Version 2

If you emulated Version 1 precisely you will have noticed that you did a lot of unnecessary work when the mark was 96. Your simulation should have revealed that the grade passed the test in the first if statement and output "A+". So far so good. Next though it went on to evaluate the second if test, and the third, and the fourth and so on, all the way down the list. But, once we determined that the grade was an A+ we didn't have to do any of the other tests, because if the grade is between 95 and 100, it can't be in any of the other ranges.

The way to avoid the unnecessary tests is to perform the tests following the first one with elif statements. The structure of the code is:

# Version 2.
print " ... " # Instructions to user
grade = input( " ... " )
if grade >= 95 and grade <= 100:
    print "A+"
elif grade >= 86 and grade <= 94:
    print "A"
elif grade >= 80 and grade <= 85:
    print "A-"
# ...
# and so on down to
# ...
elif grade >= 50 and grade <= 54:
    print "D"
elif grade >= 0 and grade <= 50:
    print "F"
else:
    print "Error: The grade must be between 0 and 100."

Before we analyze this code, again verify that it works by simulating its execution for a couple of sample numerical grades, e.g. 65 and 93.

This is an improvement over the original code. For one thing we have eliminated the final error condition test completely: if the mark doesn't fall in any of the specified ranges then it must be out of range. But it also, on average does fewer tests. Consider, if the grade is an A+, e.g. 96, it does only the first if and skips the rest. Of course if the grade is an F it still does all the tests as it bumps its way down the list. So it never does more tests and often does fewer. On average (if the average grade is in the middle of the list) it will do half as many if tests as the first version, making it twice as fast.

However, careful examination reveals that we still have more tests than are required, and those unnecessary tests waste processor time and slow down the program. What are the unnecessary tests? This time it's not unnecessary tests being evaluated, it's unnecessary comparisons being performed inside the tests.