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 builtin 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.