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 i

Answer 1

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 2

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 3

#include<stdio.h>
void main() {
    int array[] = {0,1,2,3,4,5,6,7,8,9,10,12,13};
    int i,j;
    unsigned long int len = sizeof(array)/sizeof(array[i]);
    for(i=0; i<len/2; i++) {
        j = array[i];
        array[i] = array[(len-1)-i];
        array[(len-1)-i] = j;
    }
    for(i=0; i<len; i++) printf("%d ",array[i]);
    printf("\n");
}

Answer 4

#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

Answer 5

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 6

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])