for loop in Python

Question

In C/C++, I can have the following loop

for(int k = 1; k <= c ; k +=2)

How do the same thing in Python?

I can do this

for k in range(1,c):

In Python, which would be identical to

for(int k = 1; k <= c ; k++)

in C/C++.

Answer 1

You should also know that in Python, iterating over integer indices is bad style, and also slower than the alternative. If you just want to look at each of the items in a list or dict, loop directly through the list or dict.

mylist = [1,2,3]
for item in mylist:
    print item

mydict  = {1:'one', 2:'two', 3:'three'}
for key in mydict:
    print key, mydict[key]

This is actually faster than using the above code with range(), and removes the extraneous i variable.

If you need to edit items of a list in-place, then you do need the index, but there's still a better way:

for i, item in enumerate(mylist):
    mylist[i] = item**2

Again, this is both faster and considered more readable. This one of the main shifts in thinking you need to make when coming from C++ to Python.

Answer 2

Try using this:

for k in range(1,c+1,2):

Answer 3

The answer is good, but for the people that want this with range(), the form to do is:

range(end):

>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range(start,end):

 >>> list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

range(start,end, step):

 >>> list(range(0, 30, 5))
[0, 5, 10, 15, 20, 25]

Answer 4

If you want to write a loop in Python which prints some integer no etc, then just copy and paste this code, it'll work a lot

# Display Value from 1 TO 3  
for i in range(1,4):
    print "",i,"value of loop"

# Loop for dictionary data type
  mydata = {"Fahim":"Pakistan", "Vedon":"China", "Bill":"USA"  }  
  for user, country in mydata.iteritems():
    print user, "belongs to " ,country

Answer 5

In Python you generally have for in loops instead of general for loops like C/C++, but you can achieve the same thing with the following code.

for k in range(1, c+1, 2):
  do something with k

Reference Loop in Python.

Answer 6

In C/C++, we can do the following, as you mentioned

for(int k = 1; k <= c ; k++)
for(int k = 1; k <= c ; k +=2)

We know that here k starts with 1 and go till (predefined) c with step value 1 or 2 gradually. We can do this in Python by following,

for k in range(1,c+1):
for k in range(1,c+1,2):

Check this for more in depth.

Answer 7

The range() function in python is a way to generate a sequence. Sequences are objects that can be indexed, like lists, strings, and tuples. An easy way to check for a sequence is to try retrieve indexed elements from them. It can also be checked using the Sequence Abstract Base Class(ABC) from the collections module.

from collections import Sequence as sq
isinstance(foo, sq)

The range() takes three arguments start, stop and step.

1. start : The staring element of the required sequence
2. stop : (n+1)th element of the required sequence
3. step : The required gap between the elements of the sequence. It is an optional parameter that defaults to 1.

To get your desired result you can make use of the below syntax.

range(1,c+1,2)

Answer 8

Here are some example to iterate over integer range and string:

#(initial,final but not included,gap)
for i in range(1,10,2): 
  print(i); 
1,3,5,7,9

# (initial, final but not included)  
# note: 4 not included
for i in range (1,4): 
   print(i);
1,2,3 

#note: 5 not included
for i in range (5):
  print (i);
0,1,2,3,4 

# you can also iterate over strings
myList = ["ml","ai","dl"];  

for i in myList:
  print(i);
output:  ml,ai,dl

Answer 9

Use this instead of a for loop:

k = 1
while k <= c:
   #code
   k += 1