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# Test if an integer is a palindrome in c language

Hello mates,

Does anyone know how to write a function that tests if an integer is a
palindrome in C language?

Aug 14 '07 #1
20 13101 Wabz wrote:
Does anyone know how to write a function that tests if an integer is a
palindrome in C language?
Yes. I'm quite sure that someone does.

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Aug 14 '07 #2
On Aug 14, 1:19 pm, Kevin Handy <k...@srv.netwrote:
Wabz wrote:
Does anyone know how to write a function that tests if an integer is a
palindromein C language?

Yes. I'm quite sure that someone does.

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Could this someone possibly help me write the said program?

Aug 14 '07 #3
"Wabz" writes:
On Aug 14, 1:19 pm, Kevin Handy <k...@srv.netwrote:
>Wabz wrote:
Does anyone know how to write a function that tests if an integer is a
palindromein C language?

Yes. I'm quite sure that someone does.
Could this someone possibly help me write the said program?
Start by extracting the individual digits of the integer. You may find the
modulo operator, %, helpful. There is another way, involving sprintf() in
<stdio.h>, but if this is a school assignment, the instructor probably
expects the modulo way.
Aug 14 '07 #4
On Tue, 14 Aug 2007 17:10:51 +0000, Wabz wrote:
Hello mates,

Does anyone know how to write a function that tests if an integer is a
palindrome in C language?
I assume you mean palindrome in base 10.
sprintf() it to a string, and then check whether the string is
palindrome:
len = strlen(str);
for (i = 0; i < len/2; i++) {
if (str[i] != str[len - i - 1])
break; /*it is not palindome*/
}

--
Army1987 (Replace "NOSPAM" with "email")
No-one ever won a game by resigning. -- S. Tartakower

Aug 14 '07 #5

"Wabz" <Wa*****@gmail.comwrote in message
Hello mates,

Does anyone know how to write a function that tests if an integer is a
palindrome in C language?
What base? Eleven is a palindrome when written in base 10,
but not when written in base 8.
Create a buffer large enough to hold the characters to display the
largest integer. Use sprintf to fill it. Check whether palindrome.
--
Fred L. Kleinschmidt
Boeing Associate Technical Fellow
Aero Stability and Controls Computing
Aug 14 '07 #6
Wabz wrote, On 14/08/07 18:42:
On Aug 14, 1:19 pm, Kevin Handy <k...@srv.netwrote:
>Wabz wrote:
>>Does anyone know how to write a function that tests if an integer is a
palindromein C language?
Yes. I'm quite sure that someone does.

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----http://www.newsfeeds.comThe #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Please don't quote things you are not commenting on, such as the
annoying text appended to Kevin's post.
Could this someone possibly help me write the said program?
Yes, I'm sure one of the people meeting those requirements could help if
they were so desired. However, knowledgeable people are unlikely to do
your homework for you when you have not even made an attempt. So if you
questions you have. The more effort you put in the more people will be
--
Flash Gordon
Aug 14 '07 #7
On Aug 14, 11:20 am, Army1987 <army1...@NOSPAM.itwrote:
On Tue, 14 Aug 2007 17:10:51 +0000, Wabz wrote:
Hello mates,
Does anyone know how to write a function that tests if an integer is a
palindrome in C language?

I assume you mean palindrome in base 10.
Otherwise (for instance) FFFF is a palindrome, but 65535 will say
'nope'
sprintf() it to a string, and then check whether the string is
palindrome:
len = strlen(str);
for (i = 0; i < len/2; i++) {
if (str[i] != str[len - i - 1])
break; /*it is not palindome*/

}
It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?

Aug 14 '07 #8
On Aug 14, 2:15 pm, user923005 <dcor...@connx.comwrote:
On Aug 14, 11:20 am, Army1987 <army1...@NOSPAM.itwrote:
On Tue, 14 Aug 2007 17:10:51 +0000, Wabz wrote:
Hello mates,
Does anyone know how to write a function that tests if an integer is a
palindrome in C language?
I assume you mean palindrome in base 10.

Otherwise (for instance) FFFF is a palindrome, but 65535 will say
'nope'
sprintf() it to a string, and then check whether the string is
palindrome:
len = strlen(str);
for (i = 0; i < len/2; i++) {
if (str[i] != str[len - i - 1])
break; /*it is not palindome*/
}

It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?
It seems that what numbers are not palindromes in some base might be a
more interesting question. Of course if we include bases up to that
number, then the answer is obviously 'none'.

