Checking if a number is a palindrome

Question

I've tried to check whether a number is a palindrome with the following code:

unsigned short digitsof (unsigned int x)
{
    unsigned short n = 0;
    while (x)
    {
        x /= 10;
        n++;
    }
    return n;
}

bool ispalindrome (unsigned int x)
{
    unsigned short digits = digitsof (x);

    for (unsigned short i = 1; i <= digits / 2; i++)
    {
        if (x % (unsigned int)pow (10, i) != x % (unsigned int)pow (10, digits - 1 + i))
        {
            return false;
        }
    }
    return true;
}

However, the following code isn't able to check for palindromes - false is always returned even if the number is a palindrome.

Can anyone point out the error?

(Please note: I'm not interested to make it into a string and reverse it to see where the problem is: rather, I'm interested to know where the error is in the above code.)

`digitsof` can be replaced mostly with a `log10` call. – chris Sep 17 '13 at 12:15 — chris, Sep 17 '13 at 12:15
Have you tried debugging it with simple examples? – Rob Sep 17 '13 at 12:16 — Rob, Sep 17 '13 at 12:16
Note also that `digitsof` returns wrong value for `0`. – Jarod42 Sep 17 '13 at 12:35 — Jarod42, Sep 17 '13 at 12:35
The number N is always a palindrome in base N-1 ;) – MSalters Sep 17 '13 at 12:58 — MSalters, Sep 17 '13 at 12:58

score 5 · Answer 1 · answered Sep 17 '13 at 12:32

5

I personally would just build a string from the number, and then treat it as a normal palindrome check (check that each character in the first half matches the ones at length()-index).

answered Sep 17 '13 at 12:32

Mats Petersson

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score 1 · Answer 2 · answered Sep 17 '13 at 12:33

1

x % (unsigned int)pow (10, i) is not the ith digit.

answered Sep 17 '13 at 12:33

Jarod42

203,559
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score 0 · Accepted Answer · answered Sep 17 '13 at 12:30

0

The problem is this:

x % (unsigned int)pow (10, i)

Lets try:

x =504405
i =3

SO I want 4.

x % 10^3 => 504405 %1000 => 405 NOT 4

How about

x / (unsigned int)pow (10, i -1) % 10

answered Sep 17 '13 at 12:30

Martin York

257,169
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score 0 · Answer 4 · answered Sep 17 '13 at 13:36

0

Just for more info! The following two functions are working for me:

double digitsof (double x)
{
    double n = 0;
    while (x > 1)
    {       
        x /= 10;
        n++;
    }
    return n;
}

bool ispalindrome (double x)
{
    double digits = digitsof (x);
    double temp = x;
    for(double i = 1; i <= digits/2; i++)
    {
        float y = (int)temp % 10;
        cout<<y<<endl;
        temp = temp/10;

        float z = (int)x / (int)pow(10 , digits - i);
        cout<<(int)z<<endl;
        x = (int)x % (int)pow(10 , digits - i);

        if(y != z)
            return false;

    }
    return true;        
}

answered Sep 17 '13 at 13:36

Soosh

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palindrome of `double` ? You mix `int`, `float`, `double` :/ `digitsof(0)` is still wrong. – Jarod42 Sep 20 '13 at 17:32
@Jarod42 I just post this for more info, you can change the types of variables. also this code is working. – Soosh Sep 20 '13 at 19:10

score 0 · Answer 5 · answered Jul 25 '18 at 05:37

Code to check if given number is palindrome or not in JAVA

import java.util.*;
public class HelloWorld{

private static int countDigits(int num) {
    int count = 0;
    while(num>0) {
        count++;
        num /= 10;
    }
    return count;
}
public static boolean isPalin(int num) {
    int digs = HelloWorld.countDigits(num);
    int divderToFindMSD = 1;
    int divderToFindLSD = 1;

    for (int i = 0; i< digs -1; i++)
    divderToFindMSD *= 10;

    int mid = digs/2;  

    while(mid-- != 0)
    {
        int msd = (num/divderToFindMSD)%10;
        int lsd = (num/divderToFindLSD)%10;
        if(msd!=lsd)
        return false;

        divderToFindMSD /= 10;
        divderToFindLSD *= 10;
    }
    return true;

}

 public static void main(String []args) {
    boolean isPalin = HelloWorld.isPalin(1221);
    System.out.println("Results: " + isPalin);
 }
 }

score 0 · Answer 6 · edited Feb 27 '19 at 15:24

I have done this with my own solution which is restricted with these conditions

Do not convert int to string.
Do not use any helper function.

var inputNumber = 10801

var firstDigit = 0

var lastDigit = 0

var quotient = inputNumber



while inputNumber > 0 {

    lastDigit = inputNumber % 10

    var tempNum = inputNumber

    var count = 0

    while tempNum > 0 {

        tempNum = tempNum / 10

        count = count + 1

    }

    var n = 1

    for _ in 1 ..< count {

        n = n * 10

    }

    firstDigit = quotient / n

    if firstDigit != lastDigit {

        print("Not a palindrome :( ")

        break

    }

    quotient = quotient % n

    inputNumber = inputNumber / 10

}

if firstDigit == lastDigit {

    print("It's a palindrome :D :D ")

}

Checking if a number is a palindrome

6 Answers6

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