GPT答：

7i24.cn

```matlab
% 信源符号和概率
symbols = ['a', 'b', 'c', 'd'];
probabilities = [0.1, 0.4, 0.2, 0.3];

% 消息序列
message = ['c', 'a', 'a', 'd', 'a', 'c', 'd'];

% 算术编码
lowRange = 0;
highRange = 1;
for i = 1:length(message)
    symbolIndex = find(symbols == message(i));
    rangeWidth = highRange - lowRange;
    highRange = lowRange + rangeWidth * sum(probabilities(1:symbolIndex));
    lowRange = lowRange + rangeWidth * sum(probabilities(1:symbolIndex-1));
end

encodedResult= (lowRange + highRang) /2;

disp(['Encoded result: ', num2str(encodedResult)]);
```

解码部分需要使用相同的概率和符号进行反向操作，这里不再赘述。