#include <stdlib.h>
#include <string.h>
/* ltostr is from snippets (I fixed the broken bit). */
char *ltostr(long long num, char *string, size_t max_chars, unsigned
base)
{
char remainder;
int sign = 0;
if (base < 2 || base 36)
return ((void *) 0);
if (num < 0) {
sign = 1;
num = -num;
}
if (num == 0) /* bugbug:drc formerly wrong result here... */
return "0";
string[--max_chars] = '\0';
for (max_chars--; max_chars sign && num != 0; max_chars--) {
remainder = (char) (num % base);
if (remainder < 9)
string[max_chars] = remainder + '0';
else
string[max_chars] = remainder - 10 + 'A';
num /= base;
}
if (sign)
string[--max_chars] = '-';
if (max_chars 0)
memset(string, ' ', max_chars + 1);
return string + max_chars;
}

int ispal(const char *start)
{
const char *end = start + strlen(start) - 1;
while (end start) {
if (*start != *end) {
return 0;
}
start++;
end--;
}
return 1;
}

#include <stdio.h>
static char string = {0};
int main(void)
{
long long index;
unsigned base;
for (index = 0; index < 4000000000; index++) {
for (base = 2; base < 37; base++) {
long long val = index;
char *p = ltostr(val, string, sizeof string,
base);
if (isspace(*p))
p++;
if (ispal(p)) {
printf("%llu = %s is a palindrome in base %u\n", val,
p, base);
}
}
}
return 0;
}

Aug 14 '07 #9
/* Original ltostr() was hosed. Fixed version: */
static const char B36TAB[] =
{
'0', '1', '2', '3', '4', '5', '6', '7', '8',
'9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q',
'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'
};
#include <stdlib.h>
#include <string.h>
/* ltostr is from snippets (I fixed the broken bit). */
char *ltostr(long long num, char *string, size_t max_chars, unsigned
base)
{
char remainder;
int sign = 0;
if (base < 2 || base 36)
return ((void *) 0);
if (num < 0) {
sign = 1;
num = -num;
}
if (num == 0) /* bugbug:drc formerly wrong result
here... */
return "0";
string[--max_chars] = '\0';
for (max_chars--; max_chars sign && num != 0; max_chars--) {
remainder = (char) (num % base);
string[max_chars] = B36TAB[remainder];
num /= base;
}
if (sign)
string[--max_chars] = '-';
if (max_chars 0)
memset(string, ' ', max_chars + 1);
return string + max_chars;
}

int ispal(const char *start)
{
const char *end = start + strlen(start) - 1;
while (end start) {
if (*start != *end) {
return 0;
}
start++;
end--;
}
return 1;
}

#include <stdio.h>
static char string =
{0};
int main(void)
{
long long index;
unsigned base;
for (index = 0; index < 4000000000; index++) {
for (base = 2; base < 37; base++) {
long long val = index;
char *p = ltostr(val, string, sizeof string,
base);
if (isspace(*p))
p++;
if (ispal(p)) {
printf("%llu = %s is a palindrome in base %u\n", val,
p, base);
}
}
}
return 0;
}
/*
Sample output:
....
2290134 = E656E is a palindrome in base 20
2290141 = 84548 is a palindrome in base 23
2290156 = 1IEI1 is a palindrome in base 35
2290187 = 9H1H9 is a palindrome in base 22
2290220 = 36Q63 is a palindrome in base 29
2290252 = 11022100122011 is a palindrome in base 3
2290293 = 50805 is a palindrome in base 26
2290322 = 2OOO2 is a palindrome in base 30
2290353 = 925529 is a palindrome in base 12
2290378 = 4270724 is a palindrome in base 9
2290391 = HAHAH is a palindrome in base 19
2290460 = BG6GB is a palindrome in base 21
2290499 = 2ERE2 is a palindrome in base 31
2290501 = 1D3D1 is a palindrome in base 36
2290530 = 5LEL5 is a palindrome in base 25
2290534 = E666E is a palindrome in base 20
2290627 = 3K9K3 is a palindrome in base 28
2290641 = 1000101111001111010001 is a palindrome in base 2
2290658 = 20233033202 is a palindrome in base 4
2290670 = 84648 is a palindrome in base 23
2290671 = 9H2H9 is a palindrome in base 22
2290686 = 6LGL6 is a palindrome in base 24
2290735 = 13EE31 is a palindrome in base 18
2290738 = 48A84 is a palindrome in base 27
2290752 = HAIAH is a palindrome in base 19
2290850 = 25T52 is a palindrome in base 32
2290853 = 1O9O1 is a palindrome in base 34
2290901 = BG7GB is a palindrome in base 21
2290922 = 2290922 is a palindrome in base 10
....
*/
Aug 14 '07 #10
user923005 <dc*****@connx.comwrites:
/* Original ltostr() was hosed. Fixed version: */
<string-based palindrome tester snipped>

I like this way:

int pal_aux(int n, int r, int b)
{
return n == 0 ? r : pal_aux(n / b, r * b + n % b, b);
}

int is_pal_in_base(int n, int b)
{
return pal(n, 0, b) == n;
}

(not well tested, but it *looks* sound!)

