Signed and unsigned keywords

We know that the bottom of the computer only knows 0、1. Any data at the bottom will be converted into 0、1. How to store negative numbers ？ I'm sure this “-” The number cannot be stored in memory , What do I do ？ It's easy to do , Make a mark . Free up the highest bit of the basic data type , Used to store symbols , At the same time, it is agreed as follows ： If the highest order is 1, Indicates that this number is negative , The value is added to the value of the remaining bits except the highest bit “-” Number ; If the highest level is 0, Indicates that this number is a positive number , Its value is the value of the remaining bits except the highest bit .

int main()
{
    
	char a[1000];
	int i;
	for(i=0; i<1000; i++)
	{
    
		a[i] = -1-i;
	}
	printf("%d",strlen(a));
	return 0; 
}

To do this problem, we must first consider two problems
1. How are values saved on the computer . In computer system , All values are represented by complements （ Storage ）
2. How to save after overflow . There was an overflow ,-129 need 9 Bits can be stored , and char The type data is only 8 position , So the highest bit is discarded .

Also understand the conversion between the original code and the complement

Here are some steps to solve the problem



1. answer ：
hypothesis +0 and -0 All are int data .int Type data size is 4Byte = 32 bit
We know , Integer data stored in memory is binary complement .

2. answer ：
If you press %d Output is -10; Press %u The output is a large value .

The calculated complement is then converted into the original code

In unsigned integer form （%u） Output
The complement obtained above is the same , The original code is also this , Just for %u When outputting , The highest bit is not the sign bit , Instead, it becomes a numerical digit , So it becomes a big number .

3. answer ：
Defines an unsigned integer variable i, We know that unsigned numbers are always greater than or equal to 0, therefore i>=0 Your judgment has always been true . The program execution will be endless .

In specific cases, we can still use complement ,. We know -1 The complement of is 32 individual 1,i Shaping for unsigned , So its corresponding 10 Decimal digits ：2^32 -1 =4294967295. So the running result of the program is ：9,8, 7, 6,5,4, 3,2, 1,0,4294967295,4294967294 …

Let's test it

#include<stdio.h>
#include<Windows.h>

int main()
{
    
    unsigned i;
    for (i = 9; i >= 0; i--)
    {
    
        printf("%u\n", i);
        Sleep(1000);
    }
    return 0;
}