11.6. A histogram¶
It is often useful to take the data from the previous tables and store
them for later access, rather than just print them. What we need is a
way to store 10 integers. We could create 10 integer variables with
names like how_many_ones, how_many_twos, etc. But that would require
a lot of typing, and it would be a real pain later if we decided to
change the range of values.
A better solution is to use a vector with size 10. That way we can create all ten storage locations at once and we can access them using indices, rather than ten different names. Here’s how:
std::size_t num_values = 100000;
int upper_bound = 9;
std::vector<int> numbers = make_vector (num_values, upper_bound);
std::vector<std::size_t> histogram (static_cast<std::size_t>(upper_bound) + 1);
for (std::size_t i = 0; i < histogram.size(); ++i) {
histogram[i] = how_many (numbers, static_cast<int>(i));
}
I called the vector histogram because that’s a statistical term for a vector of numbers that counts the number of appearances of a range of values.
The tricky thing here is that I am using the loop variable in two
different ways. First, it is an argument to how_many, specifying
which value I am interested in. Second, it is an index into the
histogram, specifying which location I should store the result in.
We are making a major assumption with our histogram:
we assume we will remember that each index position in histogram
also corresponds to the value counted.
In other words, index position 0 stores the count
for how many times the value 0 was randomly generated.
The value 0 is not explicitly stored anywhere -
it is implied from the position.
If we reorder the data in the histogram, our results are invalid.
Q1
Which of the following statements are true about using vectors to store data?