How to Read Inputs as Numbers

How can I read inputs as numbers?

Solution

Since Python 3, input returns a string which you have to explicitly convert to ints, with int, like this

x = int(input("Enter a number: "))
y = int(input("Enter a number: "))

You can accept numbers of any base and convert them directly to base-10 with the int function, like this

>>> data = int(input("Enter a number: "), 8)
Enter a number: 777
>>> data
511
>>> data = int(input("Enter a number: "), 16)
Enter a number: FFFF
>>> data
65535
>>> data = int(input("Enter a number: "), 2)
Enter a number: 10101010101
>>> data
1365

The second parameter tells what is the base of the numbers entered and then internally it understands and converts it. If the entered data is wrong it will throw a ValueError.

>>> data = int(input("Enter a number: "), 2)
Enter a number: 1234
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: invalid literal for int() with base 2: '1234'

For values that can have a fractional component, the type would be float rather than int:

x = float(input("Enter a number:"))

Differences between Python 2 and 3

Summary

• Python 2's input function evaluated the received data, converting it to an integer implicitly (read the next section to understand the implication), but Python 3's input function does not do that anymore.
• Python 2's equivalent of Python 3's input is the raw_input function.

Python 2.x

There were two functions to get user input, called input and raw_input. The difference between them is, raw_input doesn't evaluate the data and returns as it is, in string form. But, input will evaluate whatever you entered and the result of evaluation will be returned. For example,

>>> import sys
>>> sys.version
'2.7.6 (default, Mar 22 2014, 22:59:56) \n[GCC 4.8.2]'
>>> data = input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
(22, <type 'int'>)

The data 5 + 17 is evaluated and the result is 22. When it evaluates the expression 5 + 17, it detects that you are adding two numbers and so the result will also be of the same int type. So, the type conversion is done for free and 22 is returned as the result of input and stored in data variable. You can think of input as the raw_input composed with an eval call.

>>> data = eval(raw_input("Enter a number: "))
Enter a number: 5 + 17
>>> data, type(data)
(22, <type 'int'>)

Note: you should be careful when you are using input in Python 2.x. I explained why one should be careful when using it, in this answer.

But, raw_input doesn't evaluate the input and returns as it is, as a string.

>>> import sys
>>> sys.version
'2.7.6 (default, Mar 22 2014, 22:59:56) \n[GCC 4.8.2]'
>>> data = raw_input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
('5 + 17', <type 'str'>)

Python 3.x

Python 3.x's input and Python 2.x's raw_input are similar and raw_input is not available in Python 3.x.

>>> import sys
>>> sys.version
'3.4.0 (default, Apr 11 2014, 13:05:11) \n[GCC 4.8.2]'
>>> data = input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
('5 + 17', <class 'str'>)

Convert input str to int in python

By default, the input type is string. To convert it into integer, just put int before input. E.g.

number = int(input("Please guess what number I'm thinking of. HINT: it's between 1 and 30: "))
print(type(number))

Sample of the output:

Please guess what number I'm thinking of. HINT: it's between 1 and 30: 30
<class 'int'>   # it shows that the input type is integer

ALTERNATIVE

# any input is string
number = input("Please guess what number I'm thinking of. HINT: it's between 1 and 30: ")   
try:                      # if possible, try to convert the input into integer
    number = int(number)
except:                   # if the input couldn't be converted into integer, then do nothing
    pass
print(type(number))       # see the input type after processing

Sample of the output:

Please guess what number I'm thinking of. HINT: it's between 1 and 30: 25    # the input is a number 25
<class 'int'>   # 25 is possible to convert into integer. So, the type is integer

Please guess what number I'm thinking of. HINT: it's between 1 and 30: AAA   # the input is a number AAA
<class 'str'>   # AAA is impossible to convert into integer. So, the type remains string

C++ How to read input N and then read a series of numbers N long?

Just don't judge harshly, please. I suggest another way to solve this problem.

#include <iostream>
#include <vector>
using namespace std;
int special, count;
void dfs(int current, int previous, vector<int>& visited, vector<int>& input)
{
    if(visited[current]==1)
    {
        return;
    }
    visited[current]=1;
    if(current==(input.size()-1))
    {
        special=input[current];
    }
    for(int next=(current+1); next<input.size(); ++next)
    {
        if(next==previous)
        {
            continue;
        }
        dfs(next, current, visited, input);
    }
    if(special==input[current])
    {
        ++count;
    }
    if(current==0)
    {
        --count;
        cout<<count;
    }
    return;
}
void solve()
{
    int quantity;
    cin>>quantity;
    int number;
    vector<int> numbers;
    for(int i=0; i<quantity; ++i)
    {
        cin>>number;
        numbers.push_back(number);
    }
    vector<int> visited(quantity);
    dfs(0, -1, visited, numbers);
    return;
}
int main()
{
    solve();
    return 0;
}

Input:

9
1 7 3 2 4 7 5 8 7

Here is the result:

2

How to check if string input is a number?

Simply try converting it to an int and then bailing out if it doesn't work.

try:
    val = int(userInput)
except ValueError:
    print("That's not an int!")

See Handling Exceptions in the official tutorial.

Read user input numbers and perform a calculation in Python

in fact you need just:

num1 = int(input())
num2 = int(input())
num3 = num1+num2
print(num3)

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