Python: Loops

Loops

There are two types of loops in Python, for and while.

The “for” loop

For loops iterate over a given sequence. Here is an example:

primes = [2,3,5,7]
for prime in primes:
    print prime

For loops can iterate over a sequence of numbers using the “range” and “xrange” functions. The difference between range and xrange is that the range function returns a new list with numbers of that specified range, whereas xrange returns an iterator, which is more efficient. (Python 3 uses the range function, which acts like xrange). Note that the xrange function is zero based.

# Prints out the numbers 0,1,2,3,4
for x in xrange(5):
    print x

# Prints out 3,4,5
for x in xrange(3,6):
    print x

“while” loops

While loops repeat as long as a certain boolean condition is met. For example:

# Prints out 0,1,2,3,4

count = 0
while count < 5:
    print count
    count += 1  # This is the same as count = count + 1

“break” and “continue” statements

break is used to exit a for loop or a while loop, whereas continue is used to skip the current block, and return to the “for” or “while” statement. A few examples:

# Prints out 0,1,2,3,4

count = 0
while True:
    print count
    count += 1
    if count >= 5:
        break

# Prints out only odd numbers - 1,3,5,7,9
for x in xrange(10):
    # Check if x is even
    if x % 2 == 0:
        continue 

print x