# Approximating $\pi$ using Monte Carlo integration

I need to estimate $\pi$ using the following integration:

$$\int_{0}^{1} \!\sqrt{1-x^2} \ dx$$

using monte carlo

Any help would be greatly appreciated, please note that I'm a student trying to learn this stuff so if you can please please be more indulging and try to explain in depth..

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could it be a typografic error? You have $$\frac{\pi}{4} = \int_0^1 \sqrt{1-x^2} dx$$ – Esteban Crespi Jan 26 '11 at 19:58
Yes!! can you explain how you reached that answer? – Zapacila Jan 26 '11 at 20:03
A picture may help. $x^2 + y^2 = 1$ gives the unit circle; if $y \ge 0$ and $x \in [0, 1]$, then we can rewrite this as $y = \sqrt{1 - x^2}$. The unit circle has area $\pi * 1^2 = \pi$, and we're taking just the upper right quarter of it. – user4689 Jan 29 '11 at 4:37

## 4 Answers

Generate a sequence $U_1,U_2,\ldots$ of independent uniform$[0,1]$ random variables. Let $Y_i = f(U_i)$, where $f(x)=\sqrt{1-x^2}$, $0 \leq x \leq 1$. Then, for sufficiently large $n$, $$\frac{{\sum\nolimits_{i = 1}^n {Y_i } }}{n} \approx \int_0^1 {f(x)\,{\rm d}x = } \int_0^1 {\sqrt {1 - x^2 } \,{\rm d}x} = \frac{\pi }{4}.$$

EDIT: Elaborating.

Suppose that $U_1,U_2,\ldots$ is a sequence of independent uniform$[0,1]$ random variables, and $f$ is an integrable function on $[0,1]$, that is, $\int_0^1 {|f(x)|\,{\rm d}x} < \infty$. Then, the (finite) integral $\int_0^1 {f(x)\,{\rm d}x}$ can be approximated as follows. Let $Y_i = f(U_i)$, so the $Y_i$ are independent and identically distributed random variables, with mean (expectation) $\mu$ given by $$\mu = {\rm E}[Y_1] = {\rm E}[f(U_1)] = \int_0^1 {f(x)\,{\rm d}x}.$$ By the strong law of large numbers, the average $\bar Y_n = \frac{{\sum\nolimits_{i = 1}^n {Y_i } }}{n}$ converges, with probability $1$, to the expectation $\mu$ as $n \to \infty$. That is, with probability $1$, $\bar Y_n \to \int_0^1 {f(x)\,{\rm d}x}$ as $n \to \infty$.

To get a probabilistic error bound, suppose further that $f$ is square-integrable on $[0,1]$, that is $\int_0^1 {f^2 (x)\,{\rm d}x} < \infty$. Then, the $Y_i$ have finite variance, $\sigma_2$, given by $$\sigma^2 = {\rm Var}[Y_1] = {\rm E}[Y_1^2] - {\rm E}^2{[Y_1]} = {\rm E}[f^2{(U_1)}] - {\rm E}^2{[f(U_1)]} = \int_0^1 {f^2 (x) \,{\rm d}x} - \bigg[\int_0^1 {f(x)\,{\rm d}x} \bigg]^2 .$$ By linearity of expectation, the average $\bar Y_n$ has expectation $${\rm E}[\bar Y_n] = \mu.$$ Since the $Y_i$ are independent, $\bar Y_n$ has variance $${\rm Var}[\bar Y_n] = {\rm Var}\bigg[\frac{{Y_1 + \cdots + Y_n }}{n}\bigg] = \frac{1}{{n^2 }}{\rm Var}[Y_1 + \cdots + Y_n ] = \frac{n}{{n^2 }}{\rm Var}[Y_1 ] = \frac{{\sigma ^2 }}{n}.$$ By Chebyshev's inequality, for any given $\varepsilon > 0$, $${\rm P}\big[\big|\bar Y_n - {\rm E}[\bar Y_n]\big| \geq \varepsilon \big] \leq \frac{{{\rm Var}[\bar Y_n]}}{{\varepsilon ^2 }},$$ so $${\rm P}\big[\big|\bar Y_n - \mu \big| \geq \varepsilon \big] \leq \frac{{\sigma^2}}{{n \varepsilon ^2 }},$$ and hence $${\rm P}\bigg[\bigg|\bar Y_n - \int_0^1 {f(x)\,{\rm d}x} \bigg| \geq \varepsilon \bigg] \leq \frac{1}{{n \varepsilon ^2 }} \bigg \lbrace \int_0^1 {f^2 (x) \,{\rm d}x} - \bigg[\int_0^1 {f(x)\,{\rm d}x} \bigg]^2 \bigg \rbrace.$$ So if $n$ is sufficiently large, with high probability the absolute difference between $\bar Y_n$ and $\int_0^1 {f(x)\,{\rm d}x}$ will be smaller than $\varepsilon$.

