How can I exclude negative numbers?

Question

I am not sure how to prevent a negative number from affecting the final calculations.

I don't know what else to try.

for month in months:
    rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
    if rainfall_answer < 0:
        print("Input must be positive.")
        rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
    elif rainfall_answer > 0:
        rainfall.append(rainfall_answer)

I expect the invalid input to not be included in the final results.

Possible duplicate of [Asking the user for input until they give a valid response](https://stackoverflow.com/questions/23294658/asking-the-user-for-input-until-they-give-a-valid-response) — Peter Wood, Apr 16 '19 at 22:39

score 1 · Answer 1 · answered Apr 16 '19 at 22:11

If I understand you correctly, you can use a while loop to repeatedly prompt the user until the input is non-negative:

for month in months:
    rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
    while rainfall_answer < 0:
        print("Input must be positive.")
        rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))

    rainfall.append(rainfall_answer)

score 1 · Answer 2 · answered Apr 16 '19 at 22:14

You're almost there, one trick you can use is to set rainfall_answer to a negative (invalid) value, then you can repeat input reading until you have a positive value in rainfall_answer:

for month in months:
    rainfall_answer = -1
    while rainfall_answer < 0:
        rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
    rainfall.append(rainfall_answer)

score 1 · Answer 3 · edited Apr 16 '19 at 23:17

You can make use of while loop to loop over the input and also you need to use a try except blocks to handle and raise exception in this case excluding negative numbers

for month in months:
  while True:
    try:
        rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
        if rainfall_answer < 0:
            raise ValueError
        break
    except ValueError:
        print ("Input must be positive")
rainfall.append(rainfall_answer)

score 0 · Answer 4 · answered Apr 16 '19 at 22:13

You need to loop over the input.

for month in months:
    while True:
        rainfall_answer = int(input("What was the rainfall in {}? ".format(month)))
        if rainfall_answer >= 0:
            break  # exit while loop because you got valid input
        else:
            print('Input must be positive')
    rainfall.append(rainfall_answer)

How can I exclude negative numbers?

4 Answers4