finding if string has unique characters

[venkatacplpl]

bool isUnique(string _str)
{
bool char_set[256];
int len = _str.length();

memset(char_set, '\0', 256);
for(int i = 0; i < len; ++i)
{
int val = _str[i]- '0';
if(char_set[val])
{
return false;
}
char_set[val] = true;
}

return true;
}

Hi....I came across this code to find if string has unique characters...i didnt understand why they subracted ascii value of character '0' in the statement int val = _str[i]- '0' and what is happening with the statements...if(char_set[val])
{
return false;
}
char_set[val] = true;


i take each character in the sting and traverse the whole string .and if count is 2 i use break and conclude that its not unique and not otherwise...can i use this method or this is not efficient????

please help....i am new to c++...thanks..

[writetonsharma]

This is just trying to create an index value for each printable character.

lets say _str has "hello world" in it.
int val =_str[0] - '0' will give the ascii value of the first letter;
this will convert '0' to its ascii value 48 (is it 47 ?) and h to 104. So you will get 104 - 47 = 57;

similarly, for each character a unique value will be generated and then that index in char_set will be set to true;
in case that index is again generated (this will happen if that character will come again), then that index in char_set will already be true and it will be assumed that the character has already been seen and its not a unique string.

[venkatacplpl]

thanks @ writetonsharma (1243)

cant i work it out through alternative method like this...... i take each character in the sting and traverse the whole string .and if count is 2 i use break and conclude that its not unique and not otherwise...can i go this method or this is not efficient????

[venkatacplpl]

using this statement int val =_str[0] - '0' what if one of the characters in the string has ascii value less than 48 and val turns into negative....then particular character will not get any place in char_set for its boolean......why cant we leave it like this...int value=str[0]????

thanks

[JLBorges]

> int val =_str[0] - '0' what if one of the characters in the string has ascii value less than 48 '0' and val turns into negative

Undefined behaviour.


> why cant we leave it like this...int value=str[0]????

What if std::numeric_limits<char>::is_signed is true and one of the characters in the string has a negative value?

[writetonsharma]

We are talking about printable characters or only numeric/alphabets. So, nothing like -ve will come.. because anything below ascii 48 will not be in the input. obviously you can change the code to include non-printable characters too.

You can do any method you like and which is easy to understand. the idea is to learn programming and not write optimal code at the moment.

[JLBorges]

> We are talking about printable characters or only numeric/alphabets.
> So, nothing like -ve will come.
> because anything below ascii 48 will not be in the input.

Don't be asinine. You have no idea of what you are talking about.


@venkatacplpl - You have the right idea; now try to work out a correct way of doing this.

[writetonsharma]

This is what the program is suppose to do (or written).. not taking special characters and others... obviously there are printable characters below 48..

[venkatacplpl]

everyone....thanks for replying.