Thread: how to convert a numeric string to integer

The following code is supposed to convert a numeric string to an integer, but it does not work. I am trying to use GDB to see what is going on, but still can not follow the program. When I put in 1, the program return the right number but when I put in 12, the program returns -429. Here is the code:

Code:

#include <stdio.h>

int main ()

{

int a;

a = getint ();

printf ("You entered ...%d", a);

printf ("Press any key to continue ...");

getch();

}

getint ()

{

int val,i,j,k;

char str[6];

i = 0;

while ( i <= 5)

  {

    str[i] = getche();

    if ( str[i] == '\r')

      {

        str[i]='\0';

        break;

      }

    if ( str[i] == '\b')

      {

        printf ("\b");

        i--;

      }

    else 

      i++;

  } 

val = 0;

k = 1;

for ( j = i-1; j >=0; j--)

  {

    val = val + ( str[j] - 48 * k);

    k = k * 10; 

  }

return val;

}

I am also wondering what are the GDB outputs which start with $. I will try to do a more detailed search online; the GDB tutorials online do not seem to have anything about this. Below is a capture of what I mean:

Code:

(gdb) p k
$3 = 1
(gdb)
$4 = 1
(gdb)
$5 = 1
(gdb) s
33      for ( j = i-1; j >=0; j--)
(gdb) s
35          val = val + ( str[j] - 48 * k);
(gdb) p j
$6 = 4
(gdb) p val
$7 = 6
(gdb) s
36          k = k * 10;
(gdb) s
33      for ( j = i-1; j >=0; j--)
(gdb) s
35          val = val + ( str[j] - 48 * k);
(gdb) quit

I figured out my problem. I needed to add a ()s in line 38 to first subtract 48 from the value in the array and then multiply by k.

Code:

      1 #include <stdio.h>
      2 int main ()
      3 {
      4 int a;
      5 a = getint ();
      6 printf ("You entered ...%d", a);
      7 printf ("Press any key to continue ...");
      8 getch();
      9 }
     10 getint ()
     11 {
     12 int val,i,j,k;
     13 char str[6];
     14 i = 0;
     15 while ( i <= 5)
      6 printf ("You entered ...%d", a);
      7 printf ("Press any key to continue ...");
      8 getch();
      9 }
     10 getint ()
     11 {
     12 int val,i,j,k;
     13 char str[6];
     14 i = 0;
     15 while ( i <= 5)
     16   {
     17     str[i] = getche();
     18     if ( str[i] == '\r')
     19       {
     20         str[i]='\0';
     21         break;
     22       }
     23     if ( str[i] == '\b')
     24       {
     25         printf ("\b");
     26         i--;
     27       }
     28     else
     29       {
     30         printf ("You entered ....%d", str[i]);
     31         i++;
     32       }
     33   }
     34 val = 0;
     35 k = 1;
     36 for ( j = i-1; j >=0; j--)
     37   {
     38     val = val + ( (str[j] - 48) * k);
     39     k = k * 10;
     40     printf ("J is %d", j);
     41     printf ("Here is what val is %d", val);
     42   }
     43 return val;
     44 }

In modern C we always explicitly declare the return value of a function. We also always use function prototypes where appropriate. You should do so for getint(). And you should probably explicitly return 0 at the end of main and declare getint and main as taking a void (although this is not technically necessary in C99 or higher).

Since you've used non-portable functions getch and getche, and also the non-standard line ending character '\r' I can't actually run your code.

It's not very handy to have an explicit "press enter to continue" wait for a keypress at the end of main. Why do you need to do that? If you're using an IDE that doesn't hold the console when the program ends, maybe there's a setting for that. You could also run the program from a console.

Instead of 48, use '0'. And your math is wrong. Recall that the precedence of multiplication is higher than subtraction, so you need parentheses:

Code:

        val += (str[j] - '0') * k;

Normally it would be done like so:

Code:

val = 0;
for (j = 0; j < i; j++)
    val = val * 10 + (str[j] - '0');

Thank you so much. This helped a lot. This is not my code. I first try to do the problem, and then look up the answer. The book is an old turbo C book. I was lucky to find MinGw, which supports more dos functions than regular gcc.