算法：从键盘输入能够构成三角形的三条边长，编程计算该三角形的面积

程序分析：三角形面积海伦公式：√[ p ( p - a ) ( p - b ) ( p - c ) ] .
其中 p = (a + b + c) / 2 .a、b、c分别是三角形的三边长。
分别用if语句和switch语句实现。
用if语句实现：

#include "stdio.h"#include "math.h"int main(void){    float a = 0, b = 0, c = 0, p = 0;    float area = 0;    printf("Please input three sides of triangle:");    scanf_s("%f %f %f", &a, &b, &c);    if((a + b) > c && (a + c) > b && (b + c) > a)    {        p = (a + b + c) / 2;        area = sqrt(p * (p - a) * (p - b) * (p - c));    }    else        printf("Triangle does not exist！\n");    printf("The area of triangle is:%f\n", area);    return 0;}

用switch语句实现：

#include "stdio.h"#include "math.h"int main(void){    float a = 0, b = 0, c = 0;    float p = 0;    printf("Please input three sides of triangle:");    scanf_s("%f %f %f", &a, &b, &c);    switch (a + b > c  &&  a + c > b && b + c > a)    {    case 0:printf("Triangle does not exist！\n"); break;    case 1:        p = (a + b + c)*0.5;        printf("The area of triangle is:%f\n", sqrt(p * (p - a) * (p - b) * (p - c)));        break;    }    return 0;}