C-00001-字符处理

#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);
void rearrange(char *output,char const *input,int n_columns,int const columns[]);

int main(void)
{
    int n_columns;
    int columns[MAX_COLS];
    char input[MAX_INPUT];
    char output[MAX_INPUT];

n_columns = read_column_numbers(columns,MAX_COLS);

while(gets(input) != NULL)
{
    printf("Original input:%s\n",input);
    rearrange(output,input,n_columns,columns);
    printf("Rearranged line:%s\n",output);
}
    return EXIT_SUCCESS;
}
    int read_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.");
        exit(EXIT_FAILURE);
    }
    while((ch = getchar())!=EOF && ch != '\n')
    ;
}
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(columns[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';
}