shift elements in array

Question

This is elementary, but my googling just doesn't cut it. I know I have to do something else to shift the values of an array one by one, but the below coding gives me the same values for items[k] to items[infinity] all equaling items[k]. What I don't understand is how to preserve the original k+1 value while I copy the k value into the k+1 slot.

if ( i < numItems) //if i is inside the used boundaries of the array
{
    for (int k = i; k < numItems; k++) //shift the array values from point i
    {
                double temp = 0.0;
        temp = items[k];
        items[k+1] = temp;
    }

    items[i] = value; //and insert value into i
}

Does it has to be a recursive method?

Teudimundo · Answer 1 · 2013-01-28T10:16:20.937

39

You can as well use memmove, that handles overlap of regions.

memmove(&items[k+1], &items[k], (numItems-k-1)*sizeof(double));
items[k] = value;

edited Jan 28 '13 at 10:16

answered Sep 28 '12 at 07:48

Teudimundo

2,610
20
28

Of course, after the `memmove`, you should set `items[0] = value;` – Frerich Raabe Sep 28 '12 at 08:33
You are right, and I've also corrected the answer to reflect the question (insert at the k-th position). – Teudimundo Jan 28 '13 at 10:21
2

You can find a nice discussion about this question here: http://stackoverflow.com/questions/7776085/why-is-memcpy-and-memmove-faster-than-pointer-increments – Teudimundo Apr 17 '15 at 09:06

score 10 · Accepted Answer · edited Aug 03 '17 at 15:20

An easy option would be to iterate through the array in reverse

for (int k = numItems; k > i; k--){        
    items[k]=items[k-1];
}

Option 2:

If you want to keep your method intact then you can also use the temp variable differently

before your for loop initialize temp to

double temp = items[i];

and then in the loop you can use temp to store the [k+1] value in temp rather than storing the [k] value.

items [k+1] = temp;
temp = items [k+1];
items[k+1] = items[k];

also you should watch your boundaries so that k+1 is not going past the last element in the array. You could use something like numItems - 1 with a check before, to ensure that the array is not empty.

score 0 · Answer 3 · answered Nov 03 '16 at 07:06

Can you try reversal method

this is an example.

// reverse array from start to end
void reverse(int a[], int start, int end)
{
  int i;
  int temp;
  while(start++ < end--)
  {
    temp = a[start];
    a[start] = a[end];
    a[end] = temp;
  }
}

// function that will rotate array by d elements
void rotateArray(int a[], int d, int n)
{
  reverse(a, 0, d-1);
  reverse(a, d, n-1);
  reverse(a, 0, n-1);
}

score -2 · Answer 4 · answered Oct 29 '17 at 19:47

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

int main() {

    int i,j=0,s;
    int n,k;
    int A[n];

    scanf("%d %d",&n,&k);
    if(((n>=0) && (n<=100000))&&(k>=0)){
        for(i=0;i<n;i++){
            scanf(" %d", &A[i]);
        }
        if(k>=n){
            k=k-n;
        }else{
        for(j=0;j<n;j++){
            s=j+k;
            if(s>n){
                s-=n;
                A[j]=A[s];
            }else{
            A[j]=A[s];
            }

        }
        for(i=0;i<n;i++){
            printf("%d ",A[i]);
        }
      }
    }
    return 0;
}

shift elements in array

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