Free A-Level Computer Science guide

OCR pseudocode: strings, files and language differences

Trace OCR Appendix 5d pseudocode, then preserve its meaning in executable code.

Content owner: Michael Print · Written for A-Level learners · Checked against official specifications

The idea to start with

OCR substring uses a start and character count. FOR includes both endpoints; DO...UNTIL stops when true. Translate these meanings into the chosen language.

Use openRead/readLine/endOfFile/close for input and openWrite/writeLine/close for output. Scalar byVal changes a copy; byRef can change the caller's variable. Python sharing differs.

OCR H446 · 2.2.1(a,d), 2.3.1(a); Appendix 5d, printed pages 33–36; supporting inline CSS from 1.3.4(a) and Appendix 5d page 39

Before you start

Useful foundations

Variables, selection and iteration
Functions, procedures and basic text files

By the end, you should be able to

Translate substring, arithmetic and casting operations accurately
Read and write text lines with an EOF-controlled loop
Preserve inclusive endpoints and post-test stopping conditions
Trace scalar parameters passed by value and reference
Recognise switch branches and an inline CSS declaration

Arithmetic and casts: the symbols have a contract

OCR ^ means exponentiation; Python/JavaScript use **. JavaScript ^ is bitwise XOR: 2^5 gives 7, not 32. For non-negative integers and a positive divisor, MOD becomes %; DIV becomes Python // or JavaScript Math.floor(a/b). Negative operands need separate conventions.

OCR int, str and float convert types; Python uses these names. JavaScript Number converts valid numeric strings; String produces text. Number("14") and Number("2.75") both produce Number values: it is not a truncating integer cast. Languages accept different inputs, so validate separately.

Strings: a count is different from an end position

OCR indexes from zero. BADGER has length 6 and positions 0…5. The appendix spells the method subString and substring; both descriptions use a start and character count.

Translate the meaning rather than copying argument numbers. Assume ASCII and valid non-negative positions: languages can count complex Unicode text differently.

Select three characters starting at position 2

Find the start
B=0, A=1, D=2, G=3, E=4, R=5. Begin at D.
Count three characters
Take positions 2,3,4: DGE. OCR text.subString(2,3) uses a count.
Translate the end boundary
Python text[2:5] and JavaScript text.substring(2,5) exclude position 5. JavaScript (2,3) would take only D.

Extract three characters starting at position 2
Notation	Expression	Result
OCR	text.subString(2,3)	DGE
Python	text[2:5]	DGE
JavaScript	text.substring(2,5)	DGE

Loops and branches: preserve the condition's meaning

OCR for i=2 to 5, ending next i, visits 2,3,4,5. Python range(2,6) excludes its stop; JavaScript can use i<=5.

Python can implement a post-test stopping condition with a break after the body. Preserve both test position and condition meaning.

OCR switch entry: chooses a case or default, ending endswitch. JavaScript break prevents completed branches falling into later cases. An if/elseif/else chain can express the same choices.

Where and how does each loop test?

WHILE: test first: Continue while true. The body can run zero times.
OCR DO...UNTIL: test afterwards: Run the body first, then stop when true. At least one execution.
JavaScript do...while: test afterwards: Run the body first, then continue when true. Translate UNTIL count==3 into while(count!==3).

Files: distinguish a blank record from the end

names.txt contains LYNX, a blank line, OWL and OTTER. prefixes.txt should receive LY, OW and OT: take two-character prefixes only from names of length at least three.

A blank line is a record, not EOF. It fails the length test but must not prevent reading OWL and OTTER. openWrite overwrites previous contents; close both handles afterwards.

Python file iteration stops at EOF. Remove the newline before checking length; a with block closes its handle even if an exception occurs.

One file-processing iteration

Check EOF
Continue only while NOT file.endOfFile().
Read one record
Call readLine once. A blank record still counts as input.
Apply the length rule
If length≥3, write its first two characters. Otherwise skip this record.
Return to the EOF check
Keep processing later records; finish only at the end of the input.

