How to input pi

How can i enter pi into an equation on matlab?

[Vignesh Shetty]

Hi Anthony!

Its very easy to get the value of π. As π is a floating point number declare a long variable then assign 'pi' to that long variable you will get the value.

Eg:-

format long

p=pi

[Walter Roberson]

@Vignesh Shetty

That is what @Geoff Hayes suggested years before. But it does not enter π into the calculation, only an approximation of π

Answer 1

[Geoff Hayes]

Anthony - use pi which returns the floating-point number nearest the value of π. So in your code, you could do something like

 sin(pi) 

For more information, see https://www.mathworks.com/help/matlab/ref/pi.html.

[Walter Roberson]

Also see sinpi and cospi

Answer 2

[Essam Aljahmi]

28t2e−0.3466tcos(0.6πt+π3)ua(t).

[Walter Roberson]

Guessing a number of times about what that is intended to mean:

28 * t.^2 .* exp(−0.3466 .* t .* cos(0.6 .* pi .* t + pi/3) .* ua(t))

[James Tursa]

How is this answer relevant?

[Roger Pou]

because it is maybe a joke. maybe its the same value as pi but with a long formula.

[Image Analyst]

Attached is code to compute Ramanujan's formula for pi, voted the ugliest formula of all time.

.

Actually I think it's amazing that something analytical that complicated and with a variety of operations (addition, division, multiplication, factorial, square root, exponentiation, and summation) could create something as "simple" as pi.

Unfortunately it seems to get to within MATLAB's precision after just one iteration - I'd have like to see how it converges as afunction of iteration (summation term). (Hint: help would be appreciated.)

[John D'Errico]

As I recall, these approximations tend to give a roughly fixed number of digits per term. I'll do it using HPF, but syms would also work.

DefaultNumberOfDigits 500
n = 10;
piterms = zeros(n+1,1,'hpf');
f = sqrt(hpf(2))*2/9801*hpf(factorial(0));
piterms(1) = f*1103;
hpf396 = hpf(396)^4;
for k = 1:n
  hpfk = hpf(k);
  f = f*(4*hpfk-3)*(4*hpfk-2)*(4*hpfk-1)*4/(hpfk^3)/hpf396;
  piterms(k+1) = f*(1103 + 26390*hpfk);
end
 
piapprox = 1./cumsum(piterms);
pierror = double(hpf('pi') - piapprox))
pierror =
  -7.6424e-08
  -6.3954e-16
  -5.6824e-24
  -5.2389e-32
  -4.9442e-40
   -4.741e-48
  -4.5989e-56
     -4.5e-64
  -4.4333e-72
  -4.3915e-80
  -4.3696e-88

So roughly 8 digits per term in this series. Resetting the default number of digits to used to 1000, then n=125, so a total of 126 terms in the series, we can pretty quickly get a 1000 digit approximation to pi:

pierror = hpf('pi') - piapprox(end + [-3:0])
pierror =
HPF array of size: 4  1
|1,1| -1.2060069282720814803655e-982
|2,1| -1.25042729756426e-990
|3,1| -1.296534e-998
|4,1| -8.e-1004

So as you see, it generates a very reliable 8 digits per term in the sum.

piapprox(end)

ans =

3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381964428810975665933446128475648233786783165271201909145648566923460348610454326648213393607260249141273724587006606315588174881520920962829254091715364367892590360011330530548820466521384146951941511609433057270365759591953092186117381932611793105118548074462379962749567351885752724891227938183011949129833673362440656643086021394946395224737190702179860943702770539217176293176752384674818467669405132000568127145263560827785771342757789609173637178721468440901224953430146549585371050792279689258923542019956112129021960864034418159813629774771309960518707211349999998372978049951059731732816096318595024459455346908302642522308253344685035261931188171010003137838752886587533208381420617177669147303598253490428755468731159562863882353787593751957781857780532171226806613001927876611195909216420199

hpf('pi')

ans =

3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381964428810975665933446128475648233786783165271201909145648566923460348610454326648213393607260249141273724587006606315588174881520920962829254091715364367892590360011330530548820466521384146951941511609433057270365759591953092186117381932611793105118548074462379962749567351885752724891227938183011949129833673362440656643086021394946395224737190702179860943702770539217176293176752384674818467669405132000568127145263560827785771342757789609173637178721468440901224953430146549585371050792279689258923542019956112129021960864034418159813629774771309960518707211349999998372978049951059731732816096318595024459455346908302642522308253344685035261931188171010003137838752886587533208381420617177669147303598253490428755468731159562863882353787593751957781857780532171226806613001927876611195909216420199

I also ran it for 100000 digits, so 12500 terms. It took a little more time, but was entirely possible to compute. I don't recall which similar approximation I used some time ago, but I once used it to compute 1 million or so digits of pi in HPF. HPF currently stores a half million digits as I recall.

As far as understanding how to derive that series, I would leave that to Ramanujan, and only hope he is listening on on this.

Answer 3

[Walter Roberson]

If you are constructing an equation using the symbolic toolbox use sym('pi')

[Walter Roberson]

MATLAB changed the rules in R2020a (I think it was). sym('pi') now creates a symbolic variable named pi that is nothing special. To create the symbolic π now, you should use

sym(pi)

ans = 

π

and count on the fact that sym() recognizes values "close" to pi as being π

[James Emmanuelle Galvan]

sym(pi) prints out "pi".

[Steven Lord]

That's correct. There are four different conversion techniques the sym function uses to determine how to convert a number into a symbolic expression. The default is the 'r' flag which as the documentation states "converts floating-point numbers obtained by evaluating expressions of the form p/q, p*pi/q, sqrt(p), 2^q, and 10^q (for modest sized integers p and q) to the corresponding symbolic form."

The value returned by the pi function is "close enough" to p*pi/q (with p and q both equal to 1) for that conversion technique to recognize it as π. If you wanted the numeric value of the symbolic π to some number of decimal places use vpa.

p = sym(pi)

p = 

π

vpa(p, 30)

ans = 

3.14159265358979323846264338328

Answer 4

[Dmitry Volkov]

Easy way:

format long

p = pi

[Walter Roberson]

@Dmitry Volkov

That is what @Geoff Hayes suggested years before. But it does not enter π into the calculation, only an approximation of π

Answer 5

[AKHIL TONY]

using pi will give an approximate value

[Walter Roberson]

Yes, multiple people pointed that out years ago

Answer 6

[Meghpara]

it is easy to ge pi

in p=PI.

[Walter Roberson]

p=PI

Unrecognized function or variable 'PI'.

If you meant

p=pi

then @Vignesh Shetty suggested exactly that https://www.mathworks.com/matlabcentral/answers/303687-how-to-input-pi#comment_822235 several years ago, which in turn is functionally equivalent to what @Geoff Hayes suggested in 2016 https://www.mathworks.com/matlabcentral/answers/303687-how-to-input-pi#answer_235320