I am trying to find a character within a string but I am getting unexpected results. My understanding is that string::find(char c)
returns -1
when it is not found. However, I am getting some unexpected results.
Even though the string does not include an '8'
, it is still returning true
.
std::string s = "123456799";
if(s.find('8')<0)
cout << "Not Found" << endl;
else
cout << "Found" << endl;
//Output: Found
However, when using ==
instead the code works as expected.
std::string s = "123456799";
if(s.find('8')==-1)
cout << "Not Found" << endl;
else
cout << "Found" << endl;
//Output: Not Found
My understanding is that
string::find(char c)
returns-1
when it is not found.
It's not accurate. According to the documentation :
Return value
Position of the first character of the found substring or npos if no such substring is found.
So to be precise, when not found std::string::find
will return std::string::npos . The point is that the type of std::string::npos
is std::string::size_type
, which is an unsigned integer type. Even it's initialized from value of -1
, it's not -1
; it's still unsigned. So s.find('8')<0
will always be false
because it's not possible to be negative.
Documentation of std::string::npos :
static const size_type npos = -1;
This is a special value equal to the maximum value representable by the type
size_type
.
So you should use std::string::npos for checking the result, to avoid such kind of confusing.
if (s.find('8') == std::string::npos)
cout << "Not Found" << endl;
else
cout << "Found" << endl;
if(s.find('8')==-1)
works fine, because the left-hand operand of operator== here is unsigned, the right-hand one is signed. According to the rules for arithmetic operators ,
- Otherwise, if the unsigned operand's conversion rank is greater or equal to the conversion rank of the signed operand, the signed operand is converted to the unsigned operand's type.
So -1
will be converted to unsigned, which is the value of std::string::npos
and then all work as expected.
string::find()
返回size_t
,这是一个unsigned int,所以它永远不会是负数。
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.