如何提取不同长度的子字符串

Question

我有一个n by 2矩阵，它包含指定字符串的子字符串的开始和结束索引。如何在没有for循环的情况下提取子字符串的n by 1单元数组？

string = 'Hello World!';
ranges = [1 1;
2 3;
4 5;
3 7];
substrings = cell(size(ranges, 1), 1);
for i=1:size(ranges, 1)
substrings{i} = string(ranges(i, 1):ranges(i, 2));
end

预期结果：

substrings = 
'H'
'el'
'lo'
'llo W'

Answer 1

您可以使用cellfun使其成为单行操作：

str = 'Hello World!';
ranges = [  1 1;
2 3;
4 5;
3 7];
% first convert "ranges" to a cell object
Cranges = mat2cell(ranges,ones(size(ranges,1),1),2);
% call "cellfun" on every row/entry of "Cranges"
cellfun(@(x)str(x(1):x(2)),Cranges, 'UniformOutput',false)

ans=

4×1单元阵列

{'H'    }
{'el'   }
{'lo'   }
{'llo W'}

我已将变量string更改为str，因为string是MATLAB中的本机函数(将输入转换为字符串类型(。

虽然这是单线操作，但并不意味着它更高效：

Num = 1000000;

substrings = cell(size(ranges, 1), 1);
% time for-loop
tic
for j = 1:Num
for i = 1:size(ranges, 1)
substrings{i} = str(ranges(i, 1):ranges(i, 2));
end
end
toc;
Cranges = mat2cell(ranges,ones(size(ranges,1),1),2);
% time function-call
tic
for j = 1:Num
substrings = cellfun(@(x)str(x(1):x(2)),Cranges, 'UniformOutput',false);
end
toc;

Elapsed time is 3.929622 seconds. 
Elapsed time is 50.319609 seconds.