Create a char count array for a String using java8 features

Question

I wish to create a int[] of count for a particular String (comprising of only lowercase English Alphabets) using Java 8 stream API. Where arr[i] denotes the count of i-th character of English dictionary (e.g. arr[0] = count of 'a' in String str while arr[2] = count of 'c' in String str. This can be simply done by:

int[] arr = new int[26];
for(char c : str.toCharArray())
       arr[c-'a']++;

Or using IntSream in the 2nd way:

int[] arr = IntStream.range('a','z'+1).map(i -> (int)str.chars().filter(c -> c == i).count()).toArray();

But the problem with the second approach is that the String is traversed 26 times for each of the characters from 'a' to 'z'

Can you suggest a better way of achieving the same using java8-stream API?

PS: I know this can be done using Map but I need int[]

Your iterative approach is much better. Don't use streams. Keep what you have. — Ravindra Ranwala
– Ravindra Ranwala, Commented Apr 15, 2020 at 4:08

Andrew · Accepted Answer · 2020-04-15 09:00:39Z

6

int[] r = str.chars()
             .boxed()
             .reduce(new int[26], 
                     (a, c) -> { ++a[c - 'a']; return a; }, 
                     (a1, a2) -> a1);

You know the former is simpler and better. My answer just proves it's feasible with the Stream API, and doesn't suggest that you should go with it. Personally, I would choose the map approach as the most intuitive one.

As pointed out by @Holger, collect is a better option here

str.chars()
   .map(c -> c - 'a')
   .collect(() -> new int[26], 
            (a, i)-> a[i]++, 
            (a1, a2) -> /* left as an exercise to the reader*/);

edited Apr 15, 2020 at 9:00

answered Apr 14, 2020 at 21:16

Andrew

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3 Comments

David Conrad Over a year ago

I would throw .filter(i -> i >= 'a' && i <= 'z') after .chars() just in case someone calls it with out-of-range characters in str, to avoid the potential out of bounds exception.

Andrew Over a year ago

@DavidConrad I agree, but the OP seems to be confident with the new int[26], maybe str is being validated before this

Holger Over a year ago

Modifying incoming arguments in a reduction function is an abuse of the API. Obviously, this would break with a parallel stream. This is a perfect use case for collect, which can even be done without boxing,

str.chars().map(c -> c - 'a').collect(() -> new int[26], (a,i)->a[i]++, (a1, a2) -> /* left as an exercise to the reader*/)

pero_hero · Accepted Answer · 2020-04-15 07:06:13Z

2

if you want to use streams and keep the iterative approach, you could do it as well like this:

final int count[] = new int[26];
test.chars().forEach(c -> count[c-'a']++);

edited Apr 15, 2020 at 7:06

answered Apr 15, 2020 at 6:42

pero_hero

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2 Comments

Samrat Chakraborty Over a year ago

I found this working. Can you explain why is this not violation of the "Effective final" clause of lambda functions?

pero_hero Over a year ago

As arrays are objects in java, the count array is de facto final; its reference is not changed during the iteration ie. it is not assigned to a new object. it stays final even if you change values (members) of an object.

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