Reversing an array In place

Question

Okay so I've tried to print and Array and then reverse is using another array But I'm trying to create a For Loop that will take an array and reverse all of the elements in place without me having to go through the process of creating an entirely new array.

My for loop is running into some problems and I'm not sure where to go from here...i'm using i to take the element at the end and move it to the front and then j is being used as a counter to keep track of the elements...if there is an easier way to do this Any suggestions would be appreciated.

I'm New to this programming language so any extra info is greatly appreciated.

#include <stdlib.h>
#include <time.h>

int Random(int Max) {
  return ( rand() % Max)+ 1;
}

void main() {
  const int len = 8;
  int a[len];
  int i;
  int j = 0;
  Randomize() ;

  srand(time(0));
  //Fill the Array
  for (i = 0; i < len; ++i) {
    a[i] = rand() % 100;
  }

  //Print the array after filled
  for (i = 0; i < len; ++i) {
    printf("%d ", a[i]);
  }
  printf("\n");
  getchar();

  //Reversing the array in place.
  for (i = a[len] -1; i >= 0, --i;) {
    a[i] = a[j];
    printf("%d ", a[j]);
    j++;
  }


}

Answer 1

A while loop may be easier to conceptualize. Think of it as starting from both ends and swapping the two elements until you hit the middle.

  i = len - 1;
  j = 0;
  while(i > j)
  {
    int temp = a[i];
    a[i] = a[j];
    a[j] = temp;
    i--;
    j++;
  }

  //Output contents of now-reversed array.
  for(i = 0; i < len; i++)
    printf("%d ", a[i])

Answer 2

void reverse_range(int* buffer, int left, int right)
{
    while (left < right)
    {
        int temp = buffer[left];
        buffer[left++] = buffer[right];
        buffer[right--] = temp;
    }
}

call it to reverse array

int a[3] = {1, 2, 3};
reverse_range(a, 0, 2);

Answer 3

You are on the right track but need to think about that last for loop a little more and the assignment operation inside. The loop initialization is off, since i = a[len] - 1 will copy the value of the last entry to i. Since that value is a random number, your index will probably start out of bounds.

Next, you're copying half of the array to the other half and then back. That loop does the following: a[7] = a[0] a[6] = a[1] a[5] = a[2] a[4] = a[3] ...

At this point you've lost all of the initial values in a[4] through a[7].

Try this:

for( i = 0; i < len / 2; i++ ){
    int temp = a[i];
    a[i] = a[len - i];
    a[len - i] = temp;
}

Use a debugger and step through the loop watching the value of i, temp, and each element in the array

Answer 4

Just my 2 cents...

#include <stdlib.h>
#include <stdio.h>

int main() {
      int arry[] = {0, 1, 2, 3, 4, 5};
      int* s = arry;
      int* e = arry + (sizeof(arry) / sizeof(arry[0])) - 1;
      while (s < e) {
        *e ^= *s;
        *s ^= *e;
        *e ^= *s;
        s++;
        e--;
      }
      for (size_t i = 0; i < (sizeof(arry) / sizeof(arry[0])); i++) {
        fprintf(stderr, "%d, ", arry[i]);
      }
      fprintf(stderr, "\n");
   }

Answer 5

For starters, instead of this:

for (i = a[len] -1; i >= 0, --i;) {

you want this:

for (i = len-1; i >= 0, --i;) {

but you also only want to go half-way through the array, so it would be

for (i = len-1; i > j, --i;) {

Answer 6

Try this;

#include <stdlib.h>
#include <time.h>

int Random(int Max) {
  return ( rand() % Max)+ 1;
}

void main() {
  const int len = 8;
  int a[len];
  int i,end;
  int j = 0;
  Randomize() ;

  srand(time(0));
  //Fill the Array
  for (i = 0; i < len; ++i) {
    a[i] = rand() % 100;
  }

  //Print the array after filled
  for (i = 0; i < len; ++i) {
    printf("%d ", a[i]);
  }
  printf("\n");
  getchar();

  for (i = 0; i < n/2; i++) {
      t = a[i];
      a[i]   = a[end];
      a[end] = t;
      end--;
  }

}

Hope this helps... :)

Just for suggestion. Try to use meaningful variable name instead of just i,a.... That will help you while writing a bigger code. :)

Answer 7

If you are not interested in writing functions for any numeric type, try macros for this task. This code same working with any built-in numeric type: int, float, double.

It has not a support for strings, since any string is ending on the character the NULL character \0. More a controlled version my similar answer is here https://stackoverflow.com/a/42063309/6003870 and contains solution for reverse a string.

---

A full code

#include <stdio.h>

// print items of an array by a format
#define PRINT_ARRAY(array, length, format) \
{ \
    putchar('['); \
    for (size_t i = 0; i < length; ++i) { \
        printf(format, array[i]); \
        if (i < length - 1) printf(", "); \
    } \
    puts("]"); \
}


// reverse an array in place
#define REVERSE_ARRAY(array, length, status) \
    if (length > 0) { \
        for (int i = 0; i < length / 2; ++i) { \
            double temp; \
            temp = array[i]; \
            array[i] = array[length - i - 1]; \
            array[length - i - 1] = temp; \
        } \
        *status = 0; \
    } \
    else if (length < 0) *status = -1; \
    else *status = 1;

#define SUCCESS_REVERSE_ARRAY_MSG "An array succefully reversed"
#define FAILED_REVERSE_ARRAY_MSG "Failed reverse for an array"
#define NO_CHANGED_REVERSE_ARRAY_MSG "An array no changed"


