可能这是一些基本问题。但我无法弄清楚并找到答案。
据我所知，int的最高编号是:-2147483648
当我超过该限制时，编译器将始终向我显示该最大值。
对于float，它是:3.4e+38(或减号，但这不是重点)
当我通过 float 限制时，编译器总是显示inf。为什么不使用3.4e+38或-3.4e-38 ???
我的代码是这样的:
int bytesT=8*5;
int a=0;
for (int i=0; i<bytesT; i++)
{
a += pow(2, i);
}
std::cout << "a = " << a << std::endl;
float b= 3.4 * pow(10, 39);
std::cout << "b = " << b << std::endl;
它给了我那个结果:
a = -2147483648
b = inf
有人可以解释这种行为吗？提前致谢。
最佳答案
I'm gonna try to make this as simple as possible:
int = 32 bits:
0000 0000 0000 0000 0000 0000 0000 0000
left-most bit represents sign, so largest int32 number that you can have is:
0111 1111 1111 1111 1111 1111 1111 1111 (which is 2147483647)
for negative numbers if left-most bit is set to 1 then to get "human readable"
number you will have to do complement 2 of that number which is invert every bit
and add 1.
ex.
1111 1111 1111 1111 1111 1111 1111 1111 <- my number (i know it is negative, -?)
0000 0000 0000 0000 0000 0000 0000 0000 <- all bits converted
0000 0000 0000 0000 0000 0000 0000 0001 <- added 1
=> so that begging number was -1
Now that you know this, question is what happens if you add 1 to the
largest positive int32 number (0111 1111 1111 1111 1111 1111 1111 1111) ?
Well you will get 1000 0000 0000 0000 0000 0000 0000 0000
which is (-2147483648). What happened here was overflow caused by addition.
This was the case in your program for your 'a' variable.
- As for your confusion with 'inf':
Floating point numbers work differently then integers.
I'm not gonna go into too much detail on how bits and bytes are arranged.
Just know that there is :
- 1 bit reserved for sign (top-left)
- next 8 (for float32) bits for exponent
- and the rest for fraction
Now what is interesting is that there are certain 'special' values defined in
IEEE 754 which are the following:
- +0 = 0 00000000 00000000000000000000000
- -0 = 1 00000000 00000000000000000000000
- +inf = 0 11111111 00000000000000000000000
- -inf = 1 11111111 00000000000000000000000
- NaN = 0 11111111 00000000000000000000001
As you see these special values are not defined in int type which is the reason
why you get 'inf' in float and something weird/unexpected in int.
I hope i answered your question.