LEDA has a small number of useful functions ready, which are declared in the header file misc.h. We have already got to know some of them, for example read_string() for reading strings and print_statistics() for the output of information about the state of the memory manager.

Another frequently used function from misc.h is used_time() for measuring running times. It is used as follows: A call

float T = used_time();

without parameters returns the CPU time consumed until now. This time can be saved in a variable T for later measurings.

In order to find out at a later time how much time has passed since T, used_time() can be invoked with T as argument:

float elapsed_time = used_time(T);

The function then returns the difference between the two points in time, that is the time passed in between. In addition, it resets T to the CPU time consumed until then; thereby, calls of used_time(T) can be put in series to successively measure the times used up in individual code sections.

We demonstrate the use of used_time() by measuring the running times of individual sections in our anagram program from Section 2.3.5. This program is composed of the three partial steps reading, sorting and printing, and runs for a quite long time all in all since the number of words to be read and sorted (approximately 300,000) is quite large in our example dictionary. The function used_time() is used as follows to measure the running times of the three partial steps:

...
#include <LEDA/misc.h>
...
using leda::used_time;

// later inside main()

  // Set startTime and T to the currently used CPU time
  float startTime = used_time(); 
  float T = startTime; 

  // Read all words
  ...
  // Set T to the currently used CPU time and return the difference
  cout << "Time to read in words: " << used_time(T) << endl;
  
  // Sort according to user defined ordering
  ...
  cout << "Time to sort: " << used_time(T) << endl;

  // Print all sequences of equivalent words of length > 1
  ...
  cout << "Time to find all sequences of length > 1: " 
       << used_time(T) << endl;

  cout << "Total time: " << used_time(startTime) << endl;

The output of the program on the author's machine is:

Time to read in words: 7.51
Time to sort: 4.08
Time to find all sequences of length > 1: 0.4
Total time: 11.99