Why is int rather than unsigned int used for C and C++ for loops?

Question

This is a rather silly question but why is int commonly used instead of unsigned int when defining a for loop for an array in C or C++?

for(int i;i<arraySize;i++){}
for(unsigned int i;i<arraySize;i++){}

I recognize the benefits of using int when doing something other than array indexing and the benefits of an iterator when using C++ containers. Is it just because it does not matter when looping through an array? Or should I avoid it all together and use a different type such as size_t?

Answer 1

Using int is more correct from a logical point of view for indexing an array.

unsigned semantic in C and C++ doesn't really mean "not negative" but it's more like "bitmask" or "modulo integer".

To understand why unsigned is not a good type for a "non-negative" number please consider these totally absurd statements:

• Adding a possibly negative integer to a non-negative integer you get a non-negative integer
• The difference of two non-negative integers is always a non-negative integer
• Multiplying a non-negative integer by a negative integer you get a non-negative result

Obviously none of the above phrases make any sense... but it's how C and C++ unsigned semantic indeed works.

Actually using an unsigned type for the size of containers is a design mistake of C++ and unfortunately we're now doomed to use this wrong choice forever (for backward compatibility). You may like the name "unsigned" because it's similar to "non-negative" but the name is irrelevant and what counts is the semantic... and unsigned is very far from "non-negative".

For this reason when coding most loops on vectors my personally preferred form is:

for (int i=0,n=v.size(); i<n; i++) {
    ...
}

(of course assuming the size of the vector is not changing during the iteration and that I actually need the index in the body as otherwise the for (auto& x : v)... is better).

This running away from unsigned as soon as possible and using plain integers has the advantage of avoiding the traps that are a consequence of unsigned size_t design mistake. For example consider:

// draw lines connecting the dots
for (size_t i=0; i<pts.size()-1; i++) {
    drawLine(pts[i], pts[i+1]);
}

the code above will have problems if the pts vector is empty because pts.size()-1 is a huge nonsense number in that case. Dealing with expressions where a < b-1 is not the same as a+1 < b even for commonly used values is like dancing in a minefield.

Historically the justification for having size_t unsigned is for being able to use the extra bit for the values, e.g. being able to have 65535 elements in arrays instead of just 32767 on 16-bit platforms. In my opinion even at that time the extra cost of this wrong semantic choice was not worth the gain (and if 32767 elements are not enough now then 65535 won't be enough for long anyway).

Unsigned values are great and very useful, but NOT for representing container size or for indexes; for size and index regular signed integers work much better because the semantic is what you would expect.

Unsigned values are the ideal type when you need the modulo arithmetic property or when you want to work at the bit level.

Answer 2

This is a more general phenomenon, often people don't use the correct types for their integers. Modern C has semantic typedefs that are much preferable over the primitive integer types. E.g everything that is a "size" should just be typed as size_t. If you use the semantic types systematically for your application variables, loop variables come much easier with these types, too.

And I have seen several bugs that where difficult to detect that came from using int or so. Code that all of a sudden crashed on large matrixes and stuff like that. Just coding correctly with correct types avoids that.

Answer 3

Not much difference. One benefit of int is it being signed. Thus int i < 0 makes sense, while unsigned i < 0 doesn't much.

If indexes are calculated, that may be beneficial (for example, you might get cases where you will never enter a loop if some result is negative).

And yes, it is less to write :-)

Answer 4

It's purely laziness and ignorance. You should always use the right types for indices, and unless you have further information that restricts the range of possible indices, size_t is the right type.

Of course if the dimension was read from a single-byte field in a file, then you know it's in the range 0-255, and int would be a perfectly reasonable index type. Likewise, int would be okay if you're looping a fixed number of times, like 0 to 99. But there's still another reason not to use int: if you use i%2 in your loop body to treat even/odd indices differently, i%2 is a lot more expensive when i is signed than when i is unsigned...