Loops and Character Manipulation

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• Loops and Character Manipulation
• 4.1 Control Constructs Loops
• While Loops - code block is repeated as long as
  loop condition is satisfied
• Iterative Loops - code block repeated a
  pre-specified number of times (hopefully and/or
  maybe)
• 4.1.1 The WHILE Loop
• DO
• Statement Block 1
• IF (logical expression) EXIT
• Statement Block 2
• END DO
• Execution Steps
• Execute Statement Block 1
• If logical expression is True, transfer to
  Statement after END DO.
• If logical expression is False, Execute Statement
  Block 2.
• Repeat Steps 1-4.

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• The DO WHILE Loop
• General Form
• DO WHILE (logical expression)
• Statement Block
• END DO
• If the logical expression is True at the point of
  the DO WHILE, the Statement Block is executed.
  If it is False, the statement after the END DO is
  executed.
• 4.1.3 The Iterative or Counting Loop
• DO indexistart, iend, increment
• Statement Block
• END DO
• Index gt loop index (counter)
• Istart gt beginning value of index
• Iend gt terminating value of index
• Increment gt increment added to index after each
  execution of Statement Block.

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• Execution Steps
• index set to istart.
• if indexincrement lt iendincrement, go to Step
  3.
• If indexincrement gt iendincrement, go to
  Statement after END DO.
• Statement Block executed.
• algebraically add increment to index, go to Step
  2.
• What parameters would allow a DO Loop to never
  execute?

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• Hints on Looping
• Indent Statement Block on all Block Constructs,
  i.e., IF, DO, etc.
• Never change the DO Loop parameters within a DO
  loop. In many cases, compilers cannot detect
  this change!
• Number of Iterations (iend istart incr) /
  incr
• If the Number of Iterations lt 0, Statement Block
  is not executed.
• Parameters should be integer variables. (Some
  compilers allow)
• Branching out of the range of a DO Loop is
  permissible. The index variable maintains its
  last value.
• After a normal DO Loop completion, the index
  variable is undefined!
• Branching into the range of a DO Loop is not
  permissible! Compilers will catch this one

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• CYCLE and EXIT Statements
• CYCLE Statement inside DO or WHILE loops
• 1) Increment that loops index variable
• Resume execution at top of that loop, i.e.,
  check index variable for loop completion and
  execute that loop if appropriate
• EXIT Statement inside DO or WHILE loops
  Immediate transfer to first statement after that
  loop.

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• Named Loops, Nested Loops and Block IF Constructs
• L1 DO
• L2 DO
• IF (logical expression 1) THEN
• Statement 1
• Statement 2
• L3 DO IL312,7,-3
• Statement 1
• Statement 2
• END DO L3
• ELSE IF (logical expression 2) THEN
• Statement 1
• Statement 2
• END IF
• DO IL41,100,3
• Statement 1
• Statement 2

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• Character Assignments and Character
  Manipulations
• 4.2.1 Character Assignments
• General Form Character variable name character
  expression
• A B If A is longer than B, A is padded with
  blanks.
• If A is shorter than B, extra characters in B are
  truncated.
• 4.2.2 Substring Specifications
• Character_variable_name (char1 char2)
• Read as The character string in
  character_variable_name from
• char1 to char2, inclusive.

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• The Concatenation Operator (//)
• ConcatenationgtTack one character string onto the
  tail end of another string.

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• 4 Relational Operators with Character Data
• Comparison result dependent on the collating
  sequence on the computer.
• See Text, Appendix A, ASCII and EBCDIC Coding
  Systems, p 719.

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Character Intrinsic Functions
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• DeBugging Fortran Loops
• WHILE Loops Probable error in Logical
  Expression.
• Iterative Loops Probable error in loop
  parameters.