math

#include <stdio.h>
#include <string.h>
#include <math.h>

int main(int argc, char **argv)
{
    char input[20];
    float s, val1, val2, val3;

    printf("input shape(cirlcle, triangle, rectangle): ");
    scanf("%20s", &input);

    if (!strcmp(input, "circle"))
    {
        printf("input radius: ");
        scanf("%f", &val1);
        printf("S = %f\n", M_PI * val1 * val1);
    }

    else if (!strcmp(input, "rectangle"))
    {
        printf("input a, b: ");
        scanf("%f %f", &val1, &val2);
        printf("S = %f\n", val1 * val2);
    }

    else if (!strcmp(input, "triangle"))
    {
        printf("input a, b, c: ");
        scanf("%f %f %f", &val1, &val2, &val3);
        s = (val1 + val2 + val3) / 2;
        printf("S = %f\n", sqrt(s * (s - val1) * (s - val2) * (s - val3)));
    }
    else
        printf("error: wrong data format\n");

    return 0;
}