Question

In a system I am maintaining, users request elements from a collection from a 1-based indexing scheme. Values are stored in 0-based arrays in C++ / C.

Is the following hypothetical code portable if 0 is erroneously entered as the input to this function? Is there a better way to validate the user’s input when converting 1-based numbering schemes to 0-based?

const unsigned int arraySize;
SomeType array[arraySize];    

SomeType GetFromArray( unsigned int oneBasedIndex )
{
  unsigned int zeroBasedIndex = oneBasedIndex - 1;

  //Intent is to check for a valid index.
  if( zeroBasedIndex < arraySize )
  {
    return array[zeroBasedIndex];
  }

  //else... handle the error
}

My assumption is that (unsigned int)( 0 - 1 ) is always greater than arraySize; is this true?

The alternative, as some have suggested in their answers below, is to check oneBasedIndex and ensure that it is greater than 0:

const unsigned int arraySize;
SomeType array[arraySize];    

SomeType GetFromArray( unsigned int oneBasedIndex )
{
  if( oneBasedIndex > 0 && oneBasedIndex <= arraySize )
  {
    return array[oneBasedIndex - 1];
  }

  //else... handle the error
}

Answer 1

For unsigned types, 0-1 is the maximum value for that type, so it’s always >= arraySize. In other words, yes that is absolutely safe.