c++ - Number changing value when assigned to variable? -

i have little problem when assigning result variable, happening first time me now. call convert() "aaa" parameter, here output:

aaa  **676** *(value  cout)* = 26^(3-1)*1        **675** *(value of variable)*  +26 = 26^(3-2)*1 700  +1 = 26^(3-3)*1  701  701 

and here code:

string alphabet="abcdefghijklmnopqrstuvwxyz";  unsigned long long convert(string &str){   unsigned long long wvalue=0;   for(int i=0;i<str.size();++i){     size_t found=alphabet.find(str[i]);     if(found==string::npos)       cout<<"please enter lowercase letters of english alphabet!"<<endl;      unsigned long long add=((found+1)*pow(26,(str.size()-(i+1))));     wvalue+=add;     if(i>0)cout<<"+";     cout<<"\t"<<((found+1)*pow(26,(str.size()-(i+1))))<<" = "<<"26^("<<str.size()<<"-"<<(i+1)  <<")*"<<(found+1)<<"\t"<<wvalue<<endl;   }   return wvalue; } 

chances i'm missing awfully obvious, cannot figure out.

((found+1)*pow(26,(str.size()-(i+1))))  

is doing calculation, , doing supposed to, result within cout-statment correct. variable substracted 1 in first 2 assignments.

pow floating-point function. takes , returns floating point numbers. assigning floating-point number integer variable truncates integer number, might have been 675.9999999 before assignment, turn 675 when assigned integer variable add.

cout rounds floating-point numbers, depending on configuration example 6 significant digits. 676.0 better approximation 675.999, see 676 in output.

since don't want calculate real numbers integral numbers, better stay integral functions. take 26 power of n, better use multiplication n times. since use loop, , have next power of 26 every character, best add variable in keep current power value, this:

unsigned long long currentfactor = 1; (...) {     ...     unsigned long long add = currentfactor * (found+1);     wvalue += add;     currentfactor *= 26; } 

also note don't have find character in alphabet string. can use character arithmetic this:

int charnumber(char c) {     if (c >= 'a' && c <= 'z')         return c - 'a';   // calculate position of c relative 'a'     else         return -1;        // error }