Inline CSS is a declaration inside an HTML attribute

style is the HTML attribute; color is the CSS property; #124A66 is its hexadecimal value. Colon separates property/value; semicolon ends the declaration. External h1 { color: #124A66; } targets elements through a selector.

Original HTML with inline CSShtml

<h1 style="color:#124A66;">File summary</h1>

Worked example

Trace arithmetic and type conversion

Python prints 32 4 5, then 14 2.75 '14'. Quotes distinguish the final string from the integer. JavaScript implements the same traced operations below.

Original OCR arithmetic/casting trace
OCR expression	Value/type
2^5	32, integer
29 DIV 6	4, integer quotient
29 MOD 6	5, remainder
int("14")	14, integer
float("2.75")	2.75, real
str(14)	"14", string

Runnable Python 3: arithmetic and castspython

print(2 ** 5, 29 // 6, 29 % 6)
number = int("14")
amount = float("2.75")
label = str(number)
print(number, amount, repr(label))

Worked example

Read, filter and write in OCR pseudocode

Assume names.txt contains the four stated lines. File variables hold handles. Each iteration consumes one line.
Skipping the blank preserves later OWL and OTTER processing. Empty input produces empty output. Test lengths 2 and 3 at the boundary.

Original algorithm using Appendix 5d file/string operationstext

source = openRead("names.txt")
target = openWrite("prefixes.txt")
while NOT source.endOfFile()
    name = source.readLine()
    if name.length >= 3 then
        target.writeLine(name.subString(0,2))
    endif
endwhile
source.close()
target.close()

Worked example

Execute the text-file algorithm in Python

A temporary folder contains the example files.
Outputs are LY, OW and OT on separate lines. [0:2] excludes position 2. Empty files and short names produce no selected lines.

Runnable Python 3: strings and text-file processingpython

from pathlib import Path
from tempfile import TemporaryDirectory

def write_prefixes(source_path, target_path):
    with source_path.open("r", encoding="utf-8") as source:
        with target_path.open("w", encoding="utf-8") as target:
            for line in source:
                name = line.rstrip("\n")
                if len(name) >= 3:
                    target.write(name[0:2] + "\n")

with TemporaryDirectory() as folder:
    source = Path(folder) / "names.txt"
    target = Path(folder) / "prefixes.txt"
    source.write_text("LYNX\n\nOWL\nOTTER\n", encoding="utf-8")
    write_prefixes(source, target)
    print(target.read_text(encoding="utf-8"), end="")

Worked example

Translate OCR operations into JavaScript

Initial outputs are 6 DGE; 2,3,4,5; 3; and Borrow. The substring end is start+length; counting includes 5; the post-test stops at 3. Arithmetic then prints 32 4 5; conversions print 14 2.75 "14"; XOR prints 7.
The switch breaks after B, preserving Borrow; default handles unrecognised choices.

Runnable JavaScript: preserve OCR behaviourjavascript

const text = "BADGER";
const start = 2, length = 3;
console.log(text.length, text.substring(start, start + length));
const visited = [];
for (let i = 2; i <= 5; i++) visited.push(i);
console.log(visited.join(","));
let count = 0;
do { count++; } while (count !== 3);
console.log(count);
const entry = "B";
let result;
switch (entry) {
  case "A": result = "Add"; break;
  case "B": result = "Borrow"; break;
  default: result = "Unknown";
}
console.log(result);
console.log(2 ** 5, Math.floor(29 / 6), 29 % 6);
console.log(Number("14"), Number("2.75"), JSON.stringify(String(14)));
console.log(2 ^ 5);

Worked example

Trace traditional scalar byVal and byRef separately

left and right begin at 5. The procedure prints 7 for both parameters internally. Afterwards left is 5 and right is 7.
OCR defaults to by-value. Reassigning a Python integer parameter leaves the caller’s binding unchanged. Python shares object references, so list mutation can be visible even though rebinding a parameter is not.
Python object sharing differs from aliasing the caller’s scalar variable in traditional by-reference passing.

