C Control Structures Repetition Loops

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C Control Structures Repetition (Loops)

• R. Johnson

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Control Structurescontrolling the flow
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Remember

• Three-pronged approach
• General concepts
• C syntax
• Common applications/algorithms

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Repetition with Loops

• Selection has the program branching, deciding
  which blocks of code to execute or skip, going
  forward.
• Repetition has the control point moving back to
  execute the body of the loop again, while a
  test condition is true.
• Post-test do while(cond)Test at end,
  body executes at least once!
• Pre-test while(cond) Test at beginning,
  may run zero times.

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General Form of Do-While Loop
do //always comment which loop //statements
to be repeated //called body of the
loop while(condition)

• Body executes at least once (blue arrow)
• Repeats, goes back, on true (green arrow)
• Exits loop on false (red arrow)
• Note closing semicolon.

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General Form of While Loop
while(condition) //statements to be
repeated //called body of the loop //always
comment which loop

• Body may not execute at all (test at beginning,
  could be false first time).
• ALWAYS repeats, goes back, at end of body (blue
  arrow).
• Re-enters body on true (green arrow)
• Exits loop, skipping body, on false (red arrow)
• Note NO closing semicolon.

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Run Again Loop

• Add to Temperature Conversion program to re-run
  for multiple temps.
• Use a character variable for yes/no answer (as
  y or n)
• Case issues? ltcctypegt and tolower()
• Code

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A Simple Run Again? Loopwith post-test do-while
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Input Validation

• Make temp conversion program smarter about bad
  input for conversion choice (menu has only 1 or
  2)
• Can prevent (or deal with) bad input before using
  it, instead of after the fact with an else.
• Why a while loop instead of an if stmt?

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Input Validation w/ Error Msgusing pre-test
while loop
Basic Algorithm Get input While input is bad
Error message Get input again End of loop Rest
of program that Uses the input
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Advice on Writing Loops

• Properly indenting loop body aids finding errors.
• KEEP IT SIMPLE ! Only have the body of the loop
  include code directly related to your purpose in
  adding the loop in the first place.
• An input validation loop would NOT include
  calculations (other than those for the validity
  test).
• If you find yourself adding if statements or
  other code trying to force the loop into
  behaving the way you wish, consider changing it
  from pre-test (while) to post-test (do-while) or
  vice versa.

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Infinite Loops

• Common logic error stuck in loop
• Once a loop is entered, there must be SOMETHING
  inside it that can change and make the loop test
  go false for exit.
• Often caused by forgetting to get a new input to
  check.

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Loops and Accumulation

• Instructions are repeated, but same variables
  (memory storage locations!) used entire time.
• Trivial re-use new inputs/results overwrite old
  ones
• Something new make use of the old values
• Accumulate counts, sums, products
• May count all inputs in loop, or just some.

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Counting

• How do we count by hand?
• Count students in front row

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Counting by Hand vs C

• Need storage for the number a hand
• Need an int counter variable
• No fingers up before we start
• Initialize counter to zero (before loop- why?)
• Count by adding one to existing number raise
  a finger
• Increment counter inside loop
• Final count available after loop.
• CODE GPA averaging example

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Accumulation Operators

• Example accumulate a sum
• Normal sum sum result
• Shortcut sum result
• Meaning add result to current value of sum,
  save (assign) back into same variable
• Others
• - /
• Counting by 1 Increment shortcuts
• count same as count 1
• Also count-- decrement

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Lab 5 - Elevator

• Loading an elevator
• Instead of run again loop
• Loop watches total weight versus capacity
• Issues when getting out of the loop last rider
  exceeds limit
• Two versions do-while and while
• Second version handles situation better.

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Lab 5 Recap

• Apparently odd order of while loop elevator
  solution
• Unroll the loop
• First input before loop - common technique
• Problem with elevator run out of riders before
  elevator is full?

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Sentinel Loop for Input Processing
Basic Algorithm Get first input While input is
not the sentinel value Use the current
input Get next input End of loop

• Sentinel Value
• Must be same data type as data input
• But not a valid real wordl value

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Lab 6 Height Converter

• Enter a height in inches, convert to feet and
  inches
• Example 73in becomes 6ft, 1 in
• Declare 3 int variables height, feet, inches
• Use integer division, modulus ()
• Print a message if tall (over 76in)
• Add a sentinel loop for multiple inputs
• use 0 as sentinal value be sure to tell user!
• Go back and count inputs and sum heights for
  average calc at end (once, after loop).

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Common Loop Logic Errors

• Errors with vs. , and vs. or
• Infinite loop (never goes false once inside)
• Forget to ask for next input inside sentinel loop
• While loop never entered (never true at start)
• Counts or sums off
• Forgot to initialize to zero
• Increment or add at wrong point inside the loop
• General leaving things outside of loop that
  should repeat.
• TRY TO STICK WITH STANDARD FORMS IF POSSIBLE
  DONT RE-INVENT THE WHEEL!!!