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Loops (DO, DO WHILE, EXIT, CYCLE)

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• Types of loop statements
  • Running index (For-loops)

  • Condition-based (While-loops)

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• FOR-loops (Fortran: DO-loops)
  • Syntax:

    (NOTE: The older syntax forms using line number will NOT be discussed !!!)

    DO variable = startValue, StopValue [, StepValue] one or more statments END DO

  • StepValue is optional , the default is 1 (ONE)

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  • Example:

    integer i, j do i = 1, 100, 7 j = i*i print *, "i = ", i, ", i^2 = ", j end do Output: i = 1 , i^2 = 1 i = 8 , i^2 = 64 i = 15 , i^2 = 225 i = 22 , i^2 = 484 i = 29 , i^2 = 841 i = 36 , i^2 = 1296 i = 43 , i^2 = 1849 i = 50 , i^2 = 2500 i = 57 , i^2 = 3249 i = 64 , i^2 = 4096 i = 71 , i^2 = 5041 i = 78 , i^2 = 6084 i = 85 , i^2 = 7225 i = 92 , i^2 = 8464 i = 99 , i^2 = 9801

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• WHILE-loops (Fortran: DO-WHILE-loops)
  • Syntax:

    DO WHILE (condition) one or more statments END DO

    Example:

    l1 = 1; l2 = x/l1 do while ( abs(l1 - l2) > 1e-10 ) l1 = (l1 + l2)/2.0 l2 = x/l1 end do

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Infinite loop...
  • The following construct will loop forever

    do ... ... end do

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• EXIT: break out of loops (C/C++'s break statement)
  • The EXIT statement executed inside a DO loop, will terminate the loop immediately

  • Example:

    DO ! FOREVER !!! .... IF ( ... ) THEN EXIT END IF END DO

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• CYCLE: skip a single loop cycle (C/C++'s continue statement)
  • When a CYCLE statement is executed inside a DO loop, one single DO loop cycle is prematurely terminated and execution of the program continues with the start of the next loop

  • Example:

    DO X = 1, 100 .... IF ( X == 44 ) THEN CYCLE ! Skip when X == 44 END IF END DO

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• The lost satelite...
  • This is a true story:
    A satelite was lost due to a one-keystroke mistake in a Fortran guidance system program

  • The syntax of the DO loop in F77 is as follows:

    DO line# variable = startValue, StopValue multiple statements line# CONTINUE

    Example: SUM = 0 DO 10 I = 1, 10 SUM = SUM + I 10 CONTINUE

  • Example Program: (F77 Do-loop) --- click here
    The tracking system program was written in F77, and there was a typo in the DO loop: the comma (,) was mistakenly typed as a period (.)

    Guess what the new F77 program did...