← IGCSE Computer Science Paper 2

Matching the variable's data type to the file data

The variable used with READFILE or WRITEFILE needs a data type that suits the data: STRING for a line of text, INTEGER for a whole number. A wrong type causes errors.

Part of: IGCSE Computer Science 0478 — 8.3 File Handling — Module 14

Understanding Matching the variable's data type to the file data in IGCSE Computer Science Paper 2

What it does: The variable is declared as STRING first, so it matches the text that READFILE reads. Total is declared as INTEGER, which suits the whole number that WRITEFILE stores. The data type must suit the data being read, otherwise errors can occur. "Hello class" is text, so STRING is needed. An INTEGER can only store whole numbers. The type is chosen in the DECLARE statement, before the variable is used with the file.

Common mistakes: Total is an INTEGER read from the file, so you can only add a whole number to it. "10" is a string. It should be Total ← Total + 10. Score is declared as INTEGER, so it must hold a whole number. A string cannot be stored in it. It should be Score ← 85.

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This module's key terms are also in a printable crossword (Modules 489–499) and word search (Modules 479–499).

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Learning

Duo

Which declaration suits a variable that will hold a line of text read from a file?

DECLARE LineOfText : INTEGERDECLARE LineOfText : STRING

Matches

DECLARE Age : STRING READFILE "ages.txt", Age Total ← Age + 1

↓

Age holds a whole number but is declared as STRING, so using it in arithmetic causes an error. It should be declared as INTEGER.

Applying

Meanings

Term

DECLARE LineOfText : STRING

Meaning

The variable used with READFILE or WRITEFILE needs a data type that suits the data: STRING for a line of text, INTEGER for a whole number. A wrong type causes errors.

Practice

DECLARE LineOfText : STRING

Testing

Gaps

DECLARE LineOfText :

Check Answer

Spots

3READFILE "total.txt", Total

4Total ← Total + "10"

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Quick Questions

1. Which declaration suits a variable that will hold a line of text read from a file?

A. DECLARE LineOfText : INTEGER

B. DECLARE LineOfText : STRING ✓

A line of text is made of characters, so the variable needs the STRING data type.

2. A file stores the whole number 42. Which declaration suits the variable used with READFILE?

A. DECLARE Score : INTEGER ✓

B. DECLARE Score : BOOLEAN

42 is a whole number, so INTEGER is the suitable data type. BOOLEAN only holds TRUE or FALSE.

3. Which statement about the variable used with READFILE is correct?

A. Its data type should suit the data in the file ✓

B. Its data type does not matter, any type works

The data type must suit the data being read, otherwise errors can occur.

4. A student wants to write the line "Hello class" to a file. Which variable type should they declare?

A. STRING ✓

B. INTEGER

"Hello class" is text, so STRING is needed. An INTEGER can only store whole numbers.

5. What is the problem with this code? DECLARE Age : STRING then WRITEFILE "Ages.txt", Age, when Age should be a whole number.

A. WRITEFILE can never be used with a variable

B. The type should be INTEGER to suit a whole number ✓

WRITEFILE can use variables. The error is the mismatched type: a whole number should be stored in an INTEGER variable.

Match the Pairs

DECLARE LineOfText : STRING → Creates a variable that can hold a line of text read from a file or written to a file

DECLARE Score : INTEGER → Creates a variable that can hold a whole number stored in a file

Choose the type when you declare the variable: a line of text needs a STRING variable before it is used with READFILE → DECLARE Name : STRING READFILE "names.txt", Name

A whole number written to a file should be held in an INTEGER variable → DECLARE Total : INTEGER WRITEFILE "total.txt", Total

DECLARE Age : STRING READFILE "ages.txt", Age Total ← Age + 1 → Age holds a whole number but is declared as STRING, so using it in arithmetic causes an error. It should be declared as INTEGER.

DECLARE Line : INTEGER READFILE "poem.txt", Line → Line is meant to hold a line of text but is declared as INTEGER, so the text cannot be stored. It should be declared as STRING.

Key Term

DECLARE LineOfText : STRING

The variable used with READFILE or WRITEFILE needs a data type that suits the data: STRING for a line of text, INTEGER for a whole number. A wrong type causes errors.

Fill in the Blanks

1. DECLARE LineOfText : STRING READFILE "story.txt", LineOfText

Hint: Type the data type that suits a whole line of text read from the file.

2. DECLARE Score : INTEGER READFILE "scores.txt", Score

Hint: Type the data type that suits a stored whole number.

3. DECLARE Mark : INTEGER READFILE "marks.txt", Mark

Hint: Type the name of the declared variable that will hold the data read from the file.

4. DECLARE Total : INTEGER Total ← 250 WRITEFILE "total.txt", Total

Hint: Type the data type that matches the whole number being written to the file.

5. DECLARE Line : STRING OPENFILE "poem.txt" FOR READ READFILE "poem.txt", Line

Hint: Type the command that reads one item from the open file into the variable.

Spot the Error

Question 1

A DECLARE Total : INTEGER

B OPENFILE "total.txt" FOR READ

C READFILE "total.txt", Total

D Total ← Total + "10"

E CLOSEFILE "total.txt"

Line D — Total is an INTEGER read from the file, so you can only add a whole number to it. "10" is a string. It should be Total ← Total + 10.

Question 2

A DECLARE Score : INTEGER

B OPENFILE "score.txt" FOR WRITE

C Score ← "eighty-five"

D WRITEFILE "score.txt", Score

E CLOSEFILE "score.txt"

Line C — Score is declared as INTEGER, so it must hold a whole number. A string cannot be stored in it. It should be Score ← 85.

Question 3

A DECLARE Name : STRING

B DECLARE Age : STRING

C OPENFILE "pupil.txt" FOR READ

D READFILE "pupil.txt", Name

E READFILE "pupil.txt", Age

F OUTPUT Age + 1

Line B — Age holds a whole number and is used in the sum Age + 1, so it must be declared as INTEGER, not STRING. It should be DECLARE Age : INTEGER.

Question 4

A DECLARE Total : INTEGER

B DECLARE Sentence : TEXT

C OPENFILE "story.txt" FOR READ

D READFILE "story.txt", Sentence

E Total ← LENGTH(Sentence)

F OUTPUT Total

Line B — TEXT is not a data type in 0478 pseudocode. A line of text read from a file needs the STRING type. It should be DECLARE Sentence : STRING.

Question 5

A DECLARE Message : STRING

B OPENFILE "msg.txt" FOR WRITE

C Message ← 100

D WRITEFILE "msg.txt", Message

E CLOSEFILE "msg.txt"

Line C — Message is a STRING variable for a line of text, so it cannot be given a whole number. It should be Message ← "Hello".

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