Stretch tasks Data Structures, Strings and Files

Round 1: Quiz Blitz

1. Many values of the same type under one name, found by index One loop can then handle any number of items.
2. Several related values of different types, each in a named field For example, a student's name, year and average, kept together.
3. 21 Indexing starts at 0: 14 is at 0, 9 at 1, 21 at 2.
4. False Its indexes are 0 to 4. LEN is 5, but the last index is LEN - 1.
5. 1 Row 1 is [0, 1, 0], and position 1 in it is 1.
6. 3 C is 0, o is 1, m is 2 and p is 3.
7. Com Indexes 0, 1 and 2, both ends included.
8. 65 Characters are stored as numbers: A is 65.
9. The integer 25 It converts the string to a number you can do arithmetic with.
10. Buffered output may not be written otherwise Output is often held in a buffer until the file is closed.
11. True Numbers must be converted before arithmetic.
12. They come from an algorithm and a seed, so they can be predicted The same seed gives the same sequence every time.

Round 2: Quick Fire

1. • An array holds many values of the same type, found by index
   • A record holds several values of different types, in named fields
   Model answer

   An array holds many values of the same type under one name, each found by its index. A record holds several related values of different types, each in a named field.

2. • LEN gives the number of items
   • The last index is one less, because indexing starts at 0
   Model answer

   LEN gives the number of items, but indexing starts at 0, so the last index is LEN(scores) - 1. Looping to LEN(scores) asks for one item too many.

3. • CHAR_TO_CODE('B') gives 66
   • Add 32 to get 98, then CODE_TO_CHAR turns it back into a letter
   Model answer

   CHAR_TO_CODE('B') gives 66. Lower case letters are always 32 further on, so 66 + 32 = 98, and CODE_TO_CHAR(98) gives 'b'.

4. • Everything read from a text file is a string
   • So it must be cast to a number, for example with STRING_TO_INT
   Model answer

   Everything read from a text file arrives as a string, so it must be cast to a number, for example with STRING_TO_INT, before any arithmetic.

Round 3: Big Think

1. • A record keeps one student's different values together in fields
   • The fields are a string, an integer and a real
   • An array holds one record per student, all the same type
   • So a loop can process every student
   Model answer

   Each student needs a name, a year group and an average mark, which are different types: a string, an integer and a real. A record can hold them together in named fields. An array then holds one record for each student, all of the same record type, so a FOR loop can go through every student by index.

2. • Opens the file
   • Reads it with readLine
   • Loops until the end of the file
   • Closes the file at the end
   Model answer

   AQA pseudo-codeCopy

   results ← OPEN('results.txt')
   line ← results.readLine()
   WHILE NOT results.endOfFile()
       OUTPUT line
       line ← results.readLine()
   ENDWHILE
   results.close()

This or That

1. A record An array holds many of the same thing. A record holds the different things about one thing.
2. 8 There are 9 characters, so the indexes run from 0 to 8.
3. B A is 65, so 66 is B. Lower case b would be 98.
4. 'Computing' Concatenation joins strings end to end, adding nothing between them.
5. An error That is why the loop must test for the end of the file.
6. The same random sequence Pseudo-random numbers are fully determined by the seed.
7. The first item in row 2 The first index chooses the row, the second the position in it.
8. 32 A is 65 and a is 97, and every letter pair is 32 apart.

Memory Match

• A 2 LEN('Computing'): 9
• B 4 POSITION('Computing', 'p'): 3
• C 6 SUBSTRING(0, 2, 'Computing'): Com
• D 1 CHAR_TO_CODE('A'): 65
• E 3 INT_TO_STRING(25): '25'
• F 5 'Comp' + 'uting': Computing

Sort It

1. Array Every item is found by an index counted from 0 Arrays are indexed from 0.
2. Text file Read line by line until endOfFile() is true Reading past the end of a file is an error.
3. Text file Everything read from it arrives as a string Text files hold text, so numbers must be converted.
4. Record student.name ← 'Amira' Setting the name field of a record.
5. Array scores[3] ← 11 Changing the item at index 3 of an array.
6. Record Several fields of different types about one thing That is the definition of a record.
7. Array Many values of the same type under one name That is the definition of an array.
8. Array A grid, such as a seating plan A two-dimensional array is an array of arrays.
9. Text file Keeps data after the program closes Writing to a file makes data persist.
10. Text file Must be opened before use and closed afterwards Files are opened and closed.
11. Record A name, a year group and an average, kept together Different types for one student: a record.