The two scalar parameter routes

copy:byVal receives left’s value: Changes affect the local copy: 5 becomes 7 inside. The caller’s left stays 5.
original:byRef refers to right: Changes affect the caller’s variable: right becomes 7 both inside and afterwards.

Original OCR scalar parameter tracetext

procedure change(copy:byVal, original:byRef)
    copy = copy + 2
    original = original + 2
    print(copy)
    print(original)
endprocedure
left = 5
right = 5
change(left,right)
print(left)
print(right)

Original A-Level practice

8 original questions total 34 marks. Attempt each before opening the independently written indicative marking guidance.

Question 1

7 marks

Evaluate OCR 3^4, 23 DIV 5 and 23 MOD 5 and translate them into JavaScript. Give JavaScript equivalents of int("8"), float("1.5") and str(8), with resulting values/types. Explain why copying ^ is wrong.

Show solution and marking guidance

Indicative answer

1 mark: 81; JavaScript 3**4.
1 mark: quotient 4; Math.floor(23/5).
1 mark: remainder 3; 23%5.
1 mark: Number("8") gives numeric 8.
1 mark: Number("1.5") gives numeric 1.5.
1 mark: String(8) gives string "8".
1 mark: JavaScript ^ is XOR, so 3^4 is 7 rather than 81.

Question 2

4 marks

For text = BADGER, give its length, OCR text.subString(2,3), and equivalent Python and JavaScript expressions.

Show solution and marking guidance

Indicative answer

1 mark: length 6.
1 mark: substring DGE.
1 mark: Python text[2:5].
1 mark: JavaScript text.substring(2,5).

Question 3

6 marks

Write OCR pseudocode to read labels.txt, write two-character prefixes of lines of length at least three to short.txt, process every line and close both files.

Show solution and marking guidance

Indicative answer

1 mark: openRead labels.txt and openWrite short.txt into distinct handles.
1 mark: loop while NOT the input handle's endOfFile().
1 mark: readLine exactly once per iteration into a variable.
1 mark: test that the line length is at least 3.
1 mark: writeLine using subString(0,2) for accepted lines.
1 mark: close both handles after the loop; accept equivalent clear solutions.

Question 4

3 marks

OCR for i=2 to 5 visits which values? Give the equivalent Python range and a JavaScript loop condition, assuming i starts at 2 and increases by 1.

Show solution and marking guidance

Indicative answer

1 mark: 2,3,4,5 (four iterations).
1 mark: range(2,6).
1 mark: i<=5 (or an equivalent i<6).

Question 5

3 marks

Starting count at 0, an OCR DO loop adds 1 UNTIL count==3. Give the iteration count, equivalent JavaScript continuation condition, and explain why this kind of loop runs at least once.

Show solution and marking guidance

Indicative answer

1 mark: three iterations.
1 mark: count!==3, because the JavaScript loop continues while its condition is true.
1 mark: the body runs before the first condition test.

Question 6

4 marks

Design mutually exclusive JavaScript switch branches: A selects Add, R selects Remove, and any other entry selects Reject. Explain what happens for X and how to prevent fall-through.

Show solution and marking guidance

Indicative answer

1 mark: switch(entry) with distinct cases for A and R.
1 mark: the case branches select Add and Remove respectively.
1 mark: default selects Reject, including for X.
1 mark: break after each completed case prevents fall-through; equivalent direct returns in a function are acceptable.

Question 7

4 marks

Trace adjust(a:byVal,b:byRef), which sets a=a*2 and b=b+3. After x=4,y=7 and adjust(x,y), give a,b inside, x,y afterwards, and explain why.

Show solution and marking guidance

Indicative answer

1 mark: a=8 and b=10 inside.
1 mark: afterwards x=4.
1 mark: afterwards y=10.
1 mark: a changes a local copy; b aliases the caller's y, so its assignment affects y.

Question 8

3 marks

Write a blue h1 Results heading using inline CSS. Identify the HTML attribute and CSS property/value.

