The sum of the first and the last digits in the number (Editorial)

Algorithm Analysis

The last digit of the number $n$ equals last = n \% 10.

We divide the number $n$ by 10 until it is greater than 9. Now $n$ will contain the first digit of the input number: first = n.

Find and output the sum of the first $first$ and last $last$ digit.

Solution using a string. Read the number as a string s. If the string starts with the ‘-‘ symbol (the input number is negative), then the first digit of the number is located in s[1]. Find the last digit by calculating the length of the string.

Algorithm Implementation

Read the input number $n$.

scanf("%d", &n);

If the number $n$ is negative, then change the sign of the number to the opposite.

if (n < 0) n = -n;

The last digit of the number $n$ equals last = n \% 10.

last = n % 10;

Divide the number $n$ by 10 until it is greater than 9. Now $n$ contains the first digit of the input number.

while (n > 9) n = n / 10;

first = n;

Calculate and output the answer.

res = first + last;
printf("%d\n", res);

String-based Algorithm Implementation

Read the input number into a string s.

char s[100];

Read the input number. Set pos equal to the index indicating the first digit of the number.

gets(s);
pos = (s[0] == '-') ? 1 : 0;

Find the sum of the first s[pos] – ‘0’ and the last s[strlen(s) – 1] – ‘0’ digit.

sum = s[pos] + s[strlen(s)-1] - 2*'0';

Output the answer.

printf("%d\n",sum);

Java Implementation

import java.util.*;

public class Main
{
    public static void main(String[] args) 
    {
        Scanner con = new Scanner(System.in);
        char[] s = con.nextLine().toCharArray();
        int pos = 0;
        if (s[0] == '-') pos = 1;

        int res = s[pos] + s[s.length - 1] - 2 * '0';      
        System.out.println(res);
        con.close();
    }
}

Python Implementation

Read the input number $n$.

n = int(input())

If the number $n$ is negative, then change the sign of the number to the opposite.

if n < 0: n = -n

Convert the number $n$ to a string.

n = str(n)

Output the sum of the first and last digit.

print(int(n[0]) + int(n[-1]))