How to remove substring from string in Python 3

Question

I am new to Python 3. Currently, I am working on a project that requires ne to go through a csv file (without using the csv modules) and extract numbers. While I have been able

Answer 1

Current code issues

replace doesn't work in-place. Instead it returns a string with the replacement done. Hence, for fix #1 you should change your statement from:

row[i].replace("\\n", " ")

to:

row[i] = row[i].replace("\\n", " ")

However, the bigger problem is the iteration over the list obtained from the .split(",") operation.

In fact, your iteration runs short of 1 element, hence never touches the last item, and thus never removes the \n. Let's do some math:

row = ['13.9', ' 5.2', ' 3.4\n']
# len(row)  == 3
# len(row) - 1 == 2
# range(len(row) - 1) == [0 1], which will do 2 iterations instead of 3

So, fix #2 would be to correct that for loop, which should look something like:

for row in open(filename):
    row = row.split(",")
    for i in range(len(row)):  # notice the absence of -1
        row[i] = row[i].replace("\n", "")
        row[i] = float(row[i])
    lines.append(row)

Better way

Since every line of a CSV file has an ending \n, you might as well strip it before you split the columns and perform the conversion str to float via map, like this:

lines = []
for row in open(filename):
    row = row.strip().split(",")  # first remove the "\n" then split
    row = list(map(float, row))   # [13.9, 5.2, 3.4]
    lines.append(row)

Answer 2

Strings are immutable in Python, so you will need to always assign row[i] back to the modified version of itself:

for row in open(filename):
    row = row.split(",")
    for i in range(len(row) - 1):
        row[i] = row[i].replace("\n", "")  # CHANGE HERE
        row[i] = float(row[i])
    lines.append(row)

Note: You don't need to double escape the backslash in \n when using regular string replace.