/*
    Print message about status reverse an array
 */
static void
print_msg_reverse_array_status(const int status)
{
    if (status == 0) printf("Status: %s\n", SUCCESS_REVERSE_ARRAY_MSG);
    else if (status == -1) printf("Status: %s\n", FAILED_REVERSE_ARRAY_MSG);
    else if (status == 1) printf("Status: %s\n", NO_CHANGED_REVERSE_ARRAY_MSG);
}


int
main (const int argc, const char *argv[])
{
    // keep value of status
    int status;

    puts("\tExample reverse of an integer array");
    int arr_int[5] = {1, 2, 3, 4, 5};

    status = 0;
    PRINT_ARRAY(arr_int, 5, "%d");
    REVERSE_ARRAY(arr_int, -1, &status);

    // will be an error, since a length is less 0, and the array is not changed
    print_msg_reverse_array_status(status);
    PRINT_ARRAY(arr_int, 5, "%d");

    status = 0;
    REVERSE_ARRAY(arr_int, 0, &status);

    // a length is equal to 0, so an array is not changed
    print_msg_reverse_array_status(status);
    PRINT_ARRAY(arr_int, 5, "%d");

    status = 0;
    REVERSE_ARRAY(arr_int, 5, &status);
    print_msg_reverse_array_status(status);
    PRINT_ARRAY(arr_int, 5, "%d");

    puts("\n\tExample reverse of an float array");
    float arr_float[5] = {0.78, 2.1, -3.1, 4, 5.012};

    status = 0;
    PRINT_ARRAY(arr_float, 5, "%5.3f");
    REVERSE_ARRAY(arr_float, 5, &status);
    print_msg_reverse_array_status(status);
    PRINT_ARRAY(arr_float, 5, "%5.3f");

    puts("\n\tExample reverse of an double array");
    double arr_double[5] = {0.00001, 20000.002, -3, 4, 5.29999999};

    status = 0;
    PRINT_ARRAY(arr_double, 5, "%8.5f");
    REVERSE_ARRAY(arr_double, 5, &status);
    print_msg_reverse_array_status(status);
    PRINT_ARRAY(arr_double, 5, "%8.5f");
    return 0;
}

---

I am used the GCC for compilation and your result must be as next

    Example reverse of an integer array
[1, 2, 3, 4, 5]
Status: Failed reverse for an array
[1, 2, 3, 4, 5]
Status: An array no changed
[1, 2, 3, 4, 5]
Status: An array succefully reversed
[5, 4, 3, 2, 1]

    Example reverse of an float array
[0.780, 2.100, -3.100, 4.000, 5.012]
Status: An array succefully reversed
[5.012, 4.000, -3.100, 2.100, 0.780]

    Example reverse of an double array
[ 0.00001, 20000.00200, -3.00000,  4.00000,  5.30000]
Status: An array succefully reversed
[ 5.30000,  4.00000, -3.00000, 20000.00000,  0.00000]

---

Testing environment

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 8.6 (jessie)
Release:    8.6
Codename:   jessie
$ uname -a
Linux localhost 3.16.0-4-amd64 #1 SMP Debian 3.16.36-1+deb8u2 (2016-10-19) x86_64 GNU/Linux
$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2

Answer 8

You can reverse an array in place you don't need an auxiliary array for that, Here is my C code to do that

#include <stdio.h>
int main(void)
 {
    int arr[5]={1,2,3,4,5};
    int size=sizeof(arr)/sizeof(int);   
    int success= reverse(arr,size);
    if(success==1)
        printf("Array reversed properly");
    else
        printf("Array reversing failed");   

    return 0;
}


int reverse(int arr[], int size)
{
    int temp=0;
    int i=0;
    if(size==0)
        return 0;
    if(size==1)
        return 1;

    int size1=size-1;
    for( i=0;i<(size/2);i++)
    {
        temp=arr[i];
        arr[i]=arr[size1-i];
        arr[size1-i]=temp;
    }

    printf("Numbers after reversal are ");
    for(i=0;i<size;i++)
    {
        printf("%d ",arr[i]);
    }
    return 1;

}

Answer 9

Here's an easy and clean function for flipping arrays of all sizes. Change the parameters according to your type of array:

void flipArray(int *a, int asize){
            int b[asize];
            int *b_p = b;

            for(int i=0; i<asize; i++){
                    //backwardsOrientation = (arraySize-1)-increment
                    b_p[asize-1-i] = a[i];
            }
            for(int i=0; i<asize; i++){
                    a[i] = b_p[i];
            }
    }

Answer 10

 public static void ReverseArrayInPlace()
    {
        int[] arr = new int[] { 1, 2, 3, 4, 5, 6, 7, 8 };
        int end = arr.Length - 1;
        foreach (var item in arr)
        {
            Console.WriteLine(item);
        }

        for (int i = 0; i < arr.Length/2; i++)
        {
            var temp = arr[i];
            arr[i] = arr[end];
            arr[end] = temp;
            end--;
        }
        Console.WriteLine("--------------------------");

        foreach (var item in arr)
        {
            Console.WriteLine(item);
        }
    }

Answer 11

Here is how I did it in Java

It will be exactly same in C or C++ too

class Solution {
    private void reverseHelper(char s[],int i, int j){
        if(i >= j) return;
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
        reverseHelper(s,i+1,j-1);
    }
    
    public void reverseString(char[] s) {
        reverseHelper(s,0,s.length-1);
    }
}

The space complexity here is O(1)

Answer 12

#include<Stdio.h>
#include<string.h>
#define max 25
int main()
{ 
  char arr[max]="0123456789";
  strrev(arr);
  atoi(arr);

  return 0;
}
//you can also use built in functions such as strrev(); string reverse atoi just 
//changes string into integer