how to use cell function to convert string to number

I have a cell array a = { 'a' '0' ; 'b' '1' ; 'c' '2' ; 'd' '3'}

so a = 'a' '0'

    'b''1'

    'c''2'

how can I convert the second column from string to number and should retain the cell array?

Answer 1

[Ingrid]

a(:,2) = mat2cell(cellfun(@str2num,a(:,2)),ones(4,1),1)

Answer 2

[Azzi Abdelmalek]

a = { 'a' '0' ; 'b' '1' ; 'c' '2' ; 'd' '3'}
a(:,2)=cellfun(@str2double,a(:,2),'un',0)

[Supraja Murali]

Can you explain what 'un' and '0' are in the answer above?

[madhan ravi]

un-> UniformOutput 0-> false