Loops and Iteration
An Infinite Loop
n = 5 while n > 0 : print 'Lather’ print 'Rinse' print 'Dry off!'
n = 0 while n > 0 : print 'Lather' print 'Rinse' print 'Dry off!'
Breaking Out of the Loop
• The break statement ends the current loop and jumps to the statement immediately following the loop • It is like a loop test that can happen anywhere in the body of the loop
while True: line = raw_input('> ') if line == 'done' : break print line print 'Done!'
Continue
The continue statement ends the current iteration and jumps to the top of the loop and starts the next iteration
while True: line = raw_input('> ') if line[0] == '#' : continue if line == 'done' : break print line print 'Done!'
Indefinite Loops
• While loops are called “indefinite loops” because they keep going until a logical condition becomes False • The loops we have seen so far are pretty easy to examine to see if they will terminate or if they will be “infinite loops” • Sometimes it is a little harder to be sure if a loop will terminate
Definite Loops
• Quite often we have a list of items of the lines in a file - effectively a finite set of things • We can write a loop to run the loop once for each of the items in a set using the Python for construct • These loops are called “definite loops” because they execute an exact number of times • We say that “definite loops iterate through the members of a set”
for i in [5, 4, 3, 2, 1] : print i print 'Blastoff!'
The "is" and is not " Operator
• Python has an is operator that can be used in logical expressions • Implies “is the same as” • Similar to, but stronger than == • is not also is a logical operator
smallest = None print 'Before' for value in [3, 41, 12, 9, 74, 15] : if smallest is None : smallest = value elif value < smallest : smallest = value print smallest, value print 'After', smallest
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