c - 在 scanf 之后，gets() 如何工作? [英] c - how gets() work after scanf?

问题描述

我有两个问题:

1. 为什么只有当我在 "%d " --> scanf("%d ", &num); 中做空格时才有效?

我在 scnaf 和 get 之间尝试了 fflush(stdin) \ _flushall() 但它不起作用，它跳过了 get.

2. 当我做空格时，它首先扫描，然后获取，然后打印数字并打印字符串.

<前>无效主(){字符 ch, str[10];整数;printf("请输入您的电话号码:");scanf("%d", #);printf("%d\n",num);获取(字符串)；把(字符串)；系统(暂停")；}

解决方案

1. 为什么只有当我在 "%d " 中做空格 --> scanf("%d ", &num);它有效吗?

scanf("%d", &num); "%d" 后没有空格，读取一个数字后停止扫描.所以输入 123Enter，'\n' 保持在stdin 用于下一个输入函数，例如现在非标准的 gets().gets() 读取单个 '\n' 并返回.通过添加一个空格，scanf("%d ", &num); 消耗了数字后面的空格，直到数字后面输入了非白字符才返回.><块引用>

2. 当我做空格时，它首先执行 scanf 然后获取，然后打印数字并打印字符串.

通过添加一个空格，scanf("%d", &num); 直到在数字后面输入非空格(如 'a' 在下面).由于 stdin 通常是行缓冲的，这意味着需要首先输入 2 行.123回车 abc输入.

<小时>

建议改为使用 fgets() 读取用户输入的行.

char str[10*2];//不需要这么小的缓冲区整数;printf("请输入您的电话号码:");fflush(标准输出)；fgets(str, sizeof str, stdin);sscanf(str, "%d", &num);printf("%d\n",num);printf("请输入数据:");fflush(标准输出)；fgets(str, sizeof str, stdin);fputs(str, stdout);

更健壮的代码将检查fgets()、sscanf() 的结果并使用strtol() 而不是sscanf().

I have two questions:

1. why only when i do space in "%d " --> scanf("%d ", &num); it works?

I tried fflush(stdin) \ _flushall() between the scnaf and the gets and it doesn't works, it skips the gets.

2. When I do the space, it first does scanf then the gets and after that it print the number and print the string.

void main()
{
    char ch, str[10];
    int num;
    printf("Enter your number : ");
    scanf("%d ", &num);
    printf("%d\n",num);
    gets(str);
    puts(str);
    system("pause");
}

解决方案

1. why only when i do space in "%d " --> scanf("%d ", &num); it works?

scanf("%d", &num); without a space after the "%d", stops scanning after reading a number. So with input 123Enter, the '\n' remains in stdin for the next input function like the now non-standard gets(). gets() reads that single '\n' and returns. By adding a space, scanf("%d ", &num); consumes the white-space after the number and does not return until non-white-scape is entered after the number.

2. When I do the space, it first does scanf then the gets and after that it print the number and print the string.

By adding a space, scanf("%d ", &num); does not return until non-white-space is entered after the number (as in 'a' in the following). Since stdin is usually line buffered, this means input of 2 lines needs to first occur. 123Enter abcEnter.

---

Recommend to instead use fgets() to read a line of user input.

char str[10*2]; // no need for such a small buffer
int num;
printf("Enter your number : ");
fflush(stdout);
fgets(str, sizeof str, stdin);
sscanf(str, "%d", &num);
printf("%d\n",num);

printf("Enter data : ");
fflush(stdout);
fgets(str, sizeof str, stdin);
fputs(str, stdout);

More robust code would check the results of fgets(), sscanf() and use strtol() rather than sscanf().

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