rearrange.c

/*from c and pointer
there maybe something wrong.
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#define MAX_COLS 20
#define MAX_INPUT 1000

int read_column_numbers(int columns[], int max); /* del "columns" is also OK!*/
void rearrange(char *output, char *const input, int n_columns, int const columns[]);

int main()
{
  int   n_columns;
  int   columns[MAX_COLS];
  char  input[MAX_INPUT];
  char  output[MAX_INPUT];
  
  n_columns = read_column_numbers(columns, MAX_COLS);
  
  /* The gets() is dangerous, because it has no means of preventing you typing the characters and so should be avoided. */
  while(fgets(input, 1000, stdin) != NULL){
    printf("Original input: %s\n", input);
    rearrange(output, input, n_columns, columns);
    printf("Rearrange line : %s\n", output);
  }
  return EXIT_SUCCESS  /* EXIT_SUCCESS is the same as 0 (stackoverflow)*/
}

int real_column_numbers(int columns[], int max)
{
  int num = 0;
  int ch;
  while(num < max && scanf("%d", &columns[num]) == 1 && columns[num] >= 0){
    num += 1;
  }
  
  if (num % 2 != 0) {
    puts("Last column number is not paired.\n");
    exit(EXIT_FAILURE);
  }
  
  while((ch = getchar()) != EOF && ch != '\n') {   /* EOF vs '\n' ??? */
    return sum;
  }
}

void rearrange(char *output, char const *input, int n_columns, int const columns[])
{
  int col;
  int output_col;
  int len;
  
  len = strlen(input);
  output_col = 0;
  
  for (col = 0; col < n_columns; col += 2) {
    int nchars = columns[col + 1] - columns[col] + 1;
    if(colums[col] >= len || output_col == MAX_INPUT - 1) {
      break;
    }
    if(output_col + nchars > MAX_INPUT - 1) {
      nchars = MAX_INPUT - output_col - 1;
    }
    
    strncpy (output + output_col, input + columns[col], nchars);
    output_col += nchars;
  }
    output[output_col] = '\0';
}