在 C 中读取命令行参数并检查它是否为数字

Question

Details about my goal

In C, my program is supposed to take an input from command line, read it, make sure that it is a digit and then print out "Success" if true, "Usage: ./caesar key" if false.

Expected results

./caesar 33
Success

or

./caesar zzz
Usage: ./caesar key

Actual results

./caesar 33
Segmentation error

What I've tried

 int main (int argc, string argv[])
 {
   if (argc == 2)
   {
      if (isdigit(argv[1]))
      {
        printf("Success\n");
        return 0;
      }

or

if (argc == 2)
{
  if (isdigit(argv[0][1]))
  {
    printf("Success\n");
    return 0;
 }

or

int main (int argc, string argv[])
{
   if (argc == 2 && isdigit(argv[1]))
   {
     printf("Success\n");
     return 0;
   }

Answer 1

What you are doing wrong here is that you want to check if the entire string consists of digits, but you are passing the string itself to isdigit , which expects a char . What you can do is manually iterate through the char array and check if each char is a digit, or you can use the strspn function as demonstrated below.

strspn(argv[1], "0123456789") == strlen(argv[1])

This will compare the span of digits in the argument and return true if it is the same as the length of the string itself. If you want to allow + and - signs in the argument, use "-+0123456789" inside strspn , though that would result in a false positive for cases like +++++ (as mentioned by @dreamcrash below), which is why defining a custom function is always a better option. Both the strspn and strlen functions are available in the string.h header.

Answer 2

If you do not need to use the isdigit function, then you can use the strspn function as suggested by @Param Siddharth. However, they are several corner-cases with using

if (strspn(string, "-+0123456789") == strlen(string))

for instance that the values like "-----" would be considered as a valid number .

If you want to use the isdigit function than you need to go through all the char s in the String (instead of passing the entire String in one go) and check if each one of them is indeed a digit:

    ...
    int i, is_number = 1;
    for (i = 0; is_number && argv[1][i] != '\0'; i++){
        if(i == 0 && (argv[1][i] == '-' || argv[1][i] == '+')){
           continue;
        }   
        is_number = isdigit(string[i]);
     }
     // It should fail for Strings that are : empty, only "-" or "+"
    if(i == 0 || (i == 1 && !isdigit(argv[1][0])) || !is_number){
        printf("The String is not a Number \n");
        return 1;
     }
     printf("Success\n");
     return 0;
}

I tested with the inputs:

• "-", not a number
• "", not a number
• "-1", number
• "+1", number
• "+1+1" not a number

Answer 3

int main (int argc, string argv[])

string data type is not defined in C. Second argument should be char* argv[] .

Please see the answer : https://stackoverflow.com/questions/14709323/does-c-have-a-string-type#:~:text=There%20is%20no%20string%20type,one%20additional%20zero%20terminating%20character .