I'm trying to write a program in C that reads in a positive integer of up to 20 digits. It should output the smallest palindromic number that is higher than the input. A palindromic number is one that is the same when its digits are reversed (e.g, 98789 is a palindrome, 12344321 is a palindrome, 12345 is not a palindrome as it becomes 54321 when reversed). A sample run, for example, would take 17 as an input and output 22. However, when I run the program and place an input, a segmentation fault: 11 error occurs. Please note that I'm not an experienced programmer, so the code might seem ugly:
#include <stdio.h>
unsigned long reversed(unsigned long palindrome);
int main() {
unsigned long palindrome;
scanf("%lu", &palindrome);
while(1) {
palindrome++;
if (palindrome == reversed(palindrome)) {
printf("%lu", reversed(palindrome));
break;
}
}
}
unsigned long reversed(unsigned long palindrome) {
int size = 0;
unsigned long copy = palindrome;
while(copy != 0) {
copy /= 10;
size++;
}
int count = 0;
unsigned long arr[size];
while (palindrome != 0) {
arr[count] = palindrome % 10;
count++;
}
unsigned long reverse = 0;
for (int i = 0; i < size; i++) {
reverse *= 10;
reverse += arr[i];
}
return reverse;
}
The error seems to suggest that I'm accessing a memory location that I shouldn't be accessing, but I don't understand the actual cause of this. Does it have to do with the unsigned longs or the array? I appreciate any advice. Thanks.
The error seems to be caused by this loop probably.
while (palindrome != 0) {
arr[count] = palindrome % 10;
count++;
}
you are not changing the value of palindrome inside this loop, but the value of count is increasing causing you to access a memory which is out of bounds when count becomes greater then size
Related
I wrote a simple program (included below) that takes a 5-digit number, reverses and prints it.
#include <stdio.h>
#include <limits.h>
#include <math.h>
int main(void)
{
unsigned short num, revnum, digit, count;
count = 4;
revnum = 0;
fprintf(stdout, "Enter a five digit number in range [0 - %hu]: ", USHRT_MAX);
fscanf(stdin, "%hu", &num);
while (count >= 0) {
digit = num % 10;
num = num / 10;
revnum += digit * pow(10, count);
count--;
}
fprintf(stdout, "The number with digits reversed is %hu.\n", revnum);
return 0;
}
However the program when run, doesn't return anything post taking input and storing it in the num variable. It didn't crash/exit either. There is nothing wrong with the while loop's body, so I suspected an infinite loop and wrote a small program just to print out the count variable from a similar while loop
#include <stdio.h>
int main(void)
{
unsigned short count;
count = 4;
while (count >= 0) {
printf("%hu", count);
count--;
}
return 0;
}
This indeed turns out to be an infinite loop... a part of the output is:
...300653005530045300353002530015300052999529985299752996529955299452993529925299152990529895298852987529865298552984529835298252981529805297952978529775297652975529745297352972529715297052969529685296752966529655296452963529625296152960529595295852957529565295552954529535295252951529^C
My assumption is that the culprit is the count's type which is unsigned short. As this means it can't decrement below 0, so the final count-- when count already == 0 sets its value to USHRT_MAX and thus can never terminate the loop. Am I correct?
Your assumption is correct: being your variable unsigned it won't ever be evaluated as negative.
In your case, if you decrement the count variable when it is equal to 0 you get the value 65535 (USHRT_MAX).
The trivial solution is getting rid of the unsigned keyword:
short count = 4;
This solution is the best for the basic code provided in the question. Anyway, since in the "real code" could happen that changing the type of the counter is not an option, a check on count == 0 before decrementing could be the solution (credits to #Elijan9):
while (true)
{
/* ...; */
if (count == 0)
break;
--count;
}
I have to write a program that finds every number (except 0) which can be factored by numbers from 2-9.
For example first such a number would be number 2520 as it can be divided by every single number from 2 to 9.
It also has to be a number that contains only 1 type of digit of its own (no multiple digits in a number). So for example 2520 will not meet this requirement since there are two same digits (2). The example of a number that meets both requirements is number 7560. That is the point I don't how to do it. I was thinking about converting value in an array to string, and then putting this string in another array so every digit would be represented by one array entry.
