c语言学习1---回文整数&&阶乘
回文整数
#define likely(x) __builtin_expect(!!(x), 1) // likely 代表x经常成立 #define unlikely(x) __builtin_expect(!!(x),0) // unlikely代表x不经常成立 // 回文整数 bool isPalindrome(int x) { // !!逻辑规划 if (__builtin_expect(!!(x < 0),0)) return false; // if(x < 0) return false 告诉cpu x小于0大概率不会出现，帮助cpu增大分支预测概率 int y = 0; z = x; while (x) { y = y*10 + x % 1; x /= 10; } return z == y; }
__builtin_ffs(x):返回x中最后一个为1的位是从后向前的第几位 __builtin_popcount(x):x中1的个数 __builtin_ctz(x):x末尾0的个数。x=0时结果未定义 __builtin_clz)(x):x前导0的个数。x=0时结果未定义 __builtin_prefetch(const void *addr,...):对数据手工预取的方法 __builtin_expect(long exp, long c): 用来引导gcc进行条件分支预测 __builtin_constant_p(exp): 判断exp是否在编译时就可以确定其为常量 __builtin_parity(x): x中1的奇偶性 __builtin_return_address(n):当前函数的第n级调用者的地址
2.两个数字进行交换
#include <stdio.h> #define swap1(a, b) {\ __typeof(a) __temp = a;\ a = b; b = __temp;\ } // __typeof是一个宏，c语言内置，作用：返回a变量类型 void swap(int *a, int *b) { // 指针变量* int temp; temp = *a; *a = *b; *b = temp; return; } int main() { int a, b; scanf("%d%d", &a, &b); printf("a = %d, b = %d\n"); // swap(&a, &b); a ^= b; b ^= a; a ^=b; printf("swap : a = %d, b = %d\n", a, b); return 0; }
3.
#include <stdio.h> int check(int y, int m, int d) { if (m < 1 || d < 1) { return 0; } if (m > 12 || d > 31) { return 0; } int month[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30,31 }; if ((y % 4 == 0 && y % 100 ==0) || y % 400 ==0) { month[2] +=1; }; return d <= month[m]; } int main() { int y, m, d; scanf("%d%d%d", &y, &m, &d); if (check(y, m, d)) { printf("YES\n"); } else { printf("NO\n"); } return 0; }
#include <stdio.h> #include <stdlib.h> #include <time.h> // 输出100以内的随机数，并计算奇数个数 int main () { printf("%ld\n",time(0)); srand(time(0)); // 生成随机种子 time(0) 当前时间戳 int n, cnt = 0; scanf("%d", &n); for (int i = 0; i < n; i++) { int val = rand() % 100; if (val % 2 != 0) { // 等价于val & 1 cnt += 1; } i && printf(" "); printf("%d", val); } printf("\n"); printf("cnt : %d", cnt); return 0; }
4. n的阶乘，递归
#include <stdio.h> int fac(int n) { if (n == 1) return 1; return n * fac(n-1); } int main() { int n; while (~scanf("%d", &n)){ printf("fac(%d) = %d\n", n, fac(n)); } return 0; }
5.函数指针（函数返回值类型 (* 指针变量名) (函数参数列表);）
int g(int (*f1)(int), int (*f2)(int), int (*f3)(int), int x) { if (x < 0) { return f1(x); } if (x < 100) { return f2(x); } return f3(x); }
6.欧拉计划45题：https://projecteuler.net/problem=45
#include <stdio.h> // 欧拉计划45题 long long Triangle(long long n) { return n * (n + 1) >> 1; } long long Pentagonal(long long n) { return n * (3*n - 1) >> 1; } long long Hexagonal(long long n) { return n * (2 * n -1); } int binary_search(long long (*arr)(long long), int n, long long x) { int head = 1, tail = n, mid; while(head <= tail){ mid = (head + tail) >> 1; if (arr(mid) == x) return 1; if (arr(mid) < x) head = mid +1; else tail = mid -1; } return 0; } int main() { int n = 143; while (1){ while (1) { n++; long long temp = Hexagonal(n); if (!binary_search(Pentagonal,temp, temp)) continue; printf("%lld\n",temp); break; } } return 0; }
7. 二分查找
#include <stdio.h> #define MAX_N 100 int binary_search(int *arr, int n, int x) { int head = 0, tail = n-1, mid; while (head <= tail) { mid = (head + tail) >> 1; if (arr[mid] == x) return mid; if (arr[mid] < x) head = mid + 1; else tail = mid -1; } return -1; } // 递归版写法 int binary_search1(int *arr, int l, int r, int x) { if (l > r) return -1; int mid = (l + r) >> 1; if (arr[mid] == x) return mid; if (arr[mid] < x) l = mid + 1; else r = mid -1; return binary_search(arr, l, r, x); } int main() { int arr[MAX_N +5] = {0}, n; scanf("%d", &n); for (int i = 0; i < n; i++) { scanf("%d", &arr[i]); } int x; while (~scanf("%d",&x)){ int ret = binary_search(arr, n, x); if (ret != -1) { printf("success to find %d at arr[%d]\n", x, ret); } else { printf("fail to find %d\n", x); } } return 0; }
8.欧几里得算法（辗转相除）
