -> Math.max(0, x - 1))
    .collect(Collectors.toList());

In your case:

list.stream() // 1,2,0,5,0
    .filter(x -> x > 0) // 1,2,5
    .map(x -> x - 1) // 0,1,4
    .collect(Collectors.toList()); // will return list with three elements [0,1,4]
user_47438690

A non-stream version is using of replaceAll

list.replaceAll(x -> x != 0 ? x - 1 : x);
user_36696980

Another solution :

IntStream.range(0, list.size())
         .map(e -> list.get(e) == 0 ? list.get(e) : list.get(e) - 1)
         .forEach(System.out::println);

Output :

29 32 28 0 33 0 44

user_61395270
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