Returning to your specific question, letting $f(x)=\sqrt{1-x^2}$ thus gives $${\rm P}\Big[\Big|\bar Y_n - \frac{\pi }{4} \Big| \geq \varepsilon \Big] \leq \frac{1}{{n \varepsilon ^2 }} \bigg \lbrace \int_0^1 {(1 - x^2) \,{\rm d}x} - \frac{\pi^2 }{16} \bigg \rbrace = \frac{1}{{n \varepsilon ^2 }} \bigg \lbrace \frac{2}{3} - \frac{\pi^2 }{16} \bigg \rbrace < \frac{1}{{20n\varepsilon ^2 }},$$ where $\bar Y_n = \frac{{\sum\nolimits_{i = 1}^n {\sqrt {1 - U_i^2 } } }}{n}$.

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How did you get from that integration to \frac{\pi }{4}. ? My math is very rusty.. – Zapacila Jan 26 '11 at 20:13
– Shai Covo Jan 26 '11 at 20:20
on the link abose they get (x/2)sqrt(1-x^2) + Arcsin(x)/2 how did you get \frac{\pi }{4}. ? – Zapacila Jan 26 '11 at 20:37
Let $F(x)=(x/2)\sqrt{1-x^2} + \arcsin(x)/2$. Note that $F(1)-F(0) = \arcsin(1)/2 = \pi/4$. – Shai Covo Jan 26 '11 at 20:46

1
Concerning the evaluation of the integral, note that $\int_0^1 {\sqrt {1 - x^2 } \,{\rm d}x}$ gives the area under the curve $y = \sqrt{1-x^2}$ as $x$ goes from $0$ to $1$, hence equal to $\pi/4$. – Shai Covo Jan 26 '11 at 21:28
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Let's also elaborate on Ross Millikan's answer, adapted to the case $f(x)=\sqrt{1-x^2}$, $0 \leq x \leq 1$. Suppose that $(X_1,Y_1),(X_2,Y_2),\ldots$ is a sequence of independent uniform vectors on $[0,1] \times [0,1]$, so that for each $i$, $X_i$ and $Y_i$ are independent uniform$[0,1]$ random variables. Define $Z_i$ as follows: $Z_i = 1$ if $X_i^2 + Y_i^2 \leq 1$, $Z_i = 0$ if $X_i^2 + Y_i^2 > 1$, so the $Z_i$ are independent and identically distributed random variables, with mean $\mu$ given by $$\mu = {\rm E}[Z_1] = {\rm P}[X_1^2 + Y_1^2 \leq 1] = {\rm P}\big[(X_1,Y_1) \in \lbrace (x,y) \in [0,1]^2 : x^2+y^2 \leq 1\rbrace \big] = \frac{\pi }{4},$$ where the last equality follows from ${\rm P}[(X_1,Y_1) \in A] = {\rm area}A$ ($A \subset [0,1]^2$).

By the strong law of large numbers, the average $\bar Z_n = \frac{{\sum\nolimits_{i = 1}^n {Z_i } }}{n}$ converges, with probability $1$, to the expectation $\mu$ as $n \to \infty$. That is, with probability $1$, $\bar Z_n \to \frac{\pi }{4}$ as $n \to \infty$.

