How to iterate over an integer range when you don't know if it is in increasing or decreasing o-CodePudding

I am trying to iterate over a range in one loop and NOT using below two for-loops:

if (firstIndex <= secondIndex)
    for (int i = firstIndex; i <= secondIndex; i  )
    {...}
else
    for (int i = firstIndex; i >= secondIndex; i--)
    {...}

I considered using boost::irange but it does not cover secondIndex index.

Update: I need to preserve the order and CANNOT swap indices.

CodePudding user response：

In a contrived way, you can use

for (int i= first >= second ? first : second; 
  first >= second ? (i <= second) : (i >= first); 
  i = first >= last ?  1 : -1)

CodePudding user response：

Find out what is similar, and what is different; copy the former, abstract out the latter.

First we find out the direction, as the sign of the difference of indices. C does not have an integral sign function, but it is easy enough to calculate the sign by using the fact that true is 1 and false is 0. We can use this sign value to step between the values (i.e. i is equivalent to i = 1, and i-- is i = -1).

Now i <= secondIndex and i >= secondIndex can both be abstracted as i <= secondIndex * sign, and i and i-- as i = sign.

int diff = lastIndex - firstIndex;
int sign = (diff > 0) - (diff < 0);
for (int i = firstIndex; i <= secondIndex * sign; i  = sign) { ... }