C++ Example of a Complicated Pointer Expression Inside a Loop in Visual C#.NET

Generating EAN / UCC - 13 in Visual C#.NET C++ Example of a Complicated Pointer Expression Inside a Loop

for ( i = 0; i < rateCount; i++ ) { netRate[ i ] = baseRate[ i ] * rates->discounts->factors->net; }

In this case, assigning the complicated pointer expression to a well-named variable improves readability and often improves performance.

quantityDiscount = rates->discounts->factors->net; for ( i = 0; i < rateCount; i++ ) { netRate[ i ] = baseRate[ i ] * quantityDiscount; }

The extra variable, quantityDiscount, makes it clear that the baseRate array is being multiplied by a quantity-discount factor to compute the net rate. That wasn t at all clear from the original expression in the loop. Putting the complicated pointer expression into a variable outside the loop also saves the pointer from being dereferenced three times for each pass through the loop, resulting in the following savings:

Time Savings 19% 13% 43%

Except for the Java compiler, the savings aren t anything to crow about, implying that during initial coding you can use whichever technique is more readable without worrying about the speed of the code until later.

When you have a loop with a compound test, you can often save time by simplifying the test. If the loop is a search loop, one way to simplify the test is to use a sentinel value, a value that you put just past the end of the search range and that s guaranteed to terminate the search. The classic example of a compound test that can be improved by use of a sentinel is the search loop that checks both whether it has found the value it is seeking and whether it has run out of values. Here s the code:

found = FALSE; i = 0; while ( ( !found ) && ( i < count ) ) { if ( item[ i ] == testValue ) { found = TRUE; } else { i++; } } if ( found ) { ...

In this code, each iteration of the loop tests for !found and for i < count. The purpose of the !found test is to determine when the desired element has been found. The purpose of the i < count test is to avoid running past the end of the array. Inside the loop, each value of item[] is tested individually, so the loop really has three tests for each iteration. In this kind of search loop, you can combine the three tests so that you test only once per iteration by putting a sentinel at the end of the search range to stop the loop. In this case, you can simply assign the value you re looking for to the element just beyond the end of the search range. (Remember to leave space for that element when you declare the array.) You then check each element, and if you don t find the element until you find the one you stuck at the end, you know that the value you re looking for isn t really there. Here s the code:

// set sentinel value, preserving the original value initialValue = item[ count ]; item[ count ] = testValue; i = 0; while ( item[ i ] != testValue ) { i++; } // restore the value displaced by the sentinel item[ count ] = initialValue; // check if value was found if ( i < count ) { ...

When item is an array of integers, the savings can be dramatic:

Time Savings 23% 44% 65%

Performance Ratio 1.3:1 2:1 3:1

The Visual Basic results are particularly dramatic, but all the results are good. When the kind of array changes, however, the results also change. Here are the results when item is an array of single-precision floating-point numbers: