How to re-order elements of an array in C programming

Question

If I have an array of N elements in C programming, how can I exchange a particular pair of elements?

For instance given the array:

double variable[N] = {1, 2, ..., i-1, i, i+1, ..., N-1, N }

How could I exchange element i for i+1 in such a way that the new system is given by:

double variable[N] = {1, 2, ...,i-1, i+1, i, ..., N-1, N }

My attempt is to make a temporary variable:

{ double temp = element i ;     element i =element i+1 ;   element i+1 = temp;   }

I am dubious as to whether this will advance the variable by one or swap them as required? Unfortunately I am not sure how to implement this in code or even if this is the correct way to do it? In addition, I am unsure of how to cater for when i=N, so that elements cycle back down through the array.

I am very new to programming and very much appreciate any help offered. Thank you!

Look up indexing an array and assigning elements to an index in an array — Millie Smith
– Millie Smith, Commented Oct 10, 2015 at 23:23
I would suggest creating a swap routine that takes two pointers and then simply using it like this: swap(&element[i],&element[i+1]); — David Schwartz
– David Schwartz, Commented Oct 10, 2015 at 23:38

blakelead · Accepted Answer · 2015-10-10 23:36:22Z

2

if you know which elements to exchange, you could do :

if (i+1 < N)
{
  double temp = variable[i];
  variable[i] = variable[i+1];
  variable[i+1] = temp;
}

I check first if i+1 is inferior to N (remember the index of an array begins by 0) to prevent reading outside the array. Then I do the exchange.

If I understood you, if you have to exchange the last element, you would like that it be done with the first element. In that case, you could change the former if to :

if (i+1 <= N)
{
  double temp = variable[i];
  variable[i] = variable[(i+1)%N];
  variable[(i+1)%N] = temp;
}

edited Oct 10, 2015 at 23:36

answered Oct 10, 2015 at 23:20

blakelead

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5 Comments

Millie Smith Over a year ago

Or just int toswap = i + 1 < n ? i + 1 : 0; (or (i + 1) % n)

Millie Smith Over a year ago

Mod happens before addition btw.

blakelead Over a year ago

Yes you are right, and yes right again , my mistake. Thanks !

AngusTheMan Over a year ago

@Blake_Lead Thank you very much for your time Blake, I really appreciate it! Out of interest how could I make the exchanges swap back down through the loop once they reach N, for instance i=N swaps with j=N-1, to cycle back and forth through the array?

blakelead Over a year ago

You are very welcome. To perform what you ask, I think I would use a flag variable that would change value when N is first reached. I would then test the value of this flag variable : if it changed, I would change the order of operation.

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