Why won't this reverse the array?

Question

I thought about it, tried different things for about an hour but couldn't come up with anything. I didn't want to post here because I wanted to figure it out myself but there's just something i'm not getting

class Exercise5
{
public static void main ( String[] args )
{
int[] val = {0, 1, 2, 3}; 
int temp;

System.out.println( "Original Array: " 
    + val[0] + " " + val[1] + " " + val[2] + " " + val[3] );

// reverse the order of the numbers in the array
for( temp = 0; temp < val.length; temp++) {

    val[temp] = val[3 - temp];


}



System.out.println( "Reversed Array: " 
    + val[0] + " " + val[1] + " " + val[2] + " " + val[3] );
 }
}

It' prints out 3 2 2 3 with every different strategy i've tried

possible duplicate of [How do I reverse an int array in Java?](http://stackoverflow.com/questions/2137755/how-do-i-reverse-an-int-array-in-java) — Pete, Apr 07 '14 at 18:08

score 3 · Accepted Answer · answered Apr 07 '14 at 18:09

3

Problem:

temp = 0

val[0] =val[3];

temp =1

val[1] =val[2];

temp =2

val[2] =val[1];

temp =3

val[3] =val[0];

Solution:

Use another array to hold reversed values or swap two values at a time using a temporary variable (0 & 3, 1&2)

answered Apr 07 '14 at 18:09

jmj

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You nailed it. Plus one. Adding some description would make it clearer. – Crickcoder Apr 07 '14 at 18:10

score 2 · Answer 2 · answered Apr 07 '14 at 18:12

for( temp = 0; temp < val.length; temp++) {
    val[temp] = val[3 - temp];
}

What this does is this;

val = [0, 1, 2, 3]

val[0] = val[3]
=> val = [3, 1, 2, 3]

val[1] = val[2]
=> val = [3, 2, 2, 3]

val[2] = val[1]
=> val = [3, 2, 2, 3]

val[3] = val[0]
=> val = [3, 2, 2, 3]

What you want to do is loop the index through half the array and swap the low and high value.

score 0 · Answer 3 · answered Apr 07 '14 at 18:10

0

Try this

// reverse the order of the numbers in the array
for( temp = 0; temp < val.length/2; temp++) {
    int tmp = val[temp];
    val[temp] = val[3 - temp];
    val[3 - temp] = tmp;

}

answered Apr 07 '14 at 18:10

CMPS

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score 0 · Answer 4 · answered Apr 07 '14 at 18:10

0

You have to use another array. You are changing the value of the original before you want to. Or if the length is even you can swap them

answered Apr 07 '14 at 18:10

chad martin

31
1
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You can swap them in place without another array, even if the length is odd. Just don't swap the middle item (or swap it with itself, which is a no-op). – David Conrad Apr 07 '14 at 18:50

score 0 · Answer 5 · answered Apr 07 '14 at 18:11

Firstly,

val[temp] = val[3 - temp];

only assigns the value of val[3-temp] into val[temp], it does not swap them. If you want to swap them, you need to use a temporary variable.

Second,

You only need to iterate halfway through the array, not all the way through it.

score 0 · Answer 6 · answered Apr 07 '14 at 18:12

val[temp] = val[3 - temp];

when temp =0

val[0]= val[3-0] val[0] = val[3]= 3

when temp =1

val[1]= val[3-1] val[1] = val[2]= 2

when temp =2

val[2]= val[3-2] val[2] = val[1]= 2 (previous step you can see val[1]=2)

when temp =3

val[3]= val[3-3] val[3] = val[0]= 3

result 3 2 2 3

score 0 · Answer 7 · answered Apr 07 '14 at 18:13

0

This line is culprit inside the for loop:

val[temp] = val[3 - temp];

Let's go over the iteration to see what it's doing.

val[0] = val[3] -> 3,1,2,3
val[1] = val[2] -> 3,2,2,3
val[2] = val[1] -> 3,2,2,3
val[3] = val[0] -> 3,2,2,3

answered Apr 07 '14 at 18:13

Pankaj Gadge

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score 0 · Answer 8 · answered Apr 07 '14 at 18:18

0

Most of the people have given you the analysis. This is what you can do to resolve.

Declare one more int array

int[] val = {0, 1, 2, 3}; 
int temp;
int[] reverseVal = new int[val.length];

Add to reversed values to this array

for( temp = 0; temp < val.length; temp++) {

    reverseVal [temp] = val[val.length - temp];

}

answered Apr 07 '14 at 18:18

Crickcoder

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There's no need for a second array. – David Conrad Apr 07 '14 at 18:50
@DavidConrad, then what do you suggest.. Swap a pair of values in the same array? – Crickcoder Apr 07 '14 at 18:53
Yes, of course. Use a single temporary int variable, you can swap them pairwise from the ends towards the middle. It's a very common idiom for reversing an array in place, I've found. – David Conrad Apr 07 '14 at 18:55

Why won't this reverse the array?

8 Answers8