10.10. Counting

A good approach to problems like this is to think of simple functions that are easy to write, and that might turn out to be useful. Then you can combine them into a solution. This approach is sometimes called bottom-up design. Of course, it is not easy to know ahead of time which functions are likely to be useful, but as you gain experience you will have a better idea.

Also, it is not always obvious what sort of things are easy to write, but a good approach is to look for subproblems that fit a pattern you have seen before.

Back in Section 7.9 we looked at a loop that traversed a string and counted the number of times a given letter appeared. You can think of this program as an example of a pattern called “traverse and count.” The elements of this pattern are:

In this case, I have a function in mind called howMany that counts the number of elements in a vector that equal a given value. The parameters are the vector and the integer value we are looking for. The return value is the number of times the value appears.

int howMany (const vector<int>& vec, int value) {
  int count = 0;
  for (size_t i = 0; i < vec.size(); i++) {
    if (vec[i] == value) count++;
  return count;

Take a look at the active code below which uses the howMany function. Run the code to see how many times the target appears in the vector! Feel free to modify the code and experiment around.

Construct a block of code that counts how many numbers are between lowerbound and upperbound inclusive.

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