To get a probabilistic error bound, note first that the $Z_i$ have variance $\sigma^2$ given by $$\sigma^2 = {\rm Var}[Z_1] = {\rm E}[Z_1^2] - {\rm E}^2{[Z_1]} = \frac{\pi }{4} - \Big(\frac{\pi }{4}\Big)^2 = \frac{\pi }{4} \Big(1 - \frac{\pi }{4}\Big) < \frac{10}{59}.$$ The average $\bar Z_n$ has expectation ${\rm E}[\bar Z_n] = \mu$ and variance ${\rm Var}[\bar Z_n] = \sigma^2 / n$; hence, by Chebyshev's inequality, for any given $\varepsilon > 0$, $${\rm P}\big[\big|\bar Z_n - \mu \big| \geq \varepsilon \big] \leq \frac{{\sigma^2}}{{n \varepsilon ^2 }},$$ and so $${\rm P}\bigg[\bigg|\bar Z_n - \frac{\pi }{4} \bigg| \geq \varepsilon \bigg] < \frac{{10}}{{59n\varepsilon ^2 }}.$$

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I'll definitely have to review my math!! – Zapacila Jan 28 '11 at 7:28

No longer applicable as the integral has been corrected: I don't understand how this integral gets you $\pi$ and if you pull the $x$ out you get $\int_0^1x\sqrt{10}dx=\frac{\sqrt{10}}{2}$

The general idea of a Monte Carlo integration of $\int_0^1 f(x)dx$ is to take random pairs $(x,y)$ with $0\le x\le 1$ and $0\le y \le y_{max}$ and check for how many of the pairs $f(x)\le y$. The integral is then the number of pairs under the curve divided by the number of trials and multiplied by the area of the box ($x y_{max}$)

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hey i am very sorry i misspelled (first time using the alfabet) it's actually sqrt (1-x^2) inside the integration – Zapacila Jan 26 '11 at 19:54
So  i need to make a c++ program that simulates this. i would random generate x from 0 to 1  and y from 0 to lets say 10.000 (the higher the more accurate).. do the math for f(x) an check f(x)<= y. Good. bu how do i relate to pi? – Zapacila Jan 26 '11 at 19:58
y would run from 0 to 1-you just need it to be big enough to be greater than f(x) for all x.  So you would generate lots (say 10,000) pairs (x,y) in the unit square and count how many have y<sqrt(1-x^2).  Presumably this is about 7850.  The area would be 7850/10000 and multiplying by 4 gives your measurement of pi. – Ross Millikan Jan 26 '11 at 20:06
i'm missing something here:             int x = 0;              int y = 0;              int k = 0;              for (int i = 0; i < 10000; i++)             {                 x = new Random().Next(0, 1);                 y = new Random().Next(0, 1);                  if (y < Math.Sqrt(1 - x * x))                     k++;             }              Console.WriteLine("Pi={0}", (k/10000)*4); – Zapacila Jan 26 '11 at 20:54
@Zapacila:  I don't read C very well, but with k an int, doesn't dividing by 10000 give 0? – Ross Millikan Jan 26 '11 at 21:00
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C# implementation. Thanks to all who contributed!! Ross, Esteban, Shai

````class Program
{
    static void Main(string[] args)
    {
        double x = 0;
        var rd = new Random();

        double sum = 0;
        for (int i = 0; i < 1000000; i++) // the higher the more precise
        {
            x = rd.NextDouble();
            sum += Math.Sqrt(1 - x*x); 
        }

        Console.WriteLine("Pi={0}", (sum/1000000)*4);
        Console.Read();
    }
}
````
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Later on, I'll give a probabilistic error bound. I hope you'll find it interesting. – Shai Covo Jan 26 '11 at 21:46
Thanks Shai for you devotion to the subject. I'm most interested since i have a project based on this. Please don't forget to share lots of details.Again Thanks! – Zapacila Jan 26 '11 at 22:33
google searchable: sqrt(1-x^2) dx pi estimate integration algorithm – Zapacila Jan 26 '11 at 22:37
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