how to check integer and string in a list

Question

My list is ['1','2','to','3']

I need to write a logic that

• convert '1' '2' which is string to 1, 2 which is integer
• print an error msg since 'to' string is included and cannot be converted to integer

Here's the code I have right now:

def average_file(filename):
 inputFile = open(filename, "r")
 inList = []
 results = []
 n = []
 for line in inputFile:
 inList.append(line.strip()) #inList.append(line)
 n = [int(elem) for elem in inList if elem.isdigit()] #I only remove the string and leave integer in my list, but I need a check logic to print error msg
 results = list(map(int, n))
 inputFile.close()
 results = sum(results)/len(results)
 return results

• TrebledJ: Not entirely sure what your question is right now and how it ties in with your code. Why are you computing results every time a line is read? (It's not exactly wrong, but it's fishy.)
• Prayson W. Daniel: What do you want to do if you get non numbe? Do you want to throw an error and continue or break?

Answer 1

Few things:

• The pythonic way to do it is to expect it to be an all digit value and handle the error when it is not.
• You can use with to handle your file lifetime.
• You can calculate sum and count of elements during the reading without saving additional array (and therefore know the average).
• strip is redundant when parsing to int like that int(variable):

There you go:

def average_file(filename):
 summary = 0
 count = 0
 with open(filename, "r") as inputFile:
 for line in inputFile:
 try:
 summary += int(line)
 count += 1
 except ValueError as e:
 print('Can not parse "{0}" to a number'.format(line))
 # If reached here one of the values in the file is not a number and None is returned immediately
 return None
 # If count is 0 return None, otherwise return the average
 return (summary / count) if count else None

---

The answer was edited after some clarifications from OP:
Immediately return None when one of the values is not a number.

• potentialwjy: I input a test file (.txt) with string. I expect the error msg. However, it returned the float 51.22222222222222. the below is the content of my testing file 1 2 3 4 5 wut 6 7 8 9
• Maor Refaeli: @potentialwjy It makes no sense the average from my code came out 51.222 from input that the maximum value was 9. Something else is wrong there. For the array [1, 2, 3, 4, 5, 'wut', 6, 7, 8, 9] the result will be 5 and on the console you will see Can not parse "wut" to a number.
• Maor Refaeli: @potentialwjy Is each value on the file is in a new line?
• potentialwjy: my fault, I used the testing file in the class which is invisible..Now I use my testing file 1,2,3...wut...9, it shows: Can not parse "wut " to a number Out[192]: 5.0 the expectation is to have the error msg only. It should not have the 5.0 as average in this case. Any thought? Thanks for your help!
• Maor Refaeli: @potentialwjy I edited my answer (added a return None at the except handler), have a look

Answer 2

convert '1' '2' which is string to 1, 2 which is integer print an error msg since 'to' string is included and cannot be converted to integer

source = ['1', '2', 'to', '3']
result = []
for item in source:
 try:
 result.append(int(item))
 except ValueError as ex:
 print('Not integer: {}'.format(item))
print(result)

• grapes: Agree, just have written ex mechanically

Answer 3

Attempt to convert each item to the list of results. If the conversion fails, print an error message.

l = ['1','2','to','3']
result = []
for item in l:
 try:
 result.append(int(item))
 except ValueError:
 print(item)

Answer 4

You can use a try/except block to separate the valid integer literals from everything else:

candidates = ['1','2','to','3']
for candidate in candidates:
 try: # attempt the conversion
 value = int(candidate)
 except ValueError: # conversion failed!
 print(candidate, 'is not an integer')
 else: # conversion succeeded
 print(candidate, 'is the integer', value)

In your case, you can just collect the values in the else clause:

results = []
with open(filename) as input_file:
 for line in inputFile:
 try:
 value = int(line.strip())
 except ValueError:
 print(line.strip(), 'is not an integer')
 else:
 results.append(value)

• potentialwjy: your solution almost works...but my testing result shows: wut is not an integer 5.0 my testing file is a txt with below info: 1 2 3 4 5 wut 6 7 8 9

Answer 5

l = ['1', '2', 'word', '4']

You can do:

n = [int(i) if i.isdigit() else print('\nNot able to convert: '+i) for i in l]

Output:

Not able to convert: word
l = [1, 2, None, 4]

• TrebledJ: Wouldn't that add None elements to n... 🤔 Edit: I'm getting a syntax error for else print.
• RoadRunner: I don't think print statements are suitable for list comprehensions.
• Francesco Pegoraro: Yeah you are right. I should have tried it, I'll modify it