Iterative Structures: for Loops

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Iterative Structures for Loops

• Lecture 13 Wed, Sep 24, 2003

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Loops Controlled by a Counter

• If we know in advance the number of times a loop
  should be executed, then we can count the
  iterations and quit at the proper time.
• Establish a counter and do the following.
• Initialize the counter to 0.
• Test the counter on each iteration.
• Increment the counter.

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Loops Controlled by a Counter

• If the counter controls the loop, then the
  testing does not involve the input.
• Therefore, the pattern prompt-read-test-action is
  no longer in effect.
• Indeed, there may not be any input.

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Loops Controlled by a Counter
int limit cin gtgt limit int counter 0 while
(counter lt limit) Prompt user for input
cin gtgt number Action counter
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Example of a Counter

• CounterSum.cpp

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Combining the Methods

• A loop may be controlled by a combination of
  methods.
• For example, we may use a sentinel value of 1,
  but also test for EOF.
• This loop would be controlled by both a sentinel
  value and end-of-file.

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Combining the Methods

• The sequence should be
• Prompt user for input
• Read input
• Test input and test for EOF
• Action
• Prompt user for input
• Read input
• Test input and test for EOF
• Action
• And so on

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Combining the Methods
const int SENTINEL -1 Prompt user for
input cin gtgt number while (!cin.eof() number
! SENTINEL) Action Prompt user for
input cin gtgt number
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Example of a Sentinel Value and EOF

• SentinelEOFSum.cpp

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Combining the Methods

• As another example, we may use a sentinel value
  of 1, but also not read more than 100 numbers.
• This loop would be controlled by both a sentinel
  value and a counter.
• This is trickier, as we discover when we write
  the sequence of steps.

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Combining the Methods

• The sequence should be
• Initialize counter
• Test counter
• Prompt user for input
• Read input
• Test input
• Action
• Increment counter
• Test counter
• And so on

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Combining the Methods

• This loop requires two tests and they must occur
  at different points.
• This is one of the rare cases where it is
  justifiable to break out of a loop prematurely.
• Place one test in the while statement.
• Place the other test in the body of the loop
  together with a break statement.

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Combining the Methods
const int LIMIT 100 const int SENTINEL
-1 int counter 0 while (counter lt limit)
Prompt user for input cin gtgt number if
(number SENTINEL) break Action
counter
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Example of a Sentinel Value and a Counter

• SentinelCounterSum.cpp

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The for Statement

• Loops controlled by counters are very common.
• To facilitate their use, C provides a for
  statement.
• The for statement controls the loop by
• Initializing the counter
• Testing the counter
• Incrementing the counter

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The for Statement
for (Init counter Test counter Incr counter)
Action
for (int i 0 i lt 10 i) cout ltlt "Enter
a number " cin gtgt number sum
number
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Examples of a for Loop

• ForSum.cpp
• CountLetters.cpp