SegFault error when computing square root (Newton's method)

问题

I'm very new to C++, but I'm aware that there are tons of ways to receive a SegFault error, but I'm not sure why I'm getting one here. The goal of the program is to compute the square root of a number using Newton's method. I'm assuming it has something to do with the recursion, but I'm pretty sure that the code would run in Java.

#include <iostream>
#include <sstream>

using namespace std;

double sqrt(double eps, double num, double last_guess)
{

    if(num == 0 || num == 1)
    {
        return num;
    }

    int next = (num + (num/last_guess)) / 2;

    if(abs(last_guess - next) <= eps)
        return next;
    else
        return sqrt(eps, num, next);
}

int main(int argc, char *argv[]) {
    double eps, num;
    istringstream iss; //input string

    if(argc == 3)
    {
        iss.str(argv[1]);
        if ( !(iss >> eps) ) {
            cerr << "Error: The first argument is not a valid double." << endl;
            return 1;
        }

        iss.clear();

        iss.str(argv[2]);
        if ( !(iss >> num) ) {
            cerr << "Error: The second argument is not a valid double." << endl;
            return 1;
        }

    } else if(argc == 2) {
        iss.str(argv[1]);
        if ( !(iss >> num) ) {
            cerr << "Error: Argument is not a valid double." << endl;
            return 1;
        }

        eps = 0.000001;
    }

    cout << "sqrt(" << eps << ", " << num << ") = " << sqrt(eps,num, num) << endl;
    return 0;
}

Sample input: 0.000001 4.0

回答1:

Running your program under GDB shows that it is in infinite recursion:

(gdb) run
Starting program: /tmp/a.out 0.000001 4.0

Program received signal SIGSEGV, Segmentation fault.
0x000055555555522d in sqrt (eps=<error reading variable: Cannot access memory at address 0x7fffff7feff8>, 
    num=<error reading variable: Cannot access memory at address 0x7fffff7feff0>, last_guess=<error reading variable: Cannot access memory at address 0x7fffff7fefe8>) at foo.cc:7
7       {
(gdb) bt 20
#0  0x000055555555522d in sqrt (eps=<error reading variable: Cannot access memory at address 0x7fffff7feff8>, 
    num=<error reading variable: Cannot access memory at address 0x7fffff7feff0>, last_guess=<error reading variable: Cannot access memory at address 0x7fffff7fefe8>) at foo.cc:7
#1  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#2  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#3  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#4  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#5  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#6  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#7  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#8  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#9  0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#10 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#11 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#12 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#13 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#14 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#15 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#16 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#17 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
#18 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=2) at foo.cc:19
#19 0x00005555555552df in sqrt (eps=9.9999999999999995e-07, num=4, last_guess=3) at foo.cc:19
(More stack frames follow...)

There is no reason to make your routine recursive.
Your algorithm fails to recognize (is missing a check) that you've already computed the correct answer.

You shouldn't compare the delta between your guesses to the epsilon. You should compare the delta between your computed answer and the real answer instead.
As @PaulMcKenzie noted, you shouldn't store your successive approximations in a integer (use double instead).
To correct the program, you need to use correct formula for the next guess:

    double next = (last_guess + (num/last_guess)) / 2;

来源：https://stackoverflow.com/questions/59958672/segfault-error-when-computing-square-root-newtons-method

标签

c++

segmentation-fault