Converting an integer into an array of digits [C]

Question

I'm a beginner at programming and I've never posted on here before. I've had a search for answers to my question and one such thread was here ...but I can't get their method to work.

I'm trying to learn C by doing some of the Project Euler challenges. For one of the challenges I need to check if a number is palindromic. I thought I'd do this by loading my target integer, called product, into an array before checking pairs of elements against each other (looking for symmetry).

First I check to see how long my product is:

productLength = ceil(log10(product));

Then I have a loop to try and load that integer into my array called inspection:

for(x = productLength; x <= 0; x--) //decrement from product length and loop
{
    inspection[x - 1] = product % 10; //transfer product values to inspection array
    product /= 10; //prepare product value for next pass (move digits one place to the right)
}

This doesn't seem to work. I'm aware it's probably some silly mistake but I can't spot it.

You already got answers, but generally you should explain your problem better, not just "this doesn't seem to work". What result do you expect and what do you actually get? - Also single-stepping in the debugger would probably reveal the mistake quickly. — Martin R
– Martin R, Commented Jan 7, 2014 at 17:50
Did you take into consideration debugging this you own using a real debugger? — alk
– alk, Commented Jan 7, 2014 at 18:07

Devolus · Accepted Answer · 2014-01-07 18:04:56Z

1

This line

for(x = productLength; x <= 0; x--)

should be:

for(x = productLength; x >= 0; x--)

However, here you have another problem.

    inspection[x - 1] = product % 10; //transfer product values to inspection array

The loop will reach x = 0 inclusive, so when x = 0, this will cause an invalid index.

So the loop should look like this:

for(x = productLength-1; x >= 0; x--)
{
    inspection[x] = product % 10;
    product /= 10;
}
inspection[productLength] = 0;

edited Jan 7, 2014 at 18:04

answered Jan 7, 2014 at 17:56

Devolus

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1 Comment

alk Over a year ago

for( ...; x >= 0; ...

frowningpants · Accepted Answer · 2014-01-07 17:48:14Z

0

Your for loop should be:

for(x = productLength; x >= 0; x--)

instead of for(x = productLength; x <= 0; x--)

answered Jan 7, 2014 at 17:48

frowningpants

1

3 Comments

alk Over a year ago

The loop shall start with productLength - 1.

frowningpants Over a year ago

That is probably more standard, but he is setting: inspection[x - 1]

alk Over a year ago

Then it shall also be x > 0, otherwise you get UB for x = 0.

alk · Accepted Answer · 2014-01-07 18:03:43Z

0

So this line

productLength = ceil(log10(product));

should easier be

productLength = log10(product) + 1;

As

unsigned x = <some value > 0>;
size_t s = log10(x);

is

0 for all values of x < 10     
1 for all values of x < 100
2 for all values of x < 1000
...

Using ceil() works, but I feel using it is overkill.

The solution:

size_t productLength = log10(product) + 1; 
unsigned char inspection[productLength];

for (size_t x = productLength - 1; x >= 0; --x) //decrement from product length and loop
{
    inspection[x] = product % 10; //transfer product values to inspection array
    product /= 10; //prepare product value for next pass (move digits one place to the right)
}

edited Jan 7, 2014 at 18:03

answered Jan 7, 2014 at 17:52

alk

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Comments

meaning-matters · Accepted Answer · 2014-01-07 18:17:41Z

0

If it's an integer you need to palindrome check, then you can just print it in a string and start comparing the characters from both ends:

int palindromic(int n)
{
    char str[20];
    int  result;
    int  i;

    sprintf(str, "%d", n);
    int length = strlen(str);

    for (result = 1, i = 0; i < length / 2 && result == 1; i++)
    {
        result = (str[i] == str[length - 1 - i]);
    }

    return result;
}

answered Jan 7, 2014 at 18:17

meaning-matters

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