Is it possible to cast a string as int to get the value converted?

Question

It would be nice to cast a string (array of chars) to an int when reading from files or taking input is this possible.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 500

int main(void)
{
 FILE* pfile;
 pfile = fopen("/tmp/code","r");                              //open file
 char *stringBuffer = (char *)malloc(SIZE * sizeof(char));   //dynamic string

  char ch;
  int i = 0;
  ch = getc(pfile);                                          //read file
  while(ch != EOF)                                           //until end
  {                                                          //pass into string 
        counter++;
        stringBuffer[i] = ch;
        ch = getc(pfile);
        i++;
    }

    fclose(pfile);                                              //resize string

    int stringSize = (int)(sizeof(stringBuffer)/sizeof(char));
    stringBuffer = (char *)realloc(stringBuffer,(stringSize * sizeof(char)));

    printf("the string works: %s \n",stringBuffer);

    int intger = (int)*stringBuffer;    //this is what i want to work!!!!

    //but more like
    //int intger = (int)stringBuffer; 

    printf("the intger doesn't work if more than 1 decimal place: %i \n",(intger - 48));

    free(stringBuffer);
    return(0);
    }
~                                                                                                                                          
~      

I do realize I could 1.cast the char as int & convert from ASCII to real number 2.multiplier each (now int) by is decimal place (determined by array position) 3.add ints together for example (warning this code is kinda ugly, I haven't look twice at it yet and is just a test so far)

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

    #define SIZE 500

    int main(void)
    {
    FILE* pfile;
    pfile = fopen("/tmp/code","r");                                            

    char *stringBuffer = (char *)malloc(SIZE * sizeof(char));                 

    char ch;
    int i = 0;
    ch = getc(pfile);                                                         
    while(ch != EOF)                                                          
    {
        stringBuffer[i] = ch;
        ch = getc(pfile);
        i++;
    }

    fclose(pfile);

    int stringSize = (int)(sizeof(stringBuffer)/sizeof(char));
    stringBuffer = (char *)realloc(stringBuffer,(stringSize * sizeof(char)));

    printf("the string works: %s \n",stringBuffer);
    int multi;
    int sum;

   for(int x =0; x < stringSize; x++)
   {
      int intger = ((int)stringBuffer[((stringSize - 1) - x)] - 48);

      if( x != 0 )
      {
        multi = (int)pow(10,x);
        sum += ( multi * intger );
      }
      else
        sum += intger;
    }
    printf("the int works: %i \n",sum);
    free(stringBuffer);
    return(0);
    }

but still casting as string as int would be nice(I assume c++ has this?).

Answer 1

It does not work the way you expect.

==> In case of a statement like

  int intger = (int)stringBuffer;

Casting a "string" (or, the pointer to the first element of the char array) to int is highly implementation defined behaviour and if not properly supported by the implementation, will cause undefined behaviour.

==> OTOH, a statement like

 int intger = (int)*stringBuffer; 

is an attempt to dereference the first memory location pointed by stringBuffer and then cast that value to int (which would have been done implicitly anyway). This does not convert the whole string content to an integer type. That is why you see something like

the integer doesn't work if more than 1 decimal place: % [......]

Solution: From your problem statement, looks like you can make use of strtol() or variants. You need to read a whole line with fgets() or similar and then, (after parsing, if required) pass that to strtol() to convert to a long int.

After that, notes about your code:

• In while(ch != EOF) loop body, you have an uncontrolled iteration with i, it may lead to out of bound memory access.
• int stringSize = (int)(sizeof(stringBuffer)/sizeof(char)); is not going to give you the string size, you need to null-terminate the string and use strlen() to get the length of the string.

That said, some of general comments:

• Always check the return value of fopen() (or for that matter, any library function) before using the return value.
• Please see this discussion on why not to cast the return value of malloc() and family in C..
• getc() returns an int and a return value of EOF is not going to fit in a char. You need an int to hold that.

Answer 2

In the case of your example, you could simplify the whole code using the fscanf function.

FILE* pfile;
pfile = fopen("/tmp/code","r");

int intger;
fscanf(pfile, "%i", &intger);

fclose(pfile);

If you need both the string and the integer, you can use the sscanf function in the same way, or the strtol function.