How To Think Like a Computer Scientist C++ Edition The Pretext Interactive Version

The [] operator reads and writes the elements of a vector in much the same way it accesses the characters in an string. This is called vector indexing. As with strings, the indices start at zero, so count[0] refers to the “zeroeth” element of the vector, and count[1] refers to the “oneth” element. You can use the [] operator anywhere in an expression:

count[0] = 7;
count[1] = count[0] * 2;
count[2]++;
count[3] -= 60;

All of these are legal assignment statements. Here is the effect of this code fragment:

You can use any expression as an index, as long as it has type int. One of the most common ways to index a vector is with a loop variable. For example:

int i = 0;
while (i < 4) {
  cout << count[i] << endl;
  i++;
}

This while loop counts from 0 to 4; when the loop variable i is 4, the condition fails and the loop terminates. Thus, the body of the loop is only executed when i is 0, 1, 2 and 3.

Each time through the loop we use i as an index into the vector, outputting the ith element. This type of vector traversal is very common. Vectors and loops go together like fava beans and a nice Chianti.