Java: Possible to use a single for-loop instead of two?

Question

My code is as below. It first takes input from user and prints it in reverse. I'm new to Java. I achieve this by using two 'for loops' to first iterate through the input and another for-loop to print the numbers in reverse. My question is if there's any way to improve my code - by using just a single loop perhaps? Any suggestion is appreciated. Thank you.

public static void main (String[] args) {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    int arr[] = new int[n];

    for (int arr_i = 0; arr_i < n; arr_i++) {
        arr[arr_i] = in.nextInt();
    }

    for (int reverse_i = n-1; reverse_i >= 0; reverse_i--) {
        System.out.print(arr[reverse_i]);

        if (reverse_i != 0) {
            System.out.print(" ");
        }
    }
}

An example input:

4
1 2 3 4

Expected output:

4 3 2 1

Don't you need to populate your array before you attempt to go over it in reverse order? — OneCricketeer
– OneCricketeer, Commented Jun 7, 2017 at 2:54
yes I do by using the Scanner input --> arr[arr_i] = in.nextInt(); — yoges nsamy
– yoges nsamy, Commented Jun 7, 2017 at 2:55
@cricket_007 the end goal is to print the received input in reverse. — yoges nsamy
– yoges nsamy, Commented Jun 7, 2017 at 4:00

Mr. Polywhirl · Accepted Answer · 2017-06-07 03:02:43Z

4

Use a StringBuilder and always insert at 0 index.

See: Oracle » JavaDocs » 1.7 » java.lang.StringBuilder.insert(int, int)

StringBuilder bld = new StringBuilder();

for (int arr_i = 0; arr_i < n; arr_i++) {
    int i = in.nextInt();
    bld.insert(0, i);
}

System.out.println(bld.toString());

edited Jun 7, 2017 at 3:02

Mr. Polywhirl

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answered Jun 7, 2017 at 2:54

Scary Wombat

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4 Comments

yoges nsamy Over a year ago

This code doesn't print a blank space after inserting a number. I tried adding bld.insert (0, " "); but this doesn't fulfill the requirement because there shouldn't be an empty space after the last number.

Scary Wombat Over a year ago

Just use docs.oracle.com/javase/7/docs/api/java/lang/… to delete the last char

Gray Over a year ago

Of course there are for loops being used inside of each StringBuilder.insert(...) call.

Scary Wombat Over a year ago

As OP Posted My question is if there's any way to improve my code

Gray · Accepted Answer · 2017-06-07 13:17:51Z

2

The simplest approach I found is to use String Builder here:

Scanner in = new Scanner(System.in);
StringBuilder stringBuilder = new StringBuilder();
while (in.hasNext()) {
   stringBuilder.append(in.next());
   if(in.hasNext()) {
     stringBuilder.append(" ");
   }
}

System.out.print(stringBuilder.reverse());

edited Jun 7, 2017 at 13:17

Gray

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answered Jun 7, 2017 at 3:27

Ms. Zia

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4 Comments

cs95 Over a year ago

That's still 2 loops.

Ms. Zia Over a year ago

Updated my answer for more simple solution with one loop

Ms. Zia Over a year ago

@Gray this is doing fine. Why do you think its not reversed? As mentioned in question with an example Input: 1 2 3 4 Output is: 4 3 2 1

Gray Over a year ago

Missed the reverse @Ms.Zia. Sorry.

Elliott Frisch · Accepted Answer · 2017-06-07 03:05:10Z

1

First, you might reverse your elements as you insert them in the array. Then, assuming you are using Java 8+, you could use an IntStream instead of a loop and print with a basic Collector. Like,

Scanner in = new Scanner(System.in);
int n = in.nextInt();
int[] arr = new int[n];
IntStream.range(0, n).forEachOrdered(i -> arr[n - i - 1] = in.nextInt());
System.out.println(IntStream.of(arr).mapToObj(String::valueOf)
        .collect(Collectors.joining(" ")));

answered Jun 7, 2017 at 3:05

Elliott Frisch

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Comments

Solace · Accepted Answer · 2017-06-07 03:07:57Z

1

I am not sure about the requirements of your request, but you do not necessarily have to use an array for this. You can concatenate the inputs into a String in reverse order like so.

public static void main(String[] args) {
    System in = new Scanner(System.in);
    int n = in.nextInt();
    String allNumbers = "";

    for (int i = 0; i < n; i++) {
        int current = in.nextInt();
        allNumbers = current + " " + allNumbers;
    }

    if(allNumbers != ""){
      allNumbers = allNumbers.substring(0,allNumbers.length()-1);
    }

    System.out.println(allNumbers);
}

edited Jun 7, 2017 at 3:07

answered Jun 7, 2017 at 2:54

Solace

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2 Comments

Scary Wombat Over a year ago

better to use a StringBuilder as per my answer

Solace Over a year ago

@Scary Wombat Yeah you're right. I think this method will result in an extra blank character (" ") at the end of the string.

Rohan Rayarikar · Accepted Answer · 2017-06-07 02:54:48Z

-1

Java has inbuilt String method for the same. If your input is a String, you can use the below-

String s1 = new String("new");
String s2 = s1.reverse();
System.out.println(s2) // wen

answered Jun 7, 2017 at 2:54

Rohan Rayarikar

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1 Comment

OneCricketeer Over a year ago

Question isn't completely reversing a string (you missed the spaces)

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