#define MAX_COLS 20
#define MAX_INPUT 1024

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);

	printf("输入字符串（按回车结束）：");
	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;
	printf("请输入需要显示字符的数字序列（不超过20、负数结束输入）：");
	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')
		;
	return num;
}

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';
}

输出结果：