?? ei.c
字號(hào):
#include <math.h>#define EULER 0.57721566#define MAXIT 100#define FPMIN 1.0e-30#define EPS 6.0e-8float ei(float x){ void nrerror(char error_text[]); int k; float fact,prev,sum,term; if (x <= 0.0) nrerror("Bad argument in ei"); if (x < FPMIN) return log(x)+EULER; if (x <= -log(EPS)) { sum=0.0; fact=1.0; for (k=1;k<=MAXIT;k++) { fact *= x/k; term=fact/k; sum += term; if (term < EPS*sum) break; } if (k > MAXIT) nrerror("Series failed in ei"); return sum+log(x)+EULER; } else { sum=0.0; term=1.0; for (k=1;k<=MAXIT;k++) { prev=term; term *= k/x; if (term < EPS) break; if (term < prev) sum += term; else { sum -= prev; break; } } return exp(x)*(1.0+sum)/x; }}#undef EPS#undef EULER#undef MAXIT#undef FPMIN
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