! 例題 5-7  漸化式で平方根を求める ［DO WHILE 文 ］
PROGRAM ex5_7

  IMPLICIT NONE
  REAL :: r = 0.0E0, x, d = 1.0E-5

  DO
    PRINT *, 'Input a positive real number: '
    READ *, r
    IF ( r > 0.0E0 ) EXIT
  END DO

  x = r                             ! 初期値は0以外なら何でもよい

  DO WHILE ( ABS( x ** 2 - r ) / r > d )
    x = 0.5E0 * ( x + r / x )
  END DO

  PRINT '( 1X, E12.5 )', x

END PROGRAM ex5_7