问题描述
我是编程和 C 的新手，目前正在大学学习.这是一个作业，所以我想避免直接回答，但更多的是在正确方向的提示或提示/推动之后.
I am new to programming and to C in general and am currently studying it at university. This is for an assignment so I would like to avoid direct answers but are more after tips or hints/pushes in the right direction.
我正在尝试使用 strtol 来验证我的键盘输入，更具体地说，测试输入是否为数字.我已经查看了此处和其他网站上的其他问题，并且我已经按照给其他用户的说明进行了操作，但这对我没有帮助.
I am trying to use strtol to validate my keyboard input, more specifically, test whether the input is numeric. I have looked over other questions on here and other sites and I have followed instructions given to other users but it hasn't helped me.
根据我对 strtol 的阅读/理解(long int strtol (const char* str, char** endptr, int base);)如果 endptr 是不是空指针，函数会将 endptr 的值设置为数字后的第一个字符.
From what I have read/ understand of strtol (long int strtol (const char* str, char** endptr, int base);) if the endptr is not a null pointer the function will set the value of the endptr to the first character after the number.
因此，如果我要输入 84948ldfk，endptr 将指向l"，告诉我输入中存在数字以外的字符，这将使其无效.
So if I was to enter 84948ldfk, the endptr would point to 'l', telling me there is characters other than numbers in the input and which would make it invalid.
但是，在我的情况下，无论我输入什么，我的程序都返回无效输入.这是我的代码:
However in my case, what is happening, is that no matter what I enter, my program is returning an Invalid input. Here is my code:
void run_perf_square(int *option_stats)
{
char input[MAX_NUM_INPUT + EXTRA_SPACES]; /*MAX_NUM_INPUT + EXTRA_SPACES are defined
*in header file. MAX_NUM_INPUT = 7
*and EXTRA_SPACES
*(for '
' and ' ') = 2. */
char *ptr;
unsigned num=0; /*num is unsigned as it was specified in the start up code for the
*assignment. I am not allow to change it*/
printf("Perfect Square
");
printf("--------------
");
printf("Enter a positive integer (1 - 1000000):
");
if(fgets(input, sizeof input, stdin) != NULL)
{
num=strtol(input, &ptr, 10);
if( num > 1000001)
{
printf("Invalid Input! PLease enter a positive integer between 1
and 1000000
");
read_rest_of_line(); /*clears buffer to avoid overflows*/
run_perf_square(option_stats);
}
else if (num <= 0)
{
printf("Invalid Input! PLease enter a positive integer between 1
and 1000000
");
run_perf_square(option_stats);
}
else if(ptr != NULL)
{
printf("Invalid Input! PLease enter a positive integer between 1
and 1000000
");
run_perf_square(option_stats);
}
else
{
perfect_squares(option_stats, num);
}
}
}
谁能帮我找到正确的方向?显然错误与我的 if(ptr != NULL) 条件有关，但据我所知，这似乎是正确的.正如我所说，我已经查看了与此类似的以前的问题并采纳了答案中的建议，但它似乎对我不起作用.因此，我认为最好根据自己的情况寻求我的帮助.
Can anyone help me in the right direction? Obviously the error is with my if(ptr != NULL) condition, but as I understand it seems right. As I said, I have looked at previous questions similar to this and took the advice in the answers but it doesn't seem to work for me. Hence, I thought it best to ask for my help tailored to my own situation.
提前致谢！
推荐答案
您检查 strtol 的结果顺序错误，请先检查 ptr，也不要'不根据 NULL 检查 ptr，推导它并检查它是否指向 NUL (' ') 字符串终止符.
You're checking the outcome of strtol in the wrong order, check ptr first, also don't check ptr against NULL, derference it and check that it points to the NUL (' ') string terminator.
if (*ptr == ' ') {
// this means all characters were parsed and converted to `long`
}
else {
// this means either no characters were parsed correctly in which
// case the return value is completely invalid
// or
// there was a partial parsing of a number to `long` which is returned
// and ptr points to the remaining string
}
num >1000001 也需要 num >1000000
num 也需要 num
您还可以通过一些重组和逻辑调整将您的 if 语句序列折叠为仅一个无效分支和一个正常分支.
You can also with some reorganising and logic tweaks collapse your sequence of if statements down to onlya single invalid branch and a okay branch.
这篇关于使用 strtol 验证 ANSI C 中的整数输入的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！