Show solution and marking guidance

Indicative answer

1 mark: <h1 style="color:blue;">Results</h1>.
1 mark: style is the HTML attribute.
1 mark: color is the CSS property and blue its value.

Specification and references

This guide addresses OCR H446 2.2.1(a,d), 2.3.1(a); Appendix 5d, printed pages 33–36; supporting inline CSS from 1.3.4(a) and Appendix 5d page 39. Check your examination year and the complete specification for the assessment scope.

These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.

Found an error? Email a correction.

Need help applying this to unfamiliar questions?

Use the free examples first, then explore one-to-one tuition if guided practice would help.

Explore tuition Email Michael

Free A-Level Computer Science guide

OCR pseudocode: strings, files and language differences

Trace OCR Appendix 5d pseudocode, then preserve its meaning in executable code.

Content owner: Michael Print · Written for A-Level learners · Checked against official specifications

The idea to start with

OCR substring uses a start and character count. FOR includes both endpoints; DO...UNTIL stops when true. Translate these meanings into the chosen language.

Use openRead/readLine/endOfFile/close for input and openWrite/writeLine/close for output. Scalar byVal changes a copy; byRef can change the caller's variable. Python sharing differs.

OCR H446 · 2.2.1(a,d), 2.3.1(a); Appendix 5d, printed pages 33–36; supporting inline CSS from 1.3.4(a) and Appendix 5d page 39

Before you start

Useful foundations

Variables, selection and iteration
Functions, procedures and basic text files

By the end, you should be able to

Translate substring, arithmetic and casting operations accurately
Read and write text lines with an EOF-controlled loop
Preserve inclusive endpoints and post-test stopping conditions
Trace scalar parameters passed by value and reference
Recognise switch branches and an inline CSS declaration

Arithmetic and casts: the symbols have a contract

Strings: a count is different from an end position

OCR indexes from zero. BADGER has length 6 and positions 0…5. The appendix spells the method subString and substring; both descriptions use a start and character count.

Translate the meaning rather than copying argument numbers. Assume ASCII and valid non-negative positions: languages can count complex Unicode text differently.

Select three characters starting at position 2

Find the start
B=0, A=1, D=2, G=3, E=4, R=5. Begin at D.
Count three characters
Take positions 2,3,4: DGE. OCR text.subString(2,3) uses a count.
Translate the end boundary
Python text[2:5] and JavaScript text.substring(2,5) exclude position 5. JavaScript (2,3) would take only D.

Extract three characters starting at position 2
Notation	Expression	Result
OCR	text.subString(2,3)	DGE
Python	text[2:5]	DGE
JavaScript	text.substring(2,5)	DGE

Loops and branches: preserve the condition's meaning

OCR for i=2 to 5, ending next i, visits 2,3,4,5. Python range(2,6) excludes its stop; JavaScript can use i<=5.

Python can implement a post-test stopping condition with a break after the body. Preserve both test position and condition meaning.

OCR switch entry: chooses a case or default, ending endswitch. JavaScript break prevents completed branches falling into later cases. An if/elseif/else chain can express the same choices.

Where and how does each loop test?

WHILE: test first: Continue while true. The body can run zero times.
OCR DO...UNTIL: test afterwards: Run the body first, then stop when true. At least one execution.
JavaScript do...while: test afterwards: Run the body first, then continue when true. Translate UNTIL count==3 into while(count!==3).

Files: distinguish a blank record from the end

names.txt contains LYNX, a blank line, OWL and OTTER. prefixes.txt should receive LY, OW and OT: take two-character prefixes only from names of length at least three.

A blank line is a record, not EOF. It fails the length test but must not prevent reading OWL and OTTER. openWrite overwrites previous contents; close both handles afterwards.

Python file iteration stops at EOF. Remove the newline before checking length; a with block closes its handle even if an exception occurs.

One file-processing iteration

Check EOF
Continue only while NOT file.endOfFile().
Read one record
Call readLine once. A blank record still counts as input.
Apply the length rule
If length≥3, write its first two characters. Otherwise skip this record.
Return to the EOF check
Keep processing later records; finish only at the end of the input.

