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Tutorial

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): # or range(5)
    print x

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

# Prints out 3,5,7
for x in xrange(3, 8, 2): # or range(3, 8, 2)
    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

can we use "else" clause for loops?

unlike languages like C,CPP.. we can use else for loops. When the loop condition of "for" or "while" statement fails then code part in "else" is executed. If break statement is executed inside for loop then the "else" part is skipped. Note that "else" part is executed even if there is a continue statement.

Here are a few examples:

# Prints out 0,1,2,3,4 and then it prints "count value reached 5"

count=0
while(count<5):
    print count
    count +=1
else:
    print "count value reached %d" %(count)

# Prints out 1,2,3,4
for i in xrange(1,10):
    if(i%5==0):
        break
    print i
else:
    print "this is not printed because for loop is terminated because of break but not due to fail in condition"

Exercise

Loop through and print out all even numbers from the numbers list in the same order they are received. Don't print any numbers that come after 237 in the sequence.

Tutorial Code

numbers = [ 951, 402, 984, 651, 360, 69, 408, 319, 601, 485, 980, 507, 725, 547, 544, 615, 83, 165, 141, 501, 263, 617, 865, 575, 219, 390, 984, 592, 236, 105, 942, 941, 386, 462, 47, 418, 907, 344, 236, 375, 823, 566, 597, 978, 328, 615, 953, 345, 399, 162, 758, 219, 918, 237, 412, 566, 826, 248, 866, 950, 626, 949, 687, 217, 815, 67, 104, 58, 512, 24, 892, 894, 767, 553, 81, 379, 843, 831, 445, 742, 717, 958, 609, 842, 451, 688, 753, 854, 685, 93, 857, 440, 380, 126, 721, 328, 753, 470, 743, 527 ]

your code goes here

Expected Output

402 984 360 408 980 544 390 984 592 236 942 386 462 418 344 236 566 978 328 162 758 918

Solution

numbers = [ 951, 402, 984, 651, 360, 69, 408, 319, 601, 485, 980, 507, 725, 547, 544, 615, 83, 165, 141, 501, 263, 617, 865, 575, 219, 390, 984, 592, 236, 105, 942, 941, 386, 462, 47, 418, 907, 344, 236, 375, 823, 566, 597, 978, 328, 615, 953, 345, 399, 162, 758, 219, 918, 237, 412, 566, 826, 248, 866, 950, 626, 949, 687, 217, 815, 67, 104, 58, 512, 24, 892, 894, 767, 553, 81, 379, 843, 831, 445, 742, 717, 958, 609, 842, 451, 688, 753, 854, 685, 93, 857, 440, 380, 126, 721, 328, 753, 470, 743, 527 ]

your code goes here

for number in numbers: if number == 237: break

if number % 2 == 1:
    continue

print number