10.12. 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 howManyOnes, howManyTwos, 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:

int numValues = 100000;
int upperBound = 10;
vector<int> vector = randomVector (numValues, upperBound);
vector<int> histogram (upperBound);

for (int i = 0; i < upperBound; i++) {
  int count = howMany (vector, i);
  histogram[i] = count;

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 howMany, specifying which value I am interested in. Second, it is an index into the histogram, specifying which location I should store the result in.

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