loops - c++ mathematical calculations -

i have solve problem using c++, , can't come solution.

the condition is. in skyscraper there lot of offices, on every office door there must put plate number 1 number of last office. plate can contain 1 digit, if office number contains 2 digits there must 2 plates. how many plates total needed office doors.

the text file duom.txt. value of "16". means theres 16 offices.

this managed come with:

# include <iostream> # include <cmath> # include <iomanip> # include <fstream> using namespace std; const char cdfv [] = ("duom.txt"); const char crfv [] = ("rez.txt"); int main() {   int n;   int m;   int o;   int z;   ifstream fd (cdfv);   ofstream fr (crfv);   fd>>n;    (int i=1;i<10;i++) {     m=n+1;     (int j=1;j>=10;j++) {       o=n+2;     }   }   z=m+o;   fr<<z<<" "; } fd.close(); fr.close(); return 0; } 

i know not correct, please me this. glad if tried compiling it, because codeblocks refuses work compiler if installed seperately manually. :/ thats problem.

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seems solution has been found, helped me.

# include <iostream> # include <cmath> # include <iomanip> # include <fstream> using namespace std; const char cdfv [] = ("duom.txt"); const char crfv [] = ("rez.txt"); int main() { int limit = 10; int = 1; int viso = 0; ifstream fd (cdfv); ofstream fr (crfv); fd>>n; ( int = 1; i<=n ;++i ){     if (i==limit){         limit*=10;         up+=1;     }     viso += up; }         fr<<viso<<" ";      fd.close();     fr.close();     return 0; } 

this should work non text file variant works charm, here: http://ideone.com/xofqwn

simply iterate 1 maximum number of offices n , add number of required plates total. 1 9 need 1 plate, 10 99 need 2 , on. implement using limit , step. limit indicates when need more plates. , step indicates how many plate need office.

int limit = 10; int step = 1; int total = 0; ( int = 1; i<=n ;++i ){     if (i==limit){         limit*=10;         step+=1;     }     total += step; } cout << total << endl;