In programming a loop is a statement or block of statements that
is executed repeatedly.
for loops are used to do something a
fixed number of times
(where the number is known at the start of the loop). Here is an
sumsquares = 0 # sumsquares must have a value because we increment # it later. for i in [0, 1, 2, 3, 4, 5]: print "i now equal to:", i sumsquares = sumsquares + i**2 # sumsquares incremented here print "sum of squares now equal to:", sumsquares print "------" print "Done."
The indentation of this program is essential. Copy it into an empty
module. IDLE will try and help you with the indentation but if
it doesn't get it right use the TAB key3.6. Don't
forget the colon at the end of the
for line. The role of
indentation is explained below but first let us consider the
loop. Try and work out what a for loop does from the program's output.
for loop can actually be considered as a foreach
loop: ``For each thing in the list that follows, execute some
statements''. Remember a list is enclosed in square brackets. On each
iteration of the loop, the next item in the list is assigned to
i. That is to say, the first time around,
i = 0, the
i = 1, etc.
Indentation is an intrinsic part of Python's syntax. In most
is voluntary and a matter of personal taste. This is not the case in
Python. The indented statements that follow the
for line are
called a nested block. Python understands the nested block to
be the section of code to be repeated by the
Note that the
print "Done." statement is not indented. This means
it will not be repeated each time the nested block is. The blank line
after the nested block is not strictly necessary, but is encouraged as
it aids readability.
The editor and interactive interpreter will try and indent for you. If you are using the interpreter it will keep giving you indented lines until you leave an empty line. When you do this, the condition will be tested and if appropriate the nested block will be executed.