Hi, I am learning C/C++ programming & have encountered the usage of 'Bit arrays' or 'Bit Vectors'. Am not able to understand their purpose? here are my doubts -

1. Are they used as boolean flags?
2. Can one use int arrays instead? (more memory of course, but..)
3. What's this concept of Bit-Masking?
4. If bit-masking is simple bit operations to get an appropriate flag, how do one program for them? is it not difficult to do this operation in head to see what the flag would be, as apposed to decimal numbers?

I am looking for applications, so that I can understand better. for Eg -

Q. You are given a file containing integers in the range (1 to 1 million). There are some duplicates and hence some numbers are missing. Find the fastest way of finding missing numbers?

For the above question, I have read solutions telling me to use bit arrays. How would one store each integer in a bit?