Trying to find all the commas in the list, what is wrong with this?

Question

a = [',', 'hello',',', 'pear']

for i in a:
    d = 0
    c = []
    b = a[d]
    if b == ',':
        c.append(b)
        d += 1
    else:
        d += 1

print c

This only seems to return one comma when there are two within the code. How can I fix this?

Answer 1

The problem is that you reset d to zero at the start of every iteration, and clear c.

You need to move the initialization code outside the loop.

While doing this, you could simplify the entire loop like so:

a = [',', 'hello',',', 'pear']

c = []
for b in a:
    if b == ',':
        c.append(b)

print c

Now observe that code like this lends itself very well to being expressed as a list comprehension:

a = [',', 'hello',',', 'pear']

c = [b for b in a if b == ',']

print c

Answer 2

You are reassigning an empty list to c every iteration. You can change your logic like this

c = []                # Initialized before the loop
for i in a:
    if i[0] == ',':
        c.append(b)

The best way to write this, is to use list comprehension, like this

c = [item for item in a if item == ","]

This particular way of comprehending lists is called as filtering a list.