Python Programming for Biology: Bioinformatics and Beyond

Skipping and breaking loops

Python has two ways of affecting the flow of the for or while loop inside the block. The

continue  statement  means  that  the  rest  of  the  code  in  the  block  is  skipped  for  this

particular item in the collection, i.e. jump to the next iteration.

---

values = [10, -5, 3, -1, 7]

total = 0

for v in values:

if v < 0:

continue # skip the rest of the block for this v

total += v

# At the end total is 20

So  here  the  negative  items  are  left  out  of  the  sum.  An  alternative  way  of  writing  the

above without the continue statement is to only add the numbers we do want, rather than

skipping those we don’t:

total = 0

for v in values:

if v >= 0:

total += v

# At the end total is 20

It is a matter of taste which style you prefer. Note that either way the variable v at the

end will still be the last value from the list, whether or not it is negative.

The second way of affecting the flow in a for or while block is the break statement. In

contrast  to  the  continue  statement,  this  immediately  causes  all  looping  to  finish,  and

execution is resumed at the next statement after the loop.

total = 0

for v in values:

if v == 3:

break

total += v

# At the end total is 5, v is 3

Here the third item in the list is 3,  so  at  this  point  the  loop  terminates  and  the  sum  is

only of the first two items in the list. Note that the loop variable, v, never makes it to the

rest of the list, so stays 3 when the loop terminates.

If you have a for loop inside a for loop, then a break in the inner loop only results in a

jump to the end of the inner loop, not the outer one. This can sometimes be a pain but the

alternative choice would in general have been worse.

There  is  one  extra  twist  with  for  loops  when  you  have  a  break  statement  in  the  loop.

Often you want to know if you have broken out of the loop, but otherwise execute some

code if you have not, and Python has a special syntax for adding such code. For example,

here we take values for x and test if they are greater than 5 and less than 10. If we find

such a value foundInRange is defined to be True and the loop is stopped immediately with

break.  If  no  value  passes  the  test,  then  the  loop  reaches  the  end  and  program  execution

reaches the else: block where foundInRange is alternatively defined as False.

---

for x in someList:

if 5 < x < 10: # Is x between 5 and 10?

foundInRange = True

break # Do not test any more

else:

foundInRange = False # Found nothing

print(foundInRange)

Thus this is a second context in which else can occur. Note that the else lines up with

the  for.  The  code  in  the  else  block  is  executed  only  if  the  break  statement  has  not  been

executed.