C++ Why does my code give the desired results on Ubuntu but has undefined behavior on Windows 7 -

i taking c++ course in university, every week required write program given specifications. it's basic tasks resolve around topics discussed in class week before. discussing pointers arrays , dynamic memory naturally week's program using arrays , dynamic memory allocation.

to break down, program asked write supposed take random user input (no white spaces, random characters) , divide user input in 3 categories: upper case, lower case , special characters.

the method used not efficient 1 job, considering libraries allowed use iostream , cmath, here comes problem. in class wrote program in no time, compiled , ran straight away no issues. if give user input such aaghc4 have output ac agh 4

so went ahead , downloaded program university folder on private machine @ home work on little, make more efficient practice. when compiled (unedited , original) .cpp on windows machine it's output random, undefined behavior. both in class , @ home use g++ compile files, @ home run windows 7, in class use ubuntu 14.04.

i greatful if take @ code , tell me why doesn't work on windows machine.

#include<iostream>  using namespace std;  int main(){    char input[80];   int counter = 0;    cin >> input;   for(int = 0; input[i] != '\0'; i++){   counter++;   }    char *uppercase = new char[counter];   char *lowercase = new char[counter];   char *special = new char[counter];    for(int = 0; < (counter+1); i++){   switch(input[i]){     case 'a': lowercase[i] = input[i]; break;     case 'b': lowercase[i] = input[i]; break;     // edited part goes on entire     // alphabet, first lowercase letters uppercase     case 'y': uppercase[i] = input[i]; break;     case 'z': uppercase[i] = input[i]; break;     default: special[i] = input[i]; break;     }   }    for(int = 0; <= counter; i++){     cout << uppercase[i];   }    cout << " ";   for(int = 0; <= counter; i++){     cout << lowercase[i];   }    cout << " ";   for(int = 0; <= counter; i++){     cout << special[i];   }    delete[] uppercase;   delete[] lowercase;   delete[] special;   cout << endl; } 

this entire source code, nothing more , nothing less, thank replies!

you have undefined behavior in both programs, no matter if run on ubuntu or windows 7; lucky shows behavior expected on former.

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reading uninitialized memory = undefined behavior

you assuming valid range of elements read in of uppercase, lowercase, , special of [0, counter] isn't case; reading uninitialized memory in loops iterate on elements of these arrays (and undefined behavior emerge).

the reason think it's working when running code on ubuntu because memory of 3 arrays happens zero-initialized; meaning loops print bunch of '\0' (normally not visible in terminal) when haven't assigned particular element value.

char * p = new char[counter]   ; // default-initialized `char`s (could have value) char * p = new char[counter] (); // value-initialized, elements `char()` ie. 0 

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reading past end of array = undefined behavior

there's problem loops prints contents of uppercase, lowercase, , special has off-by-one problem.

the last element offset of these loops counter-1, assume counter (i <= counter).

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solution

• don't print elements haven't, explicitly, given value.

  this can accomplished separate counter each of arrays, or other means (like initializing every element of arrays 0 , stop iterating when such value found).

• don't read elements aren't part of memory allocated.