Determine if all characters in a string are unique in C++ [closed] - c++

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Trying to implement a fairly simple program in C++. I'm kinda new to this language. But it doesn't seem to be working.
#include <iostream>
#include <string>
using namespace std;
bool isUnique(string);
int main(){
bool uniq;
string a;
cout << "Please input a string, not a very long one...."<< endl;
getline(cin, a);
uniq = isUnique(a);
if (uniq == true)
{
cout << "The string has no repeatations." <<endl;
}else{
cout << "The characters in the string are not unique." <<endl;
}
return EXIT_SUCCESS;
}
bool isUnique(string str){
int len = strlen(str);
bool uniq = true;
for (int i = 0; i <= len; ++i)
{
for (int j = i+1; j <= len; ++j)
{
if (str[j] == str[i])
{
uniq = false;
}
}
}
return uniq;
}
The program compiles but has some logical errors I suppose. Any help appreciated.

Answer 1

An simple criterion for uniqueness is that there are no repeated characters in the sorted range of characters. There are algorithms in the standard library for this:
#include <algorithm> // for std::sort, std::unique
#include <iostream> // for std::cin, std::cout
#include <string> // for std:getline, std::string
int main()
{
std::string input;
std::cout << "Please input a string, not a very long one: ";
std::getline(input, std::cin);
std::sort(input.begin(), input.end());
bool u = std::unique(input.begin(), input.end()) == input.end();
if (u) { std::cout << "Every character is unique.\n"; }
else { std::cout << "The string contains repeated characters.\n"; }
}
As an optimization, you can exit early if the string has more characters than there are unique characters, though you'd need some way to determine what that number is.

Answer 2

You can check uniqueness much easier without a nested loop: make an array of bool[256], cast char to unsigned char, and use as an index into the array. If a bool has been set, the characters are not unique; otherwise, they are unique.
bool seen[256];
for (int i = 0 ; i != str.length() ; i++) {
unsigned char index = (unsigned char)str[i];
if (seen[index]) return false;
seen[index] = true;
}
return true;
The idea is simple: you mark characters that you've seen as you go, returning false if you see a "marked" character. If you reach the end without returning, all characters are unique.
This algorithm is O(n); your algorithm is O(n2). This does not make much difference, though, because it is impossible to construct a string of unique characters that is longer than 256 characters.

Answer 3

You are using a string, so it is not necessary to convert it to a char array. Use the string to check. You can check it like this:
bool isUnique(string str){
for (std::string::size_type i = 0; i < str.size(); ++i)
{
if(i < str.size()-1){
for (std::string::size_type j = i+1; j < str.size(); ++j)
{
if (str[j] == str[i])
{
uniq = false;
}
}
}
}
return uniq;
}

Answer 4

you can try this:
int main () {
bool uniqe=false;
string a;
char arr[1024];
int count[256]={0};
cout << "Please input a string, not a very long one...."<< endl;
getline(cin, a);
strcpy(arr, a.c_str());
for(int i=0;i<strlen(arr);i++)
count[(int)(arr[i])]++; // counting the occurence of character
for(int i=0;i<256;i++){
if(count[i]>1){ // if count > 1 means character are repeated.
uniqe=false;
break;
}else{
uniqe=true;
}
}
if(uniqe)
cout << "The string has no repeatations." <<endl;
else
cout << "The characters in the string are not unique." <<endl;
return 0;
}

Answer 5

There are too many errors in your code. For example instead of
int len = sizeof(arr)/sizeof(*arr);
there shall be
size_t len = std::strlen( arr );
Or instead of
for (int i = 0; i <= len; ++i)
there shall be at least
for (int i = 0; i < len; ++i)
and so on.
And there is no any need to define a character array. Class std::string has all that is required to do the task.
Try the following function
bool isUnique( const std::string &s )
{
bool unique = true;
for ( std::string::size_type i = 0; i < s.size() && unique; i++ )
{
std::string::size_type j = 0;
while ( j < i && s[j] != s[i] ) ++j;
unique = j == i;
}
return unique;
}
Here is a demonstrative program
#include <iostream>
#include <iomanip>
#include <string>
bool isUnique( const std::string &s )
{
bool unique = true;
for ( std::string::size_type i = 0; i < s.size() && unique; i++ )
{
std::string::size_type j = 0;
while ( j < i && s[j] != s[i] ) ++j;
unique = j == i;
}
return unique;
}
int main()
{
std::string s( "abcdef" );
std::cout << std::boolalpha << isUnique( s ) << std::endl;
s = "abcdefa";
std::cout << std::boolalpha << isUnique( s ) << std::endl;
return 0;
}
The output is
true
false

Answer 6

Here is your code with the errors fixed:
#include <iostream>
using namespace std;
bool isUnique(string,int); //extra parameter
int main(){
bool uniq;
string a;
cout << "Please input a string, not a very long one...."<< endl;
getline(cin, a);
uniq = isUnique(a,a.length()); //pass length of a
if (uniq == true)
{
cout << "The string has no repeatations." <<endl;
}else{
cout << "The characters in the string are not unique." <<endl;
}
return EXIT_SUCCESS;
}
bool isUnique(string str,int len){
bool uniq = true;
for (int i = 0; i < len-1; ++i) //len-1 else j would access unitialized memory location in the last iteration
{
for (int j = i+1; j < len; ++j) //j<len because array index starts from 0
{
if (str[j] == str[i])
{
uniq = false;
}
}
}
return uniq;
}