1.4.4. The if Statement

Like most languages, C++ provides an if statement that supports conditional execution. We can use an if to write a program to count how many consecutive times each distinct value appears in the input:

#include <iostream>
int main()
{
    // currVal is the number we're counting; we'll read new values into val
    int currVal = 0, val = 0;
    // read first number and ensure that we have data to process
    if (std::cin >> currVal) {
        int cnt = 1;  // store the count for the current value we're processing
        while (std::cin >> val) { // read the remaining numbers
            if (val == currVal)   // if the values are the same
                ++cnt;            // add 1 to cnt
            else { // otherwise, print the count for the previous value
                std::cout << currVal << " occurs "
                          << cnt << " times" << std::endl;
                currVal = val;    // remember the new value
                cnt = 1;          // reset the counter
            }
        }  // while loop ends here
        // remember to print the count for the last value in the file
        std::cout << currVal <<  " occurs "
                  << cnt << " times" << std::endl;
    } // outermost if statement ends here
    return 0;
}

If we give this program the following input:

42 42 42 42 42 55 55 62 100 100 100

then the output should be

42 occurs 5 times
55 occurs 2 times
62 occurs 1 times
100 occurs 3 times

Much of the code in this program should be familiar from our earlier programs. We start by defining val and currVal: currVal will keep track of which number we are counting; val will hold each number as we read it from the input. What’s new are the two if statements. The first if

if (std::cin >> currVal) {
    // ...
} // outermost if statement ends here

ensures that the input is not empty. Like a while, an if evaluates a condition. The condition in the first if reads a value into currVal. If the read succeeds, then the condition is true and we execute the block that starts with the open curly following the condition. That block ends with the close curly just before the return statement.

Once we know there are numbers to count, we define cnt, which will count how often each distinct number occurs. We use a while loop similar to the one in the previous section to (repeatedly) read numbers from the standard input.

The body of the while is a block that contains the second if statement:

if (val == currVal)   // if the values are the same
    ++cnt;            // add 1 to cnt
else { // otherwise, print the count for the previous value
    std::cout << currVal << " occurs "
              << cnt << " times" << std::endl;
    currVal = val;    // remember the new value
    cnt = 1;          // reset the counter
}

The condition in this if uses the equality operator (the == operator) to test whether val is equal to currVal. If so, we execute the statement that immediately follows the condition. That statement increments cnt, indicating that we have seen currVal once more.

If the condition is false—that is, if val is not equal to currVal—then we execute the statement following the else. This statement is a block consisting of an output statement and two assignments. The output statement prints the count for the value we just finished processing. The assignments reset cnt to 1 and currVal to val, which is the number we just read.