• Algorithms often need to repeat a step or a sequence of steps
  • Example: Check all batting averages to find the player with the highest
• We often want to apply the same algorithm multiple times
  • Example: Convert multiple speeds in knots to the equivalent speeds in mi/hr (e.g., to create a table)

• We start by initializing a variable that gets used in one or more calculations
• We perform the calculation
• We update the variable that gets used in the calculation
• We determine if we're done

• Loop:
  • A portion of a program that repeats a statement or group of statements
• Body of a Loop:
  • The statement or group of statements that is repeated
• Iteration:
  • One "pass" through the body of a loop

• Termination/Exit Condition:
  • Can involve a pre-test (i.e., a test at the top) or a post-test (i.e., a test at the bottom)
• Sentinel:
  • A value that would normally be "invalid" that is used to signal that a loop should terminate

• Infinite Loop:
  • A loop that never terminates
  • 
    (Courtesy of Unshelved)
• Indefinite Loop:
  • A loop that terminates after a number of iterations that is unknown a priori

• Syntax:
  • while (boolean_expression) statement
• Note:
  • statement can be a simple or block statement

speedInKnots = 1.;

while (speedInKnots < 50.)
{
    feetPerHour = speedInKnots * feetPerKnot;
    speedInMPH  = feetPerHour / feetPerMile;
	    
    System.out.printf("knots: %5.2f\tmph: %5.2f\n",
                      speedInKnots, speedInMPH);
	    
    speedInKnots = speedInKnots + 1.;    		      
}			  
  

• Properties:
  • Use a pre-test
  • Use a sentinel
• Implications:
  • May have 0 iterations
  • May be infinite
  • May be indefinite

• Syntax:
  • do statement while (boolean_expression);
• Note:
  • statement can be a simple or block statement

speedInKnots = 1.;

do
{
    feetPerHour = speedInKnots * feetPerKnot;
    speedInMPH  = feetPerHour / feetPerMile;
	    
    System.out.printf("knots: %5.2f\tmph: %5.2f\n",
                      speedInKnots, speedInMPH);
	    
    speedInKnots = speedInKnots + 1.;    		      
}
while (speedInKnots < 50.);
  

• Properties:
  • Use a post-test
  • Use a sentinel
• Implications:
  • Must have one iteration
  • May be infinite
  • May be indefinite

• Syntax:
  • for (initialization; boolean_expression; update) statement
• Note:
  • statement can be a simple or block statement

for (speedInKnots=0.; speedInKnots < 50.; speedInKnots=speedInKnots+1.)
{
    feetPerHour = speedInKnots * feetPerKnot;
    speedInMPH  = feetPerHour / feetPerMile;
 
    System.out.printf("knots: %5.2f\tmph: %5.2f\n", 
                       speedInKnots, speedInMPH);
}    
  

• Properties:
  • Use a pre-test
  • Use a "built-in" initialization
  • Use a "built-in" sentinel
• Implications:
  • May have 0 iterations
  • May be infinite
  • Are most frequently definite (i.e., not indefinite)

• An Observations:
  • Arrays have a known and fixed length so are often processed using for loops
• Example:

  • for (int i=0; i<args.length; i++)
    {
        System.out.printf("Argument %d is: %s\n", i, args[i]);
    }
    

• Consider These Examples:
  • for (j=0; j<50; ++j)
  • for (int i=0; i<50; ++i)
• What's Different?
  • The scope of i and j
• Why Does It Matter?
  • j can be accessed outside of the loop, i can't
• What About the Other Kinds of Loops?
  • The same kind of issue can arise though it is less common

• An Observation:
  • The update pattern is commonly used in loops
• Which Approaches?
  • Explicit updates (e.g., age = age + 1;)
  • Compound assignment operators (e.g., age += 1;)

• Parentheses:
  • In Java, parentheses are required
• Colons:
  • In Python, the expression/range/condition is followed by a colon
• Indenting:
  • In Python, indenting is syntactically significant

• Looping over a Range:
  • In Python, a for loop can be provided a range of integers using the range() function
• Looping Over a List/Array/String:
  • In Python, a for loop can be provided a list/array/string
  • In Java, it is possible to loop over a "collection", but we won't discuss how until later in the semester
• Do Loop:
  • In Python, there is no equivalent of a do loop