Problem with converting string to integer

Question

(I am very new to both coding and C.)

I want to check if the nr one string in "string argv" is one or more decimals and if it is convert it to an integer. I think I may need to iterate over the string but so far this code is not working. It says segmentation fault.

This is my code:

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

int main (int argc, string argv [])
{

    //check command line
    if (argc == 2)
    {
        printf("success");
    }
    else
    {
        printf("Usage: ./caesar key.");
    }

    //validate number
    if (isdigit (argv[1]))
    {
        int x = atoi(argv[1]);
        printf("I is now %i\n", x);
        return 0;
    }

    if (isalpha(argv[1]))
     {
        printf("Usage: ./caesar key\n");
        return 1;
    }
}

Which line generates the error and what is the exact error message text? — Martin James
– Martin James, Commented Jan 26, 2021 at 18:20
The condition array is bigger than two is not standard C. Use strlen() with the name of your array and compare the result with the number 2 by the mean of this binary operator: > — anotherOne
– anotherOne, Commented Jan 26, 2021 at 18:21
'some of it is in pseudocode, because that part works', well, you will need to do more to convince an experienced developer than just make a claim. Compiler error messages are often misleading and generate an error on a,line far removed from the real error location. — Martin James
– Martin James, Commented Jan 26, 2021 at 18:25
@MartinJames thanks for replying. I have changed it a bit and its compiling but it does not print out the value of the integer. I just get "segmentaton fault" — Zterling
– Zterling, Commented Jan 26, 2021 at 18:36
The function isdigit tests whether a single character is a digit. If you want to test whether all characters in a string are digits, you must loop over the string. The function atoi tries to convert the whole string to a number. (The loop is in the function itself.) There is another function, strtol, which converts a string to a long. It has better possibilities for error checking. — M Oehm
– M Oehm, Commented Jan 26, 2021 at 18:52

icolak · Accepted Answer · 2021-01-26 19:43:52Z

1

main function takes (int argc, char *argv[]) parameters. argv is a pointer to pointer and argv[1] is a pointer to a char array too. You should send a character to isdigit and isalpha, not a pointer. You get a segmentation fault because of this probably.

answered Jan 26, 2021 at 19:43

icolak

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chux · Accepted Answer · 2021-01-26 21:43:19Z

check if ... string ... is one or more decimals and if it is convert it to an integer.

isdigit (argv[1]) --> argv[1] is a pointer to a string. isdigit(int ch) expects a single character, not a pointer. Enable all compiler warnings for fast feedback.

A direct approach uses strtol()

bool test_and_convert(const char *s, long *valptr) {
  errno = 0; // set to 0 to later detect overflow
  int base = 10;
  char *endptr;
  long *valptr = strtol(s, &endptr, base);
  if (s == endtpr) {
    return false; // No conversion
  }
  if (errno == ERANGE) {
    return true; /* or false, OP's choice here */
    // *valptr is clamped to LONG_MIN or LONG_MAX
  }

  // Detect trailing non-numeric text if desired
  if (*endptr) {
    return false;
  }

  return true;
}

A plodding approach akin to OP's.

bool test_and_convert_plodding(const char *s, int *valptr) {
  // Best to access characters as unsinged char for `is...()` functions.
  const unsigned char *us = (const unsigned char *) s; // 
  
  // Maybe skip leading spaces
  while (isspace(*us)) us++;

  unsigned char sign = *us;
  if (sign == '-' || sign == '+') {
    us++;
  }

  // Accumulate digits
  unsigned found = false;
  unsigned uvalue = 0;
  while (isdigit(*us)) {
    found = true;
    // Overflow detection code needed here is TBD
    uvalue = ulvaue*10 + (*us - '0');
  }

  if (!found) {
    return false; // No digits
  }

  // Maybe skip trailing spaces
  while (isspace(*us)) us++;

  // Maybe detect non-numeric junk at the end.
  if (*us) return false;
  
  // Overflow detection code needed here is TBD
  *valptr = (sign == '-'  && uvlaue > 0) ? (-(int)(uvalue - 1) - 1) : (int) uvalue;
  return true;
}

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