#include <stdio.h>
#include <math.h>
int main() {
int i, n, x, flag, y = 0;
scanf("%d", &n);
double z = pow(10, n) - 1;
int array[(int)z];
for (i = 0; i <= z; i++) {
flag = 0;
array[i] = i;
if (i > 0) {
for (x = 2; x <= 9; x++) {
if (array[i] % x != 0) {
flag = 1;
}
}
if (flag == 0) {
y = 1;
printf("%d\n", array[i]);
}
}
}
if (y == 0) {
printf("not exist");
}
return 0;
}
This should give you a base:
#include <stdio.h>
#include <string.h>
int main()
{
char snumber[20];
int number = 11235;
printf("Number = %d\n\n", number);
sprintf(snumber, "%d", number);
int histogram[10] = { 0 };
int len = strlen(snumber);
for (int i = 0; i < len; i++)
{
histogram[snumber[i] - '0']++;
}
for (int i = 0; i < 10; i++)
{
if (histogram[i] != 0)
printf("%d occurs %d times\n", i, histogram[i]);
}
}
Output:
Number = 11235
1 occurs 2 times
2 occurs 1 times
3 occurs 1 times
5 occurs 1 times
That code is a mess. Let's bin it.
Theorem: Any number that divides all numbers in the range 2 to 9 is a
multiple of 2520.
Therefore your algorithm takes the form
for (long i = 2520; i <= 9876543210 /*Beyond this there must be a duplicate*/; i += 2520){
// ToDo - reject if `i` contains one or more of the same digit.
}
For the ToDo part, see How to write a code to detect duplicate digits of any given number in C++?. Granted, it's C++, but the accepted answer ports verbatim.
If i understand correctly, your problem is that you need to identify whether a number is consisted of multiple digits.
Following your proposed approach, to convert the number into a string and use an array to represent digits, i can suggest the following solution for a function that implements it. The main function is used to test the has_repeated_digits function. It just shows a way to do it.
You can alter it and use it in your code.
#include <stdio.h>
#define MAX_DIGITS_IN_NUM 20
//returns 1 when there are repeated digits, 0 otherwise
int has_repeated_digits(int num){
// in array, array[0] represents how many times the '0' is found
// array[1], how many times '1' is found etc...
int array[10] = {0,0,0,0,0,0,0,0,0,0};
char num_string[MAX_DIGITS_IN_NUM];
//converts the number to string and stores it in num_string
sprintf(num_string, "%d", num);
int i = 0;
while (num_string[i] != '\0'){
//if a digit is found more than one time, return 1.
if (++array[num_string[i] - '0'] >= 2){
return 1; //found repeated digit
}
i++;
}
return 0; //no repeated digits found
}
// test tha function
int main()
{
int x=0;
while (scanf("%d", &x) != EOF){
if (has_repeated_digits(x))
printf("repeated digits found!\n");
else
printf("no repeated digits\n");
}
return 0;
}
You can simplify your problem from these remarks:
the least common multiple of 2, 3, 4, 5, 6, 7, 8 and 9 is 2520.
numbers larger than 9876543210 must have at least twice the same digit in their base 10 representation.
checking for duplicate digits can be done by counting the remainders of successive divisions by 10.
A simple approach is therefore to enumerate multiples of 2520 up to 9876543210 and select the numbers that have no duplicate digits.
Type unsigned long long is guaranteed to be large enough to represent all values to enumerate, but neither int nor long are.
Here is the code:
#include <stdio.h>
int main(void) {
unsigned long long i, n;
for (n = 2520; n <= 9876543210; n += 2520) {
int digits[10] = { 0 };
for (i = n; i != 0; i /= 10) {
if (digits[i % 10]++)
break;
}
if (i == 0)
printf("%llu\n", n);
}
return 0;
}
This program produces 13818 numbers in 0.076 seconds. The first one is 7560 and the last one is 9876351240.
The number 0 technically does match your constraints: it is evenly divisible by all non zero integers and it has no duplicate digits. But you excluded it explicitly.
Task is to get int using scanf("%d") then print it again using printf("%с") without standard functions like atoi , itoa .As i understood i need to divide all numbers then add \0 char and print it, however how can i divide it. I thought about loop for dividing number%10 + /0 and number/10 to decrease number for 1 character .
