9.3. Pure functionsΒΆ

A function is considered a pure function if the result depends only on the arguments, and it has no side effects like modifying an argument or outputting something. The only result of calling a pure function is the return value.

One example is the function after, which compares two times and returns a bool that indicates whether the first operand comes after the second. Take a look at the active code below.

Example pure_function_AC_1
 1#include <iostream>
 2
 3struct time {
 4   int hour, minute;
 5   double second;
 6};
 7
 8bool after (time& time1, time& time2) {
 9   if (time1.hour > time2.hour) { return true; }
10   if (time1.hour < time2.hour) { return false; }
11   if (time1.minute > time2.minute) { return true; }
12   if (time1.minute < time2.minute) { return false; }
13   if (time1.second > time2.second) { return true; }
14   return false;
15}
16
17int main () {
18   time time = { 11, 59, 3.14159 };
19   time time2 = { 1, 50, 3.14159 };
20   std::cout << after(time, time2);
21}

What is the result of this function if the two times are equal? Does that seem like the appropriate result for this function? If you were writing the documentation for this function, would you mention that case specifically?

A second example is add_time, which calculates the sum of two times. For example, if it is 9:14:30, and your bread maker takes 3 hours and 35 minutes, you could use add_time to figure out when the bread will be done.

Here is a rough draft of this function that is not quite right:

time add_time (time& t1, time& t2) {
  time sum;
  sum.hour = t1.hour + t2.hour;
  sum.minute = t1.minute + t2.minute;
  sum.second = t1.second + t2.second;
  return sum;
}

Take a look at the active code below. If current_time contains the current time and bread_time contains the amount of time it takes for your breadmaker to make bread, then you could use add_time to figure out when the bread will be done.

Example pure_function_AC_2
 1#include <iostream>
 2
 3struct time {
 4    int hour, minute;
 5    double second;
 6};
 7
 8void print_time (time& t) {
 9    std::cout << t.hour << ':' << t.minute << ':' << t.second << '\n';
10}
11
12time add_time (time& t1, time& t2) {
13    time sum;
14    sum.hour = t1.hour + t2.hour;
15    sum.minute = t1.minute + t2.minute;
16    sum.second = t1.second + t2.second;
17    return sum;
18}
19
20int main() {
21    time current_time = { 9, 14, 30.0 };
22    time bread_time = { 3, 35, 0.0 };
23    time done_time = add_time (current_time, bread_time);
24    print_time (done_time);
25}

The output of this program is 12:49:30, which is correct. On the other hand, there are cases where the result is not correct. Can you think of one?

The problem is that this function does not deal with cases where the number of seconds or minutes adds up to more than 60. When that happens we have to "carry" the extra seconds into the minutes column, or extra minutes into the hours column.

Here's a second, corrected version of this function.

The active code below is the corrected version of add_time.

Example pure_function_AC_3
 1#include <iostream>
 2
 3struct time {
 4    int hour, minute;
 5    double second;
 6};
 7
 8void print_time (time& t) {
 9    std::cout << t.hour << ':' << t.minute << ':' << t.second << '\n';
10}
11
12time add_time (time& t1, time& t2) {
13    time sum;
14    sum.hour = t1.hour + t2.hour;
15    sum.minute = t1.minute + t2.minute;
16    sum.second = t1.second + t2.second;
17    if (sum.second >= 60.0) {
18        sum.second -= 60.0;
19        sum.minute += 1;
20    }
21    if (sum.minute >= 60) {
22        sum.minute -= 60;
23        sum.hour += 1;
24    }
25    return sum;
26}
27
28int main() {
29    time current_time = { 9, 14, 30.0 };
30    time bread_time = { 3, 35, 0.0 };
31    time done_time = add_time (current_time, bread_time);
32    print_time (done_time);
33}

Although it's correct, it's starting to get big. Later, I will suggest an alternate approach to this problem that will be much shorter.

This code demonstrates two operators we have not seen before, += and -=. These operators provide a concise way to increment and decrement variables. For example, the statement sum.second -= 60.0; is equivalent to sum.second = sum.second - 60;

Q1

Match the statement to its equivalent.