Inline CSS is a declaration inside an HTML attribute

Original HTML with inline CSShtml

<h1 style="color:#124A66;">File summary</h1>

Worked example

Trace arithmetic and type conversion

Python prints 32 4 5, then 14 2.75 '14'. Quotes distinguish the final string from the integer. JavaScript implements the same traced operations below.

Original OCR arithmetic/casting trace
OCR expression	Value/type
2^5	32, integer
29 DIV 6	4, integer quotient
29 MOD 6	5, remainder
int("14")	14, integer
float("2.75")	2.75, real
str(14)	"14", string

Runnable Python 3: arithmetic and castspython

print(2 ** 5, 29 // 6, 29 % 6)
number = int("14")
amount = float("2.75")
label = str(number)
print(number, amount, repr(label))

Worked example

Read, filter and write in OCR pseudocode

Assume names.txt contains the four stated lines. File variables hold handles. Each iteration consumes one line.
Skipping the blank preserves later OWL and OTTER processing. Empty input produces empty output. Test lengths 2 and 3 at the boundary.

Original algorithm using Appendix 5d file/string operationstext

source = openRead("names.txt")
target = openWrite("prefixes.txt")
while NOT source.endOfFile()
    name = source.readLine()
    if name.length >= 3 then
        target.writeLine(name.subString(0,2))
    endif
endwhile
source.close()
target.close()

Worked example

Execute the text-file algorithm in Python

A temporary folder contains the example files.
Outputs are LY, OW and OT on separate lines. [0:2] excludes position 2. Empty files and short names produce no selected lines.

Runnable Python 3: strings and text-file processingpython

from pathlib import Path
from tempfile import TemporaryDirectory

def write_prefixes(source_path, target_path):
    with source_path.open("r", encoding="utf-8") as source:
        with target_path.open("w", encoding="utf-8") as target:
            for line in source:
                name = line.rstrip("\n")
                if len(name) >= 3:
                    target.write(name[0:2] + "\n")

with TemporaryDirectory() as folder:
    source = Path(folder) / "names.txt"
    target = Path(folder) / "prefixes.txt"
    source.write_text("LYNX\n\nOWL\nOTTER\n", encoding="utf-8")
    write_prefixes(source, target)
    print(target.read_text(encoding="utf-8"), end="")

Worked example

Translate OCR operations into JavaScript

Initial outputs are 6 DGE; 2,3,4,5; 3; and Borrow. The substring end is start+length; counting includes 5; the post-test stops at 3. Arithmetic then prints 32 4 5; conversions print 14 2.75 "14"; XOR prints 7.
The switch breaks after B, preserving Borrow; default handles unrecognised choices.

Runnable JavaScript: preserve OCR behaviourjavascript

const text = "BADGER";
const start = 2, length = 3;
console.log(text.length, text.substring(start, start + length));
const visited = [];
for (let i = 2; i <= 5; i++) visited.push(i);
console.log(visited.join(","));
let count = 0;
do { count++; } while (count !== 3);
console.log(count);
const entry = "B";
let result;
switch (entry) {
  case "A": result = "Add"; break;
  case "B": result = "Borrow"; break;
  default: result = "Unknown";
}
console.log(result);
console.log(2 ** 5, Math.floor(29 / 6), 29 % 6);
console.log(Number("14"), Number("2.75"), JSON.stringify(String(14)));
console.log(2 ^ 5);

Worked example

Trace traditional scalar byVal and byRef separately

left and right begin at 5. The procedure prints 7 for both parameters internally. Afterwards left is 5 and right is 7.
OCR defaults to by-value. Reassigning a Python integer parameter leaves the caller’s binding unchanged. Python shares object references, so list mutation can be visible even though rebinding a parameter is not.
Python object sharing differs from aliasing the caller’s scalar variable in traditional by-reference passing.