Therefore code should look smoothing like this
#include <conio.h>
#include <stdio.h>
main(void)
{
int number,reserve ;
char Array[50];
scanf_s("%d",&number);
if (number > 0 || number == 0)
{
do
{
reserve = number % 10;
printf("%c", reserve + '/0');
number /= 10;
} while (number != 0);
}
else
{
number *= -1;
printf("-");
do
{
reserve = number % 10;
printf("%c", reserve + '/0');
number /= 10;
} while (number != 0);
}
_getch();
return 0;
}
As well there can be negative number so i need some if statement to check if it is negative and in case it is loop should avoid it it so we won't get smthing like -%10
So i don't know if loop is correct (hope someone will fix it and explain me how it is supposed to be). Waiting for your advices.
One side effect of the line
number = number % 10;
is that you lose the original value of number. So when you go to do
number = number/10;
it would always get the value zero. To fix this, store the original value somewhere else, or use another variable to do your character conversion (modulo 10, then plus \0).
Also, your loop needs to be re-examined. This process of modulo, add \0, divide, repeat, should stop when the result of the division is zero (i.e. there are no more digits to print). Another thing to think about is: in what order are these digits being printed?
I'll leave it to you to to figure out how to determine if the value of an int is greater than or less than zero, since you didn't attempt that in this snippet.
this will help you, adopt for your purposes
#include <stdio.h>
int main() {
int a;
int i = 0;
int str_size = 0;
char str[11] = {};
char tmp;
scanf("%d", &a);
while (a) {
str[str_size++] = a % 10 + '0';
a /= 10;
}
str_size--;
while (i < str_size) { // rewind
tmp = str[i];
str[i++] = str[str_size];
str[str_size--] = tmp;
}
printf("%s", str);
return 0;
}
I am trying to make a program that calculates the amount of prime numbers that don't exceed an integer using the sieve of Eratosthenes. While my program works fine (and fast) for small numbers, after a certain number (46337) I get a "command terminated by signal 11" error, which I suppose has to do with array size. I tried to use malloc() but I didn't get it quite right. What shall I do for big numbers (up to 5billion)?
#include <stdio.h>
#include<stdlib.h>
int main(){
signed long int x,i, j, prime = 0;
scanf("%ld", &x);
int num[x];
for(i=2; i<=x;i++){
num[i]=1;
}
for(i=2; i<=x;i++){
if(num[i] == 1){
for(j=i*i; j<=x; j = j + i){
num[j] = 0;
}
//printf("num[%d]\n", i);
prime++;
}
}
printf("%ld", prime);
return 0;
}
Your array
int num[x];
is on the stack, where only small arrays can be accommodated. For large array size you'll have to allocate memory. You can save on memory bloat by using char type, because you only need a status.
char *num = malloc(x+1); // allow for indexing by [x]
if(num == NULL) {
// deal with allocation error
}
//... the sieve code
free(num);
I suggest also, you must check that i*i does not break the int limit by using
if(num[i] == 1){
if (x / i >= i){ // make sure i*i won't break
for(j=i*i; j<=x; j = j + i){
num[j] = 0;
}
}
}
Lastly, you want to go to 5 billion, which is outside the range of uint32_t (which unsigned long int is on my system) at 4.2 billion. If that will satisfy you, change the int definitions to unsigned, watching out that your loop controls don't wrap, that is, use unsigned x = UINT_MAX - 1;
If you don't have 5Gb memory available, use bit status as suggest by #BoPersson.
The following code checks for errors, tested with values up to 5000000000, properly outputs the final count of number of primes, uses malloc so as to avoid overrunning the available stack space.
#include <stdio.h>
#include <stdlib.h>
int main()
{
unsigned long int x,i, j;
unsigned prime = 0;
scanf("%lu", &x);
char *num = malloc( x);
if( NULL == num)
{
perror( "malloc failed");
exit(EXIT_FAILURE);
}
for(i=0; i<x;i++)
{
num[i]=1;
}
for(i=2; i<x;i++)
{
if(num[i] == 1)
{
for(j=i*i; j<x; j = j + i)
{
num[j] = 0;
}
//printf("num[%lu]\n", i);
prime++;
}
}
printf("%u\n", prime);
return 0;
}
i got a problem which i can't solve
I want to know all prime numbers below a given limit x. Allowing me to enter x and calculate the prime numbers using the method of Erastosthenes. Displaying the result on the screen and saving it to a text file.
Calculating the primenumbers below the x, printing them and saving them to a text file worked, the only problem i have is that x can't exceed 500000
could you guys help me?
