less_retarded_wiki/sqrt.md

# Square Root

Square root (sometimes shortened to *sqrt*) of [number](number.md) *a* is such a number *b* that *b^2 = a*, for example 3 is a square root of 9 because 3^2 = 9. Finding square root is one of the most basic and important operations in [math](math.md) and [programming](programming.md), e.g. for computing [distances](distance.md), solving [quadratic equations](quadratic_equation.md) etc. Square root is a special case of finding Nth [root](root.md) of a number for N = 2. Square root of a number doesn't have to be a whole number; in fact if the square isn't a whole number, it is always an [irrational number](irrational_number.md) (i.e. it can't be expressed as a fraction of two integers, for example square root of [two](two.md) is approximately 1.414...); and it doesn't even have to be a [real number](real_number.md) (e.g. square root of -1 is [i](i.md)). Strictly speaking there may exist multiple square roots of a number, for example both 5 and -5 are square roots of 25 -- the positive square root is called **principal square root**; principal square root of *x* is the same number we get when we raise *x* to 1/2, and this is what we are usually interested in -- from now on by *square root* we will implicitly mean *principal square root*. Programmers write *square root of x* as `sqrt(x)` (which should give the same result as raising to 1/2, i.e. `pow(x,0.5)`), mathematicians write it as:

```
  _    1/2
\/x = x
```

Here is the graph of square root [function](function.md) (notice it's a [parabola](parabola.md) flipped by the diagonal axis, for square root is an inverse function to the function *x^2*):

```
      ^ sqrt(x)
      |       :       :       :       :       :
    3 + ~ ~ ~ + ~ ~ ~ + ~ ~ ~ + ~ ~ ~ + ~ ~ ~ + ~
      |       :       :       :       :       :
      |       :       :       :       :       ___....--
    2 + ~ ~ ~ + ~ ~ ~ + ~ ~ ~ + ~ __..---'"""": ~
      |       :       : __..--""""    :       :
      |       :  __.--'"      :       :       :
    1 + ~ ~ _--'" ~ ~ + ~ ~ ~ + ~ ~ ~ + ~ ~ ~ + ~
      |  _-"  :       :       :       :       :
      | /     :       :       :       :       :
  ----+"------|-------|-------|-------|-------|----> x
      |0      1       2       3       4       5
      |
```

TODO

## Programming

TODO

If we need extreme speed, we may use a [look up table](lut.md) with precomputed values.

Within desired precision square root can be relatively quickly computed iteratively by [binary search](binary_search.md). Here is a simple [C](c.md) function computing integer square root this way:

```
unsigned int sqrt(unsigned int x)
{
  unsigned int l = 0, r = x / 2, m;
  
  while (1)
  {
    if (r - l <= 1)
      break;

    m = (l + r) / 2;
    
    if (m * m > x)
      r = m;
    else
      l = m;
  }

  return (r * r <= x ? r : l) + (x == 1);
}
```

TODO: Heron's method

The following is a **non-iterative [approximation](approximation.md)** of integer square root in [C](c.md) that has acceptable accuracy to about 1 million (maximum error from 1000 to 1000000 is about 7%): { Painstakingly made by me. ~drummyfish }

```
int32_t sqrtApprox(int32_t x)
{
  return
    (x < 1024) ?
    (-2400 / (x + 120) + x / 64 + 20) :
      ((x < 93580) ?
       (-1000000 / (x + 8000) + x / 512 + 142) :
       (-75000000 / (x + 160000) + x / 2048 + 565));
}
```