《Linux C 编程一站式学习》学习日记 7排序与查找

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前言

作者能力有限，仅代表个人浅显理解
所有代码经过测试均符合实际

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一、排序

1.插入排序

说明：在已排序好的序列中插入数字

以下代码

#include <stdio.h>

#define LEN 5
int a[LEN] = {1, 5, 1, 5, 7};

void ins_sort(void)
{
    int i,j,key;

    for (j = 1; j < LEN; ++j) {
        key = a[j];
        i = j - 1;
        while (i >= 0 && a[i] > key) {
            a[i+1] = a[i];
            --i;
        }
        a[i+1] = key;
    }
    for (i = 0; i < LEN; i++) {
        printf("%d   ", a[i]);
    }
}

int main(void)
{
    ins_sort();
    return 0;
}

2.归并排序

说明：sort()函数用于分割数组；merge()归并排序
从最小的分割数组sort(0,1)仅有两个元素的开始归并，确保每次归并前的数组都是有序的

函数进入过程如下

LEN = 8
a[LEN] = {5, 2, 4, 7, 1, 3, 2, 6}

sort(0, 7){
    start = 0; end = 7; mid = 3;
    >"sort(0-3, 4-7)"
    sort(0, 3){
        start = 0; end = 3; mid = 1;
        >"sort(0-1, 2-3)"
        sort(0, 1){
            start = 0; end = 1; mid = 0;
            >"sort(0-0, 1-1)"
            sort(0, 0){
                ...
            }
            sort(1, 1){
                ...
            }
            merge(0, 0, 1)
            >"merge(0-0, 1-1)"
        }
        sort(2, 3){
            start=2; end=3; mid=2;
            >"sort(2-2, 3-3)"
            ...
            ...
            merge(2,2,3)
            >"merge(2-2, 3-3)"
        }
        merge(0, 1, 3)
        >"merge(0-1, 2-3)"
    }

    sort(4, 7){

    }
}

代码如下（本人菜鸡，代码抄书）

#include <stdio.h>

#define LEN 8
int a[LEN] = { 5, 2, 4, 7, 1, 3, 2, 6 };

void merge(int start, int mid, int end)
{
    int n1 = mid - start + 1;
    int n2 = end - mid;
    int left[LEN], right[LEN];
    int i, j, k;

    for (i = 0; i < n1; i++) /* left holds
    a[start..mid] */
    left[i] = a[start+i];
    for (j = 0; j < n2; ++j) /* right holds
    a[mid+1..end] */
    right[j] = a[mid+1+j];

    i = j = 0;
    for (k = start; i < n1 && j < n2; ++k) {
        if (left[i] < right[j]) {
            a[k] = left[i];
            ++i;
        } else {
            a[k] = right[j];
            ++j;
        }
    }
    
    if (i < n1) /* left[] is not
    exhausted */
        for (; i < n1; i++) {
        a[k] = left[i];
        ++k;
        }
    if (j < n2) /* right[] is not
    exhausted */
        for (; j < n2; ++j) {
        a[k] = right[j];++k;
        }
}

void sort(int start, int end)
{
    int mid;

    if (start < end) {
        mid = (start + end) / 2;
        printf("sort (%d-%d, %d-%d) %d %d %d %d %d %d "
        "%d %d\n",start, mid, mid+1, end,
        a[0], a[1], a[2], a[3], a[4],
        a[5], a[6], a[7]);
        sort(start, mid);
        sort(mid + 1, end);
        merge(start, mid, end);
        printf("merge (%d-%d, %d-%d) to %d %d "
        "%d %d %d %d %d %d\n",
        start, mid, mid+1, end,
        a[0], a[1], a[2], a[3], a[4],
        a[5], a[6], a[7]);
    }
}

int main(void)
{
    sort(0, LEN-1);
    return 0;
}

二、查找

1.线性查找

说明：从头或者从尾一个一个查找

以下代码

#include <stdio.h>

char a[]="hello world";

int indexof(char letter)
{
	int i = 0;
	
	while (a[i] != '\0') {
		if (a[i] == letter)
			return i;
		i++;
	}
	return -1;
}
int main(void)
{
	printf("%d %d\n", indexof('o'), indexof('z'));
	return 0;
}

2.折半查找

说明：根据有序序列，折半寻找

以下代码

#include <stdio.h>

#define LEN 8
int a[LEN] = { 1, 3, 3, 3, 4, 5, 6, 7 };

int binarysearch(int number)
{
	int mid, start = 0, end = LEN - 1;
	
	while (start <= end) {
		mid = (start + end) / 2;
	if (a[mid] < number)
		start = mid + 1;
	else if (a[mid] > number)
		end = mid - 1;
	else
		return mid;
	}
	return -1;
}

int main(void)
{
	printf("%d\n", binarysearch(3));
	return 0;
}