Iteration

Template for Iterative Programs

STEP 1 Choose a variable that "counts" (could be counting through integers, or a collection of data)

STEP 2 Initialize outside the loop - where do I start?

STEP 3 Set up end test - how do I know when loop is done?

STEP 4 Construct block of code - set of instructions that will be followed each time in the loop; all that changes is the value of the variable

STEP 5 Decide what to do when done

The while statement 

#while CONDITION(S) (with an outcome of 0 or 1):
#STATEMENTS
 
#ex. Find the square root of a perfect square - without using math library!
x = 16
ans = 0
while ans*ans < x:
	ans = ans + 1
print ans
 
#The answer is right, so long as you give a perfect square that is positive.  Let's make 
#this a bit more general to accept any integer.  A good lesson in debugging is to 
#simulate the code using reasonable values (putting yourself in other people's shoes).
#Try this for x = 26.  What happens?  What happens with negative numbers?  
#Another good tactic to debug is to print out intermediate data that can tell you if 
#it matches your own simulation.
 
 
x = 189
ans = 0
if x >= 0:
	while ans*ans < x:
		ans = ans + 1
		print "ans = ", ans
	if ans*ans != x:
		print x, " is not a perfect square"
	else:
		print ans
else:
	print x, " is negative.  Please try a positive integer"

The for loop 

# for VARIABLE in SOME COLLECTION:
#		STATEMENTS
#
#	One advantage of this over the while statment is that you do not need to explicity 
#update any variable which means that the loop will terminate so long as the collection 
#is finite
# A useful built-in Python function here is the function range()
 
#ex print out all divisors of a positive integer x.  
n = 270
for i in range (1, n):
	if n%i == 0:
		print i
#you could've also done this using a while loop (try it!) but you will have to explicity 
#initiialize or increment the variable i;  incrementing is automatic using a for loop
 
#as before, you can add in the condition that checks to see that n is positive when you've 
#checked that the code is behaving as you would expect for positive input
if n > 0:
	for i in range (1, n):
		if n%i == 0:
			print i
else:
		print x, " is negative.  Please try a positive integer"
 
#rewrite the square root example using a for loop

Once you're sure that your program is working, it makes sense to take out the debugging statements and wrap these into functions. 

def mySquareRoot(xIn):
	ansOut = 0;
	if xIn >= 0:
		while ansOut*ansOut < xIn:
			ansOut = ansOut + 1
		if ansOut*ansOut != xIn:
			print xIn, " is not a perfect square"
		else:
			return ansOut			
	else:
		print x, " is negative"
 
 
print mySquareRoot(211)
print mySquareRoot(121)
 
 
def myDivisors(xIn):
	if xIn > 0:
		for candidate in range(1, xIn):
			if xIn%candidate ==0:
				print candidate
	else:
		print xIn, " is negative.  Please try a positive integer"
 
myDivisors(270)
myDivisors(-5)

Now suppose in this last example that I didn't want to print out all of the divisors as soon as I called the function but that I just wanted to store them as an output (and possibly use all the elements or some of the elements later for another purpose) The problem is that the output here isn't a single number but a collection of numbers.

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