Bleu代码matlab

Bleu（Bilingual Evaluation Understudy）是一种用于评估机器翻译结果的指标，它是一种基于n-gram的精度指标。Bleu指标的值在0和1之间，值越高表示机器翻译结果与人工翻译结果越接近。下面是一个基于MATLAB实现Bleu的代码示例：

function bleu_score = BLEU(candidate, reference, n)

% candidate: 机器翻译结果，字符串形式
% reference: 参考翻译结果，字符串形式或字符串数组形式
% n: n-gram的大小，取值范围为1-4

% 将candidate和reference转换为字符串数组形式
if ischar(reference)
    reference = {reference};
end

% 计算每个n-gram的精度权重
weights = ones(n, 1) ./ n;

% 计算candidate和reference的n-gram数值
candidate_ngram_counts = count_ngrams(candidate, n);
reference_ngram_counts = count_ngrams(reference, n);

% 初始化精度值和翻译长度
precision_scores = zeros(size(candidate_ngram_counts));
candidate_length = length(strsplit(candidate));

% 计算每个n-gram的精度值
for i = 1:length(precision_scores)
    precision_scores(i) = min(candidate_ngram_counts(i), max(reference_ngram_counts(i)));
end

% 计算geometric mean
geometric_mean = exp(mean(log(precision_scores(precision_scores~=0))));

% 计算Bleu分数
bleu_score = weights' * geometric_mean * exp(min(0, 1 - length(reference)/candidate_length));
end

function ngram_counts = count_ngrams(strs, n)
% 计算输入字符串数组中每个n-gram的数量
ngram_counts = zeros(1, n^2);
for i = 1:length(strs)
    words = strsplit(strs{i});
    for j = 1:length(words)-n+1
        ngram = strjoin(words(j:j+n-1));
        index = hash(ngram);
        ngram_counts(index) = ngram_counts(index) + 1;
    end
end
end

function index = hash(str)
% 将字符串哈希为唯一索引
prime = 5;
index = 0;
for i = 1:length(str)
    index = index + double(str(i)) * prime^(i-1);
end
end

上面的代码中，count_ngrams函数用于计算输入字符串数组中每个n-gram的数量，hash函数用于将字符串哈希为唯一索引。BLEU函数是计算Bleu分数的主要函数，它使用了precision_scores数组来存储每个n-gram的精度值，并使用geometric_mean计算geometric mean。最后，它根据输入的参考翻译结果和机器翻译结果计算Bleu分数。