Writing a method that returns the last digit of an integer

Question

I'm given the following task, and can do it if I can use a string-

Write a method named lastDigit that returns the last digit of an integer. For example, lastDigit(3572) should return 2. It should work for negative numbers as well. For example, lastDigit(-947) should return 7.

Where this problem gets tricky for me is that I am not allowed to use a String to solve this problem. Here's what I have thus far-

public static int lastDigit(int d) {  // d is the integer they call
    // i know that whatever goes here will be something like this
    int b = charAt(length - 1);
    return b;
}

Any tips? Thanks!

modulus : $a % $b Remainder of $a divided by $b. Decimal: `int % 10` — Misunderstood, May 05 '15 at 22:51
I don't really understand how this works. Can you give me more details? — vhbravo, May 07 '15 at 03:01

score 2 · Answer 1 · answered May 05 '15 at 23:01

2

Like this:

public static int lastDigit(int d) { 
     return Math.abs(d % 10);
}

answered May 05 '15 at 23:01

Kevin Walker

116
4

This won't work for negative numbers. – mrQWERTY May 05 '15 at 23:14
@Renren29: Actually, it will work with negatives just fine. That's why there is a Math.abs. – Kevin Walker May 05 '15 at 23:17
Have you tried putting in -9 for d? – mrQWERTY May 05 '15 at 23:52
However, I'm confused why this works. Normally, -9 % 10 evaluates to 1, and then abs is applied to it...which should yield 1. However, I just ran this code and it produces 9. Which confuses me. – mrQWERTY May 06 '15 at 00:06
1

@Renren29 The `%` operator in Java will return negatives. `m % n` will have a result `x` such that `-m < x < m`. So `-9 % 10` will return `-9`; `Math.abs(-9)` is trivially 9. – M. Shaw May 06 '15 at 00:09

score 0 · Answer 2 · answered May 05 '15 at 22:52

0

public static int lastDigit(int d) { 
    return Math.abs(d-((int)(d/10))*10);
}

answered May 05 '15 at 22:52

Alan Zeng

74
1
7

This works, thank you! But can you explain what d/10 * 10 does exactly? – vhbravo May 05 '15 at 22:56
1

@vhbravo This is not the best way. The comments guided you to the correct way to do it (modulus). – user1803551 May 05 '15 at 22:57
1

@vhbravo, for example (int)24/10 becomes 2, multiply 2 by 10 becomes 20. The last digit is zeroed out, basically. But using modulo is a better approach. – Alan Zeng May 05 '15 at 23:02

score 0 · Answer 3 · edited May 06 '15 at 06:00

0

public static int lastDigit(int d) { 
    // using modulo
    if (d < 0) {
        return 10 - (d % 10);
    } else {
        return d % 10;
    }
}

edited May 06 '15 at 06:00

Ziem

6,579
8
53
86

answered May 05 '15 at 23:09

Alan Zeng

74
1
7

1

Why did you add a second answer instead of editing your original one? – Marc K May 05 '15 at 23:17

Writing a method that returns the last digit of an integer

3 Answers3