CODEWITHSUNDEEP
pythonfor-loop
Cristian Diaconescu
Cristian Diaconescu

Reputation: 35681

Python: is there a C-like for loop available?

Can I do something like this in Python?

for (i = 0; i < 10; i++):
  if someCondition:
     i+=1
  print i

I need to be able to skip some values based on a condition

EDIT: All the solutions so far suggest pruning the initial range in one way or another, based on an already known condition. This is not useful for me, so let me explain what I want to do.

I want to manually (i.e. no getopt) parse some cmd line args, where each 'keyword' has a certain number of parameters, something like this:

for i in range(0,len(argv)):
    arg = argv[i]
    if arg == '--flag1':
       opt1 = argv[i+1]
       i+=1
       continue
    if arg == '--anotherFlag':
       optX = argv[i+1]
       optY = argv[i+2]
       optZ = argv[i+3]
       i+=3
       continue

    ...

Upvotes: 21

Views: 34787

Answers (12)

learning_physics
learning_physics

Reputation: 1

increment = 4                 #say
for i in range(n):
    #write your code here
    n = n + increment

This might be the simple solution to the problem if you just want to iterate through the array by skipping 4 members

Upvotes: -1

Jakobii
Jakobii

Reputation: 606

You can ensure that an index is incremented within a try...finally block. This solve the common problem of wanting to continue to the next index without having to copy/past i += 1 everywhere. Which is one of the main advantages the C-like for loop offers.

The main disadvantage to using a try...finally is having to indent your code once more. but if you have a while loop with many continue conditions its probably worth it.

Example

This example demonstrates that i still gets incremented in the finally block, even with continue being called. If i is not incremented its value will remain even forever, and the while loop will become infinite.

i = 0
while i < 10:
    try:
        print(i)

        if i % 2 == 0:
            continue

    finally:
        i += 1

without it you would have to increment i just before calling continue.

i = 0
while i < 10:
    print(i)

    if i % 2 == 0:
        i += 1 # duplicate code
        continue

    i += 1

Upvotes: 0

Netzsooc
Netzsooc

Reputation: 329

for (i = 0; i < 10; i++)
   if someCondition:
      i+=1
print i

In python would be written as

i = 0
while i < 10
   if someCondition
      i += 1
   print i
   i += 1

there you go, that is how to write a c for loop in python.

Upvotes: 14

Larry
Larry

Reputation: 314

You could first turn the argv list into a generator:

def g(my_list):
    for item in my_list:
        yield item

You could then step through the items, invoking the generator as required:

my_gen = g(sys.argv[1:]):
while True:
   try:
      arg = my_gen.next()
      if arg == "--flag1":
         optX = my_gen.next()
         opyY = my_gen.next()
         --do something
      elif arg == "--flag2":
         optX = my_gen.next()
         optY = my_gen.next()
         optZ = my_gen.next()
         --do something else
      ...
    except StopIteration:
       break

Upvotes: 1

Tony Veijalainen
Tony Veijalainen

Reputation: 5555

Your problem seems to be that you should loop not raw parameters but parsed parameters. I would suggest you to consider to change your decision not to use standard module (like the others).

Upvotes: -1

sberry
sberry

Reputation: 132088

Yes, this is how I would do it

>>> for i in xrange(0, 10):
...     if i == 4:
...         continue
...     print i,
...
0 1 2 3 5 6 7 8 9

EDIT
Based on the update to your original question... I would suggest you take a look at optparse

Upvotes: 24

catchmeifyoutry
catchmeifyoutry

Reputation: 7389

If you need to iterate over something, and need an index, use enumerate()

for i, arg in enumerate(argv):
    ...

which does the same as the questioner's

for i in range(0,len(argv)):
    arg = argv[i]

Upvotes: -1

SilentGhost
SilentGhost

Reputation: 319821

There are two things you could do to solve your problem:

  • require comma-separated arguments which are going to be grouped into the following option value, you could use getopt, or any other module then.

  • or do more fragile own processing:

    sys.argv.pop()
cmd = {}
while sys.argv:
    arg = sys.argv.pop(0)
    if arg == '--arg1':
        cmd[arg] = sys.argv.pop(0), sys.argv.pop(0)
    elif:
        pass
print(cmd)

Upvotes: 8

Teodor Pripoae
Teodor Pripoae

Reputation: 1394

 for i in xrange(0, 10):
    if i % 3 == 0
        continue
    print i

Will only values which aren't divisible by 3.

Upvotes: -1

Christian Oudard
Christian Oudard

Reputation: 49946

You probably don't actually need the indices, you probably need the actual items. A better solution would probably be like this:

sequence = 'whatever'
for item in sequence:
    if some_condition:
        continue
    do_stuff_with(item)

Upvotes: 2

Charles Beattie
Charles Beattie

Reputation: 5949

Strange way:

for x in (x for x in xrange(10) if someCondition):
    print str(x)

Upvotes: 5

kennytm
kennytm

Reputation: 523494

You should use continue to skip a value, in both C and Python.

for i in range(10):
  if someCondition:
     continue
  print(i)

Upvotes: 3

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