estions","basePath":"/gcse/computer-science/aqa/data-compression/questions","formulaUrl":null,"initialIndex":4,"initialQuestionWindow":[{"id":"01aceeda-575c-4ced-bc96-ba092de79c7e","revisionId":"942db5e3-1cbb-4697-9c7d-194887b6a2cc","questionNumber":null,"questionType":"LA","specification":"","difficulty":5,"difficultyBand":"medium","totalMarks":4,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"2adcf935-8d71-4693-bfd9-80b0f66b607a","label":null,"content":"A student was asked to describe how a Huffman tree could be created for the string in Figure 1.\n\nFigure 1\n```text\nCOCOA COCONUT\n```\n\nHer response was:\n\n\"I would count the frequency of each character in the string and write them in a table sorted alphabetically. For example, the character 'O' has the lowest frequency in this string. Then, I would take the two characters with the largest frequencies and combine them into a new parent node. The frequency of this new parent node is calculated by multiplying the two frequencies together. I would then repeat this process with the remaining nodes until the entire tree is built.\"\n\nState four mistakes the student has made in her response.","markscheme":"- The frequency table should be sorted by frequency (usually in ascending order), not alphabetically. B1\n- The character 'O' does not have the lowest frequency; 'O' has a frequency of 4, which is one of the highest (while characters 'A', 'N', 'U', 'T', and space have the lowest frequency of 1). B1\n- You should combine the two nodes with the lowest/smallest frequencies, not the largest. B1\n- The parent node's frequency is calculated by adding/summing the frequencies of the child nodes, not multiplying them. B1","marks":4,"order":0,"markingRubric":null,"explanation":null,"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"0fc37e4c-7cae-4c1a-9fb3-66f966107332","revisionId":"05d5b767-271a-4dab-9357-8cf43773c885","questionNumber":null,"questionType":"LA","specification":"The string shown in **Figure 1** can be encoded using Huffman coding or ASCII. ASCII uses 7 bits to represent each character.\n\nFigure 1\n```\nAPPALACHIA\n```\n\n**Figure 2** shows the Huffman tree created for this string.\n\n![A Huffman tree representing the string APPALACHIA](https://assets.mathsgenie.co.uk/aqa-qb/5dde0f88-ef7a-45fd-9f2a-fefe7ef8b004.png)","difficulty":5,"difficultyBand":"medium","totalMarks":4,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"ab959779-72c9-438b-b4fc-c66b51cac2df","label":null,"content":"How many bits are **saved** by using Huffman coding rather than ASCII to represent the string in **Figure 1**?\n\nYou **must** show your working.","markscheme":"- Calculates the total number of bits required for standard ASCII representation: $10 \\times 7 = 70$ bits M1\n- Correctly identifies the bit lengths or codes of the characters from the Huffman tree in Figure 2: $A$ is $1$ bit; $H, I, P$ are $3$ bits; $L, C$ are $4$ bits M1\n- Calculates the total bits for the Huffman encoding: $(4 \\times 1) + (2 \\times 3) + (1 \\times 4) + (1 \\times 4) + (1 \\times 3) + (1 \\times 3) = 24$ bits A1\n- Subtracts the Huffman bits from the ASCII bits to find the number of bits saved: $70 - 24 = 46$ bits A1","marks":4,"order":0,"markingRubric":null,"explanation":null,"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"1d080ea0-8d9f-41a4-ae48-50a80dde2388","revisionId":"e45708f8-d2be-4489-8282-57accc8943bf","questionNumber":null,"questionType":"LA","specification":"A student was asked to describe how a Huffman tree could be created for the string in Figure 1.\n\nFigure 1\n```text\nCOCOA COCONUT\n```\n\nHer response was:\n\n\"I would first write down the frequency of each character in the string and sort the list in alphabetical order. For example, the character 'C' has the lowest frequency in this string. Then, to construct the tree, I would select the two nodes with the lowest frequencies and combi

Data compression

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Data compression Questions

Data compression

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Data compression Questions