--
Ben.
Aug 14 '07 #11
On Aug 14, 3:33 pm, Ben Bacarisse <ben.use...@bsb.me.ukwrote:
user923005 <dcor...@connx.comwrites:
/* Original ltostr() was hosed. Fixed version: */

<string-based palindrome tester snipped>

I like this way:

int pal_aux(int n, int r, int b)
{
return n == 0 ? r : pal_aux(n / b, r * b + n % b, b);
}

int is_pal_in_base(int n, int b)
{
return pal(n, 0, b) == n;
/*Did you mean:*/
return pal_aux(n, 0, b) == n;
}

(not well tested, but it *looks* sound!)
Certainly much brighter than converting to character.
But it makes it harder to see the answers.

Aug 14 '07 #12
user923005 <dc*****@connx.comwrites:
On Aug 14, 3:33 pm, Ben Bacarisse <ben.use...@bsb.me.ukwrote:
>user923005 <dcor...@connx.comwrites:
/* Original ltostr() was hosed. Fixed version: */

<string-based palindrome tester snipped>

I like this way:

int pal_aux(int n, int r, int b)
{
return n == 0 ? r : pal_aux(n / b, r * b + n % b, b);
}

int is_pal_in_base(int n, int b)
{
return pal(n, 0, b) == n;

/*Did you mean:*/
return pal_aux(n, 0, b) == n;
Yes. Thanks for spotting it that.

One day I'll remember just to paste the code, and to "tidy it up" for
posting. (I added the _aux because I thought "pal" rather too sort a
name, even for a small helper function.)

--
Ben.
Aug 14 '07 #13
Wabz wrote:
Kevin Handy <k...@srv.netwrote:
>Wabz wrote:
>>Does anyone know how to write a function that tests if an integer
is a palindromein C language?

Yes. I'm quite sure that someone does.

Could this someone possibly help me write the said program?
I suggest you first find him and then ask him (or her). An
alternative would be to do your own homework.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>
--
Posted via a free Usenet account from http://www.teranews.com

Aug 15 '07 #14
Wabz wrote:
>
Hello mates,

Does anyone know how to write a function that tests if an integer is a
palindrome in C language?
/* BEGIN new.c */

#include <stdio.h>
#include <string.h>
#include <limits.h>

int integer_is_palindrome(long unsigned integer);
char *str_rev(char *s);

int main(void)
{
int integer;

for (integer = 0; 1000 integer; ++integer) {
if (integer_is_palindrome(integer)) {
printf("%d\n", integer);
}
}
return 0;
}

int integer_is_palindrome(long unsigned integer)
{
char lutoa_buff[(sizeof(long) * CHAR_BIT) / 3 + 1];
char copy[sizeof lutoa_buff];

sprintf(lutoa_buff, "%lu", integer);
strcpy(copy, lutoa_buff);
str_rev(copy);
return strcmp(copy, lutoa_buff) == 0;
}

char *str_rev(char *s)
{
char *const p = s;
char *t = p;
char swap;

if (*t != '\0' && *++t != '\0') {
t += strlen(t + 1);
do {
swap = *t;
*t-- = *s;
*s++ = swap;
} while (t s);
}
return p;
}

/* END new.c */
--
pete
Aug 15 '07 #15
pete <pf*****@mindspring.comwrites:
Wabz wrote:
>>
Hello mates,

Does anyone know how to write a function that tests if an integer is a
palindrome in C language?

/* BEGIN new.c */

#include <stdio.h>
#include <string.h>
#include <limits.h>

int integer_is_palindrome(long unsigned integer);
char *str_rev(char *s);

int main(void)
{
int integer;

for (integer = 0; 1000 integer; ++integer) {
if (integer_is_palindrome(integer)) {
printf("%d\n", integer);
}
}
return 0;
}

int integer_is_palindrome(long unsigned integer)
{
char lutoa_buff[(sizeof(long) * CHAR_BIT) / 3 + 1];
char copy[sizeof lutoa_buff];

sprintf(lutoa_buff, "%lu", integer);
strcpy(copy, lutoa_buff);
str_rev(copy);
return strcmp(copy, lutoa_buff) == 0;
}

char *str_rev(char *s)
{
char *const p = s;
char *t = p;
char swap;

if (*t != '\0' && *++t != '\0') {
t += strlen(t + 1);
do {
swap = *t;
*t-- = *s;
*s++ = swap;
} while (t s);
}
return p;
}

