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# Lab Assignment 5

 Topic: Doubles Source Code: doubles.cpp Live Archive Ref#: 2787 In-Class Day: Thursday, Oct. 13, 2011 Submission Deadline: Sunday, Oct. 16, 2011, 11:59pm Techniques: Use of fixed-sized arrays

Please review the general information about lab assignments.

# Doubles

As part of an arithmetic competency program, your students will be given randomly generated lists of from 2 to 15 unique positive integers and asked to determine how many items in each list are twice some other item in the same list. You will need a program to help you with the grading. This program should be able to scan the lists and output the correct answer for each one. For example, given the list

`1 4 3 2 9 7 18 22`

your program should answer 3, as 2 is twice 1, 4 is twice 2, and 18 is twice 9.

The input file will consist of one or more lists of numbers. There will be one list of numbers per line. Each list will contain from 2 to 15 unique positive integers. No integer will be larger than 99. Each line will be terminated with the integer 0, which is not considered part of the list. A line with the single number -1 will mark the end of the file. The example input below shows 3 separate lists. Some lists may not contain any doubles.

The output will consist of one line per input list, containing a count of the items that are double some other item.

 Example input: Example output: 1 4 3 2 9 7 18 22 0 2 4 8 10 0 7 5 11 13 1 3 0 -1 3 2 0

# Hints

This should be an easy one, but it requires a two-stage process for each group. First, we suggest having an array of boolean values that can be used to "mark" which numbers are in the set. After initializing the array to be "empty" you can read the input and mark each number in the set. Once you have finished reading the group of numbers, a loop through the array can be used to count how many times a value and its double are in the set.

Warnings: There are two common pitfalls to avoid

• Don't use illegal array indices. Remember that C++ will not stop you from doing so; instead, it will blindly access a part of memory that is not really the array.
• Make sure to reset your array for each new group. We do not want numbers from one trial to affect the next.

### Pre-lab

What would the output be if the following was entered:
1 2 4 8 16 32 0
2 4 6 8 10 0
-1