helloworld.c

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

#define MAX_COLS 20 /*最大列数*/
#define MAX_INPUT 1000 /*每个输入行的最大长度*/

int read_column_numbers(int columns[], int max);

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);

    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;
}

/**
 * 读取列标号，如果超出规定范围则不予理会
 * @param columns
 * @param max
 * @return
 */
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;
    }
    //因为后续需要，num必须是偶数
    if (num % 2 != 0) {
        puts("Last column number is not paired.");
        exit(EXIT_FAILURE);
    }
    while ((ch = getchar()) != EOF && ch != '\n');
    return num;
}

/**
 * 处理输入行,将指定列的字符连接在一起，输出行以NUL结尾
 */
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';

}