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

Question

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

    ...

Asked By: Cristian Diaconescu

||

Source

Answer 1

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

Answered By: sberry

Answer 2

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

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

Answered By: kennytm

Answer 3

Strange way:

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

Answered By: Charles Beattie

Answer 4

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)

Answered By: Christian Oudard

Answer 5

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

Will only values which aren’t divisible by 3.

Answered By: Teodor Pripoae

Answer 6

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)

Answered By: SilentGhost

Answer 7

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]

Answered By: catchmeifyoutry

Answer 8

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).

Answered By: Tony Veijalainen

Answer 9

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

Answered By: Larry

Answer 10

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.

Answered By: Netzsooc

Answer 11

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

Answered By: learning_physics

Answer 12

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

Answered By: Jakobii

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

Question:

Answers:

Example