Getting rid of punctiations in a string in python

Question

This is a code I wrote for removing commas and full stops from a string

forbidden=(",",".")
text=input("Your sentence")
for i in range(0,len(text)):
    if text[i] in forbidden:
         text=text.replace(text[i],'')
         i-=1
print text

My input was a,n

But it gives an error saying that string index is out of range

Could anyone explain why?

Answer 1

I see two problems:

• len(text) is calculated exactly once at the beginning of the loop. The fact that text shrinks as you iterate over it, doesn't have any impact on what values i will have later on.
• decrementing i within your loop has no effect on the value of i in the next iteration. Python simply steamrolls over the old value without looking at it.

If you're absolutely committed to implementing this algorithm by iterating over the indices of text, then using a while loop would resolve both of these problems. A while loops' condition evaluates at every iteration, and you have full control over the value you're incrementing/decrementing.

forbidden=(",",".")
text=raw_input("Your sentence: ")
i = 0
while i < len(text):
    if text[i] in forbidden:
        text=text.replace(text[i],'')
    else:
        i+=1
print text

Output:

Your sentence: foo,bar,baz.qux,troz,zort.
foobarbazquxtrozzort

... But if you're open to a different approach, it would save you a lot of effort if you iterated over the forbidden characters instead of text.

forbidden=(",",".")
text=raw_input("Your sentence: ")
for c in forbidden:
    text = text.replace(c, "")
print text