使用C語言編寫程序,求1到100之間的素數(還有另一種求素數:求前100個素數)。
1.每個數試除到√x
2.將素數保存,讓後面的數整除保存的素數
3.篩選:依次刪除範圍中的2的倍數、3的倍數、5的倍數…
#include<stdio.h> #include<stdlib.h> #include<math.h> #include<memory.h> void test1()//試除法(1) { //思路:每個數試除到√x const int Range = 100; int num = 0; int i = 0, j = 2; for (i = 2; i <= Range; i++) { //令每次試除都從2開始 for (j = 2; j <= sqrt(i); j++) { if (0 == i%j) break; } if (j > sqrt(i)) { printf("%2d ", i); num++; if (0 == num % 10) printf("\n"); } } printf("\n"); } void test2()//試除法(2) { //思路:將所求出的素數保存起來,然後每次試除保存的這些素數 const int Range = 100; const int num = (int)(Range / log(Range)*1.16); //素數定理(求一定範圍內的所有素數個數) int* str = (int*)calloc(num, sizeof(int));//開闢動態數組,保存以求素數 //memset(str, 0, num*sizeof(int));//沒必要 系統開闢空間自動賦值0 int i = 2, j = 0; str[0] = 2; for (i = 2; i < 100; i++) { for (j = 0; str[j] != 0 && i%str[j] != 0; j++) { } if (str[j] == 0) str[j] = i; } for (j = 0; str[j] < Range&&j < num; j++) { if (0 == j % 10) printf("\n"); printf("%2d ", str[j]); } printf("\n"); free(str); } void test3()//篩選法(size = 1B => size = 1bit) { //思路:開闢動態數組初始爲0,從2開始一直到最大值,令滿足2的倍數條件的下標所對應置爲-1,輸出值爲0的下標 const int Range = 100; int i = 0, j = 0; int num = 0; char* str = (char*)calloc(Range, sizeof(char));//開闢動態數組初始爲0 str[0] = 1; str[1] = 1; for (i = 2; i <= Range; i++) { for (j = i + 1; j <= Range; j++) { if (0 == j % i && 0 == str[j]) str[j] = 1; } } for (j = 0; j < Range; j++) { if (str[j] == 0) { if (0 == num++ % 10) printf("\n"); printf("%2d ", j); } } printf("\n"); free(str); }
可查詢更多思路
http://blog.csdn.net/program_think/article/details/7032600/