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Lec 2-3: Bits, Bytes And Integers

Why use unsigned?

Firstly, DON'T use unsigned without understanding implications.

unsigned i;
// This is actually an infinite loop
for (i = cnt - 1; i >= 0; --i) { 
    // ...
}

#define DELTA sizeof(int)

int arr[10];
// Also an infite loop,
// in that DELTA is of type `unsigned long`.
// So, `i` in `(i - DELTA)` will be cast to `unsigned long` implicitly
for (i = sizeof(arr); i - DELTA >= 0; i -= DELTA) {
    // ...
}

Counting Down with Unsigned

Second, you can do counting down with unsigned.

// When a variable of type `unsigned` is underflowed,
// it will be set to `MAX_UNSIGNED`
// according to C standard

for (unsigned i = cnt - 1; i < cnt; --i) {
    // ...
}

Even better, you can use size_t (whose size is 8 bytes, as opposed to the 4-byte unsigned int). So, even the condition is i <= cnt, it still won't become a infinite loop, as long as cnt is shorter than i.

// When a variable of type `unsigned` is underflowed,
// it will be set to `MAX_UNSIGNED`
// according to C standard

for (size_t i = cnt - 1; i < cnt; --i) {
    // ...
}

Tips

Use unsigned in the cases of

  • modular arithmetic
  • e.g. cryptography
  • using bits to represent set

Because the overflow/underflow of unsigned is well-defined in C standard, as opposed to signed types.

Words and Bytes

Any given computer has a word size.

It should roughly be

  • the largest number
  • the range that signifies how big a pointer is

in some language, or the

  • largest sort of chunk of hardware for which there's standard support for storing it for arithmetic operations and so forth

e.g. 64-bit machine has a word size of 8 bytes

Note: Actually, word size is determined by both hardware and compiler. For example, int is 4-byte, no matter whether on 32-bit machine or 64-bit machine. So, in some sense, "word" or "word size" is not a very meaningful term in precise definition. But for pointer size, this makes sense.