The two scalar parameter routes

copy:byVal receives left’s value: Changes affect the local copy: 5 becomes 7 inside. The caller’s left stays 5.
original:byRef refers to right: Changes affect the caller’s variable: right becomes 7 both inside and afterwards.

Original OCR scalar parameter tracetext

procedure change(copy:byVal, original:byRef)
    copy = copy + 2
    original = original + 2
    print(copy)
    print(original)
endprocedure
left = 5
right = 5
change(left,right)
print(left)
print(right)

Original A-Level practice

8 original questions total 34 marks. Attempt each before opening the independently written indicative marking guidance.

Question 1

7 marks

Show solution and marking guidance

Indicative answer

1 mark: 81; JavaScript 3**4.
1 mark: quotient 4; Math.floor(23/5).
1 mark: remainder 3; 23%5.
1 mark: Number("8") gives numeric 8.
1 mark: Number("1.5") gives numeric 1.5.
1 mark: String(8) gives string "8".
1 mark: JavaScript ^ is XOR, so 3^4 is 7 rather than 81.

Question 2

4 marks

For text = BADGER, give its length, OCR text.subString(2,3), and equivalent Python and JavaScript expressions.

Show solution and marking guidance

Indicative answer

1 mark: length 6.
1 mark: substring DGE.
1 mark: Python text[2:5].
1 mark: JavaScript text.substring(2,5).

Question 3

6 marks

Write OCR pseudocode to read labels.txt, write two-character prefixes of lines of length at least three to short.txt, process every line and close both files.

Show solution and marking guidance

Indicative answer

1 mark: openRead labels.txt and openWrite short.txt into distinct handles.
1 mark: loop while NOT the input handle's endOfFile().
1 mark: readLine exactly once per iteration into a variable.
1 mark: test that the line length is at least 3.
1 mark: writeLine using subString(0,2) for accepted lines.
1 mark: close both handles after the loop; accept equivalent clear solutions.

Question 4

3 marks

OCR for i=2 to 5 visits which values? Give the equivalent Python range and a JavaScript loop condition, assuming i starts at 2 and increases by 1.

Show solution and marking guidance

Indicative answer

1 mark: 2,3,4,5 (four iterations).
1 mark: range(2,6).
1 mark: i<=5 (or an equivalent i<6).

Question 5

3 marks

Starting count at 0, an OCR DO loop adds 1 UNTIL count==3. Give the iteration count, equivalent JavaScript continuation condition, and explain why this kind of loop runs at least once.

Show solution and marking guidance

Indicative answer

1 mark: three iterations.
1 mark: count!==3, because the JavaScript loop continues while its condition is true.
1 mark: the body runs before the first condition test.

Question 6

4 marks

Design mutually exclusive JavaScript switch branches: A selects Add, R selects Remove, and any other entry selects Reject. Explain what happens for X and how to prevent fall-through.

Show solution and marking guidance

Indicative answer

1 mark: switch(entry) with distinct cases for A and R.
1 mark: the case branches select Add and Remove respectively.
1 mark: default selects Reject, including for X.
1 mark: break after each completed case prevents fall-through; equivalent direct returns in a function are acceptable.

Question 7

4 marks

Trace adjust(a:byVal,b:byRef), which sets a=a*2 and b=b+3. After x=4,y=7 and adjust(x,y), give a,b inside, x,y afterwards, and explain why.

Show solution and marking guidance

Indicative answer

1 mark: a=8 and b=10 inside.
1 mark: afterwards x=4.
1 mark: afterwards y=10.
1 mark: a changes a local copy; b aliases the caller's y, so its assignment affects y.

Question 8

3 marks

Write a blue h1 Results heading using inline CSS. Identify the HTML attribute and CSS property/value.

Show solution and marking guidance

Indicative answer

1 mark: <h1 style="color:blue;">Results</h1>.
1 mark: style is the HTML attribute.
1 mark: color is the CSS property and blue its value.

Specification and references

Found an error? Email a correction.

Need help applying this to unfamiliar questions?

Use the free examples first, then explore one-to-one tuition if guided practice would help.

Explore tuition Email Michael