I have an integer that I use to hold a value, in this case world population. It starts at zero, and has values added to it by a foreach loop. at some point, I haven't gotten to count the indices yet, it changes to a negative value. I've checked the inputs, and they are all positive, so what could cause the flip to negative

the code for adding is just worldview.Population = worldview.population + j.population.

Because it's signed. Check what is the range of int in the programming language you're using.

Sounds like you got an overflow, i.e. you exceeded the maximum value that an Int32 variable can hold.
If you're working on real world population, this is absolutely normal.

A signed 32 bit integer can hold values within the range [ -(2^31), (2^31) - 1 ].
2^31 is about 2 billions, which is way less than the actual human world population that, you know, is now over 7 billions.
An unsigned 32 bit integer can go up to (2^32) - 1, about 4 billions... it won't be enough either.

I guess you're using some .NET Framework language (C# or VB, is the same).
.NET Framework offers 64 bit integer types too, namely Int64 and UInt64.
I'm pretty sure you can't overflow them just by adding some population count.

I forgot about the range. Yeah, it's well over 2 billion. I'm using population data from the CIA world factbook, for a game i'm working on. I'll change the type to a Uint64 and see what happens.

Thanks for the pointers.

EDIT: Changing int to long seems to have fixed the issue. The total population i have is a little bit under 5.5 billion, mostly because I'm using outdated census data.

The long type is implemented as a 64 bit integer on some languages and platforms.
In .NET for example long is just an alias to Int64...
Of course with 64 bit representation you won't have to worry about overflow errors, human population count will be well within that range.

PS: CIA World Factbook is updated and made freely available every year, why are you using old data?

The numbers might be a bit off because I am trying to recreate pandemic, which uses "west Europe" and "east Europe" and "north" and "south Africa" instead of splitting it into all the countries, so I just Google searched some of those numbers. I may just find all the countries in north and south Africa, and add all the populations.