Generate random exponential value correctly c++

I want to generate random number belonging to an exponential distribution. I wrote this

    int size = atoi(argv[2]);
    double *values = (double*)malloc(sizeof(double)*size);

    double gamma = atof(argv[1]);
    if(gamma<=0.0){
        cout<<"Insert gamma"<<endl;
        return 0;
    }


    for(int i=0; i<size; i++){
        values[i]=0;

    }

    srand ( time(NULL) );
    for(int i=0; i<size; i++){
        x = ((double) rand() / (RAND_MAX));
        //cout << random <<endl;
        value=(log(1.0-x)/(-gamma));
        //count each value
        values[value]=values[value]+1.0;
    }

But they do not cover all the vector's size. More or less they cover 10% of the vector, the other fields are all 0 and due to the fact that after I need to do a linear interpolation I want to reduce those 'empty space' in order to have at least one value for each cell of the array, How can I do it? for example I have a vector of 100000 only the first 60 fields are filled with values so cells from 60 to 999999 are all 0 and when I do linear regression they impact negatively on formula.