Lecture 6: for Loops

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Lecture 6 for Loops

• Yoni Fridman
• 7/6/01

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Outline

• The for statement
• The for statements format
• The for statements flow
• Comparing while, do, and for
• Loop examples
• Debugging control structures

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The for Statement

• Remember this while loop?
• int i 1
• while (i lt 5)
• System.out.println(i)
• i
• It can be described as follows
• initialization
• while (test)
• statement
• update
• The for loop condenses this
• for (initialization test update)
• statement

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The for Statements Format
for (initialization test update) statement
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The for Statements Flow
for (initialization test update)
statement
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The for Statement (Side Notes)

• The initialization, test, and update need not all
  operate on the counter variable.
• The initialization and update need not have one
  statement.
• They can be empty (have no statements), in which
  case they must be performed elsewhere. The
  semicolons must still be included.
• They can have multiple statements, separated by
  commas.
• The test can be empty it defaults to true.
• In this case, the loop must have a break, or it
  will iterate forever.
• The initialization need not declare the counter.
• IMPORTANT Variables CAN be redeclared within
  for statements.

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Comparing while, do, and for

• Why use for instead of while? Same old reasons
• Its shorter.
• It makes the program more readable all the
  vital information (initialization, test, and
  update) can easily be found in one place.
• So when do you use which loop?
• for is used in loops with counters.
• while is used in loops without counters.
• do is used in loops without counters, when you
  must guarantee at least one iteration.

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Loop Examples

• Powers of 2 (while loop)
• ltCode Examplegt
• Factorials (for loop)
• ltCode Examplegt
• Fibonacci numbers (for loop)
• ltCode Examplegt

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Debugging Control Structures

• Debugging becomes more complicated once control
  structures have been introduces.
• When testing your program, test the program on
  various input data.
• Make sure each statement of your code is executed
  at least once.
• Put println() statements within control
  structures.
• Common errors to watch out for
• Off-by-one errors, for example, using i lt n
  instead of i lt n.
• Infinite loops.
• Never-executed loops or if statements.
• Using the debugger An example.

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Homework

• Read 5.1, 5.2 (bottom of p. 193 to bottom of p.
  194), 5.3, 5.4.
• HW3 out today, due Wednesday.
• Black Jack.