It's poorly written, to say the least.
0) Use formatting!:
int f(char* p)
{
int n = 0;
while (*p != 0)
n = 10*n + *p++ - ‘0?;
return n;
}
1) ?
there is syntactically invalid. It should probably be a '
as noted by chris (and your existing ‘
is wrong too, but that's probably because you copied it from a website and not a source file), giving:
int f(char* p)
{
int n = 0;
while (*p != 0)
n = 10 * n + *p++ - '0';
return n;
}
2) The parameter type isn't as contrained as it should be. Because *p
is never modified (per our goals), we should enforce that to make sure we don't make any mistakes:
int f(const char* p)
{
int n = 0;
while (*p != 0)
n = 10 * n + *p++ - '0';
return n;
}
3) The original programmer was obviously allergic to readable code. Let's split up our operations:
int f(const char* p)
{
int n = 0;
for (; *p != 0; ++p)
{
const int digit = *p - '0';
n = 10 * n + digit;
}
return n;
}
4) Now that the operations are a bit more visible, we can see some independent functionality embedded in this function; this should be factored out (this is called reactoring) into a separate function.
Namely, we see the operation of converting a character to a digit:
int todigit(const char c)
{
// this works because the literals '0', '1', '2', etc. are
// all guaranteed to be in order. Ergo '0' - '0' will be 0,
// '1' - '0' will be 1, '2' - '0' will be 2, and so on.
return c - '0';
}
int f(const char* p)
{
int n = 0;
for (; *p != 0; ++p)
n = 10 * n + todigit(*p);
return n;
}
5) So now it's clear the function reads a string character by character and generates a number digit by digit. This functionality already exists under the name atoi
, and this function is an unsafe implementation:
int todigit(const char c)
{
// this works because the literals '0', '1', '2', etc. are
// all guaranteed to be in order. Ergo '0' - '0' will be 0,
// '1' - '0' will be 1, '2' - '0' will be 2, and so on.
return c - '0';
}
int atoi_unsafe(const char* p)
{
int n = 0;
for (; *p != 0; ++p)
n = 10 * n + todigit(*p);
return n;
}
It's left as an exercise to the read to check for overflow, invalid characters (those that aren't digits), and so on. But this should make it much clearer what's going on, and is how such a function should have been written in the first place.