Nested DO-Loops

Just like an IF-THEN-ELSE-END IF can contain another IF-THEN-ELSE-END IF (see nested IF for the details), a DO-loop can contain other DO-loops in its body. The body of the contained DO-loop, usually referred to as the nested DO-loop, must be completely inside the containing DO-loop. Note further that an EXIT statement only brings the control out of the inner-most DO-loop that contains the EXIT statement.

Suppose we have the following nested DO loops:

DO
   statements-1
   DO
      statement-2
   END DO
   statement-3
END DO

Each iteration of the outer DO starts with statements-1. When the control reaches the inner DO, statements-2 is executed until some condition of the inner DO brings the control out of it. Then, statements-3 is executed and this completes one iteration. Any EXIT in the inner DO brings the control out of the inner DO to the first statement in statement-3.

The following are a few simple examples:

• In the following nested loops, the outer one has i running from 1 to 9 with step size 1. For each iteration, say the i-th one, the inner loop iterates 9 times with values of j being 1, 2, 3, 4, 5, 6, 7, 8, 9. Therefore, with i fixed, the WRITE is executed 9 times and the output consists of i*1, i*2, i*3, ..., i*9.

  INTEGER :: i, j
  
  DO i = 1, 9
     DO j = 1, 9
       WRITE(*,*)  i*j
     END DO
  END DO
  

  Once this is done, the value of i is advanced to the next one, and the inner loop will iterate 9 times again displaying the product of the new i and 1, 2, 3,4 ..., 9.The net effect is a multiplication table. For i=1, the value if 1*1, 1*2, 1*3, ..., 1*9 are displayed; for i=2, the displayed values are 2*1, 2*2, 2*3, ..., 2*9; ...; for i=9, the displayed values are 9*1, 9*2, 9*3, ..., 9*9.
• The following shows a nested DO-loop. The outer one lets u run from 2 to 5. For each u, the inner DO lets v runs from 1 to u-1. Therefore, when u is 2, the values for v is from 1 to 1. When u is 3, the values for v are 1 and 2. When u is 4, the values for v are 1, 2, and 3. Finally, when u is 5, the values for v are 1, 2, 3 and 4.

  INTEGER :: u, v
  INTEGER :: a, b, c
  
  DO u = 2, 5
     DO v = 1, u-1
        a = 2*u*v
        b = u*u - v*v
        c = u*u + v*v
        WRITE(*,*)  a, b, c
     END DO
  END DO
  

  The above discussion can be summarized in the following table:
  

  u	Values for v
  2	1
  3	1	2
  4	1	2	3
  5	1	2	3	4

  For each pair of u and v, the inner loop computes a, b and c. Thus, it will generate the following result (please verify it):
  
  

  u	v	a	b	c
  2	1	4	3	5
  3	1	6	8	10
  	2	12	5	13
  4	1	8	15	17
  	2	16	12	20
  	3	24	7	25
  5	1	10	24	26
  	2	20	21	29
  	3	30	16	34
  	4	40	9	41

• It is obvious that the inner DO-loop computes the sum of all integers in the range of 1 and i (i.e., Sum is equal to 1+2+3+...+i). Since i runs from 1 to 10, the following loop computes ten sums: 1, 1+2, 1+2+3, 1+2+3+4, ...., 1+2+3+...+9, and 1+2+3+...+9+10.

  INTEGER :: i, j, Sum
  
  DO i = 1, 10
     Sum = 0
     DO j = 1, i
        Sum = Sum + j
     END DO
     WRITE(*,*)  Sum
  END DO
  

• The program below uses Newton's method for computing the square root of a positive number. In fact, it computes the square roots of the numbers 0.1, 0.1, ..., 0.9 and 1.0.

  REAL :: Start = 0.1, End = 1.0, Step = 0.1
  REAL :: X, NewX, Value
  
  Value = Start
  DO
     IF (Value > End)  EXIT
     X = Value
     DO
        NewX = 0.5*(X + Value/X)
        IF (ABS(X - NewX) < 0.00001)  EXIT
        X = NewX
     EBD DO
     WRITE(*,*)  'The square root of ', Value, ' is ', NewX
     Value = Value + Step
  END DO
  

  Newton's method is taken directly from the programming example, where X is the current guess, NewX is the new guess, and Value is the number for square root computation. The EXITstatement brings the execution of the inner DO to the WRITE statement.If the inner loop is removed, we have the outer loop as follows:
  

  REAL :: Start = 0.1, End = 1.0, Step = 0.1
  REAL :: X, NewX, Value
  
  Value = Start
  DO
     IF (Value > End)  EXIT
  !
  ! the inner loop computes the result in NewX
  !
     WRITE(*,*)  'The square root of ', Value, ' is ', NewX
     Value = Value + Step
  END DO
  

  It is clear that the value of Value starts with 0.1 and have a step size 0.1 until 1.0. Thus, the values of Value are 0.1, 0.2, 0.3, ..., 0.9 and 1.0. For each value of Value, the inner loop computes the square root of Value. The EXIT statement in the outer loop brings the control out of the outer loop.