#include <stdio.h>
#include <math.h>
void sieve(long x, int primes[]);
main()
{
long i;
long x=500000;
int v[x];
printf("give a x\n");
scanf("%d",&x);
FILE *fp;
fp = fopen("primes.txt", "w");
sieve(x, v);
for (i=0;i<x;i++)
{
if (v[i] == 1)
{
printf("\n%d",i);
fprintf(fp, "%d\n",i);
}
}
fclose(fp);
}
void sieve(long x, int primes[])
{
int i;
int j;
for (i=0;i<x;i++)
{
primes[i]=1; // we initialize the sieve list to all 1's (True)
primes[0]=0,primes[1]=0; // Set the first two numbers (0 and 1) to 0 (False)
}
for (i=2;i<sqrt(x);i++) // loop through all the numbers up to the sqrt(n)
{
for (j=i*i;j<x;j+=i) // mark off each factor of i by setting it to 0 (False)
{
primes[j] = 0;
}
}
}
You will be able to handle four times as many values by declaring char v [500000] instead of int v [100000].
You can handle eight times more values by declaring unsigned char v [500000] and using only a single bit for each prime number. This makes the code a bit more complicated.
You can handle twice as many values by having a sieve for odd numbers only. Since 2 is the only even prime number, there is no point keeping them in the sieve.
Since memory for local variables in a function is often quite limited, you can handle many more values by using a static array.
Allocating v as an array of int is wasteful, and making it a local array is risky, stack space being limited. If the array becomes large enough to exceed available stack space, the program will invoke undefined behaviour and likely crash.
While there are ways to improve the efficiency of the sieve by changing the sieve array to an array of bits containing only odd numbers or fewer numbers (6n-1 and 6n+1 is a good trick), you can still improve the efficiency of your simplistic approach by a factor of 10 with easy changes:
fix primes[0] and primes[1] outside the loop,
clear even offsets of prime except the first and only scan odd numbers,
use integer arithmetic for the outer loop limit,
ignore numbers that are already known to be composite,
only check off odd multiples of i.
Here is an improved version:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void sieve(long x, unsigned char primes[]) {
long i, j;
for (i = 0; i < x; i++) {
primes[i] = i & 1;
}
primes[1] = 0;
primes[2] = 1;
/* loop through all odd numbers up to the sqrt(x) */
for (i = 3; (j = i * i) < x; i += 2) {
/* skip composite numbers */
if (primes[i] == 0)
continue;
/* mark each odd multiple of i as composite */
for (; j < x; j += i + i) {
primes[j] = 0;
}
}
}
int main(int argc, char *argv[]) {
long i, x, count;
int do_count = 0;
unsigned char *v;
if (argc > 1) {
x = strtol(argv[1], NULL, 0);
} else {
printf("enter x: ");
if (scanf("%ld", &x) != 1)
return 1;
}
if (x < 0) {
x = -x;
do_count = 1;
}
v = malloc(x);
if (v == NULL) {
printf("Not enough memory\n");
return 1;
}
sieve(x, v);
if (do_count) {
for (count = i = 0; i < x; i++) {
count += v[i];
}
printf("%ld\n", count);
} else {
for (i = 0; i < x; i++) {
if (v[i] == 1) {
printf("%ld\n", i);
}
}
}
free(v);
return 0;
}
I believe the problem you are having is allocating an array of int if more than 500000 elements on the stack. This is not an efficient way, to use an array where the element is the number and the value indicates whether it is prime or not. If you want to do this, at least use bool, not int as this should only be 1 byte, not 4.
Also notice this
for (i=0;i<x;i++)
{
primes[i]=1; // we initialize the sieve list to all 1's (True)
primes[0]=0,primes[1]=0; // Set the first two numbers (0 and 1) to 0 (False)
}
You are reassigning the first two elements in each loop. Take it out of the loop.
You are initializing x to be 500000, then creating an array with x elements, thus it will have 500000 elements. You are then reading in x. The array will not change size when the value of x changes - it is fixed at 500000 elements, the value of x when you created the array. You want something like this:
long x=500000;
printf("give a x\n");
scanf("%d",&x);
int *v = new int[x];
This fixes your fixed size array issue, and also gets it off the stack and into the heap which will allow you to allocate more space. It should work up to the limit of the memory you have available.