Every k-th digit cyclic problem using strings in C - c

Question

Given some number in a form of string, I want to extract every k-th number from it. Then I go through the remaining string and extract every k-th number again. The thing I get as a result should be the number formed by these extracted ones(in a proper order). Example: 123456789, k = 3 --> 369485271
My algorithm is as follows: While the lenght of the string allows extracting every k-th number, I go through the string and store every k-th element in another string. Then I delete the extracted elements from the original string by tracking the proper index of an element and proceed forvard while the lenght of my str is sufficient.
I can't figure out what's the problem with my code. And maybe my approach isn't that good and there are some better/simpler ways of diong this?
#include <stdio.h>
#include <string.h>
void remove(char *str, unsigned int index) {
char *src;
for (src = str+index; *src != '\0'; *src = *(src+1),++src) ;
*src = '\0';
}
int main() {
char number[100];
char result[100];
int k;
printf("Enter a string: ");
scanf("%s",number);
printf("Enter a key: ");
scanf("%d",&k);
while (strlen(number)>k-1) {
for (int i = 0, p = 0; number[i] != '\0'; i++) {
if (i % k == (k-1)) {
result[p] = number[i];
p++;
}
}
for (int j = 0; number[j] != '\0'; j++){
if (j % k == (k-1)) {
remove(number, j);
j+=1; /*since the index was shifted due to removing an element*/
}
}
}
puts(result);
return 0;
}

Answer 1

You some issues:
You start writing your output from scratch again in each iteration of your while loop.
You do not handle the last digits
You do not treat the input as a cyclic input.
You do not terminate your output string.
remove is already a name of standard library function.
A shorter version could be this (untested):
#include <stdio.h>
#include <string.h>
void remove_digit(char *str, unsigned int index) {
char *src;
for (src = str+index; *src != '\0'; *src = *(src+1),++src)
;
}
int main() {
char number[100];
char result[100];
int k;
printf("Enter a string: ");
scanf("%s",number);
printf("Enter a key: ");
scanf("%d",&k);
int p = 0;
int i = 0;
int skip = k-1; // We remove 1 digit and skip k-1 digits
while (number[0] != 0) {
i = (i + skip) % strlen(number);
result[p] = number[i];
p++;
remove_digit(number, i);
}
number[p] = 0;
puts(result);
return 0;
}

Answer 2

The following code seems to be what you want:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void remove_(char *str, unsigned int index) {
char *src;
for (src = str+index; *src != '\0'; *src = *(src+1),++src) ;
*src = '\0';
}
int main(int argc, const char * argv[]) {
char number[100];
char result[100];
int tmp[100];
int k;
printf("Enter a string: ");
scanf("%s",number);
printf("Enter a key: ");
scanf("%d",&k);
int p = 0;
for (int tp = 0; strlen(number) > k-1; tp = 0) {
for (int i = 0; number[i] != '\0'; i++)
if (i % k == (k-1))result[p++] = number[i];
for (int j = 0; number[j] != '\0'; j++)
if (j % k == (k-1)) tmp[tp++] = j;
for (; tp; --tp) remove_(number, tmp[tp-1]);
}
// The newly added code
for (int index; strlen(number); ) {
index = (k-1) % strlen(number);
result[p++] = number[index];
remove_(number, index);
}
puts(result);
return 0;
}
The most important thing is that every while loop, you need to remove the elements in number at once. While ensuring the integrity of your original code, I made some changes. Unfortunately, the main idea of ​​the original code is wrong.
It should circulate from the tail (including the rest) to the head after one round. But I found that the function of the code you provided is that after each round, the next round starts from the 0th element of the head.
By the way, your algorithm is similar to the Josephus problem