【C】 Replace spaces and realize binary parity bit exchange of integers by macros

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Series column :【 from 0 To 1,C Language learning 】
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Preface

Introduce two basic questions , Use C Language implementation , The title is not just a practice in the article , Just record these ideas that are not easy for beginners ！

One . Replace blank space

1. Subject requirements

2. Their thinking

First, find out how many spaces there are in the string ,

Each space needs to use %20 To replace ,%20 Is a space of three characters , The original space only takes up one character , in other words , For each space, you need to add two characters to replace the space ;

use end1 Record the position of the last character of the original string , use end2 Record the position after adding space to replace spaces at the end of the original string

And then end1 The content of the space is assigned to end2,end1 and end2 Move forward one more space , as long as end1 The content of the space pointed to is not a space , take end1 The content of the space is assigned to end2;

When end1 When the space pointed to is a space , here end1 Then move forward to the position of the character in front of the space ;end2 Now move forward three times , Place separately 0、2、%; After placing % after end2 Move forward one bit to % Front position ;

Repeat the process until end1 and end, That is to say end1 and end2 When the pointing positions coincide , All spaces have been replaced .

3. Code implementation

class Solution {
    
public:
    void replaceSpace(char* str, int length) {
    
        char* cur = str;
        int space_count = 0;
        while (*cur)
        {
    
            if (*cur == ' ')
            {
    
                space_count++;
            }
            cur++;
        }
        int end1 = length - 1;
        int end2 = length - 1 + 2 * space_count;
        while (end1 != end2)
        {
    
            if (str[end1] != ' ')
            {
    
                str[end2--] = str[end1--];
            }
            else
            {
    
                end1--;
                str[end2--] = '0';
                str[end2--] = '2';
                str[end2--] = '%';
            }
        }

    }
};

Two . Macros realize binary parity bit exchange of integers

1. Subject requirements

Write a macro , The odd and even bits of the binary bits of an integer can be exchanged .

2. Their thinking

Swap parity bits , You need to take out the parity bits first .

Since it's macro , It's not very realistic to take out the cycle separately , Then use ** Bitwise AND （&）** These bits are the way to do .

Odd digits are taken out , That is to & On 010101010101……; Even bits , Is to & On 101010101010……, The corresponding hexadecimals are 555…… and aaa……, Generally, we default to 32 An integer ,4 Bit corresponds to a bit 16 Hexadecimal is 8 individual 5,8 individual a.

adopt & 0x55555555 The way to get odd numbers and & 0xaaaaaaa Take out even digits in the same way .

Odd number Move left （<<） One is on the even digit , Even digit Move right (>>) One goes to the odd digit , The last two numbers Add get up , The exchange is complete .

※ This macro can only complete 32 Shaping within bits , To finish 64 Bit , Then we will 5 and a Just double the number of .

3. Code implementation

#include<stdio.h>
#define SWAP_BIT(num) num = (((num&0x55555555)<<1) + ((num&0xaaaaaaaa)>>1))

int main()
{
    
	int a = 0;
	scanf("%d", &a);
	printf("%d\n", SWAP_BIT(a));
	return 0;
}

Conclusion

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