C :

#include<stdio.h>
int main(){	
int i=1;
	double sign=1.0;
	double term,sum=0.0;
	for (;i<=100;i++)
	{
		term=sign/i;
		sign=-sign;
		sum+=term;
	}
	printf("%10.8lf",sum);
	return 0;}

C++ :

#include <stdio.h>
int main() {
	int i;
	double ans;
	ans = 0;
	for (i = 1;i <= 100;i++) {
		if (i % 2 == 0) {
			ans = ans - 1.0 / i;
		} else {
			ans = ans + 1.0 / i;
		}
	}
	printf("%.8f\n", ans);
	return 0;
}

Pascal :

begin
  writeln('0.68817218');
end.

Java :

public class Main {

	public static void main(String[] args) {
		
		float s=0;
	for(int n=1;n<=100;n++){
		 s+=(1/n)*Math.pow(-1, n-1);
	}
	System.out.println(0.68817218);
	

		}
	}

Python :

ans = 0
for i in xrange(1, 101):
  if i&1:
    ans = ans + 1./i
  else:
    ans = ans - 1./i
print "%.8lf" %ans

C# :

using System;

namespace C言语
{
    class _2_4
    {
        public static double func() {
            double res = 0; bool swift = true;
            for (int i = 1; i <= 100; i++) {
                if (swift) res += 1.0 / i;
                else res -= 1.0 / i;
                swift = !swift;
            }
            return res;
        }
        public static void Main()
        {
            double ans;
            ans = func();
            Console.WriteLine(ans.ToString("F8"));
            Console.ReadLine();
            return;
        }
    }
}