/* END new.c */
slow?

something like this for the palindrome check on the string. Untested and
pretty much pseudo code.

len=strlen(p);
endp=p+len-1;
while(endp>p)
if ((*endp--)!=(*p++))
return 0;
return 1;

--
Aug 15 '07 #16
user923005 <dc*****@connx.comwrites:
It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?
I don't have an answer, but a few moments of arithmetic led me to
discover that in base B there are pow(B, -N/2) palindromic numbers
of length N, for even N. Somehow this is pleasing, even though
it's almost trivial.
--
Ben Pfaff
http://benpfaff.org
Aug 15 '07 #17
Ben Pfaff wrote:
user923005 <dc*****@connx.comwrites:
>It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?

I don't have an answer, but a few moments of arithmetic led me to
discover that in base B there are pow(B, -N/2) palindromic numbers
of length N, for even N. Somehow this is pleasing, even though
it's almost trivial.
What you calculated is the ratio of palindromic numbers in all numbers of
length N, not the amount of palindromic numbers of length N. In base 10,
there are not 0.1 but 9 palindromic numbers of length 2. I'm not sure
whether you meant to post an expression that would return 9, or whether you
meant to describe your expression as returning the ratio.
Aug 15 '07 #18
Harald van DÄ³k <tr*****@gmail.comwrites:
Ben Pfaff wrote:
>user923005 <dc*****@connx.comwrites:
>>It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?

I don't have an answer, but a few moments of arithmetic led me to
discover that in base B there are pow(B, -N/2) palindromic numbers
of length N, for even N. Somehow this is pleasing, even though
it's almost trivial.

What you calculated is the ratio of palindromic numbers in all numbers of
length N, not the amount of palindromic numbers of length N. In base 10,
there are not 0.1 but 9 palindromic numbers of length 2. I'm not sure
whether you meant to post an expression that would return 9, or whether you
meant to describe your expression as returning the ratio.
I meant the ratio. Oops.
--
"I hope, some day, to learn to read.
It seems to be even harder than writing."
--Richard Heathfield
Aug 15 '07 #19
....
It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?
It decreases fast when the numbers increase. A random number with
k base-b digits is a palindrome with probability:
1/(b ** (k/2))
and as k is floor(b_log(n) + 1), we see that it decreases fast for
all b. (To get the probablility that a number is *not* a palindrome
we multiply all factors (1 - 1/(b ** (k/2))).)

Much more interesting is that 41 is the smallest number that is not
a palindrome for any of those bases.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/
Aug 16 '07 #20
"Dik T. Winter" <Di********@cwi.nlwrites:
...
It might be interesting to try every base from 2 to 36.
Given that condition, what percentage of integers are palindromes?

It decreases fast when the numbers increase. A random number with
k base-b digits is a palindrome with probability:
1/(b ** (k/2))
and as k is floor(b_log(n) + 1), we see that it decreases fast for
all b. (To get the probablility that a number is *not* a palindrome
we multiply all factors (1 - 1/(b ** (k/2))).)

Much more interesting is that 41 is the smallest number that is not
a palindrome for any of those bases.
This got me interested... By observing that

(1) all numbers, n, are palindromic in base n - 1 (n is written "11")
(2) none are palindromic in base n (n is written "10")
(3) all are palindromic in bases b n (n is the single digit "n")

we can arrive at the idea of a number being "non-palindromic" in a
more general way: a number n is non-palindromic if it is a palindrome
only when written in the "trivial" bases b >= n-1.

The first few are 3, 4, 6, 11, 19, 47 and 53. They seem to be common
(though not as common as primes).

There are also numbers that are multi-palindromic (being palindromic
in more that their fair share of bases). For example, 21 (in bases 2,
4, and 6), 36, (in 5, 8, 11 and 17) and 60 (9, 11, 14, 19 and 29).

You might also be interested to know (I did not until yesterday) that
some bases provoke palindromism much more than others. In a quick
count for the numbers between 3 100000, it turns out that 47 makes
more of these numbers palindromic than any other base. There is a
little cluster around 47:

B Number of numbers in 3..99999 that are palindromic in base B
47 2125
46 2116
48 2081
49 2038

This "most palindromic base" creeps upwards (very slowly) as more
numbers are considered, but the presence of a cluster made me think it
might be worth plotting. Think it was. The curve is curious and
unexpected (at least to me).

Cross-posted (with followups set) to comp-programming. I hope that is
the right thing to do. We are certainly off topic now!

 An invented term, as far as I know.
 One must decide about 0, 1 and 2 simply by definition, I think.
 http://www.bsb.me.uk/palindrome/

--
Ben.
Aug 16 '07 #21

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