你問的這個問題很有意思啊，matlab要保留任意位數，是一個比較值得研究的東西。

預設format也就是format short是保留4位小數，format long 是保留14位，如果要保留兩位或者六位，就存在如何設定的問題。本人做了點總結，給出以下方法

方法1：digits+vpa

format long

a = input("please give a number which will be changed：");

n = input("how many decimal digits will you keep: ");

num = floor(a);

str = num2str(num);

len = length(str);

err = a-num;

digits(n+len); % 此處為需要的小數位+整數位數

need_num = num+vpa(err,n+len); %%%% 這裡得到的是符號型

need_str = num2str(double(need_num)) %%%%% 再次轉換獲得字元型

方法2：round 函式法

format long

a = input("please give a number which will be changed：");

n = input("how many decimal digits will you keep: ");

b = a*10^n;

aa = round(b);

need_num = aa/10^n; %%%%% 先化成整數，再轉成小數

need_str = num2str(need_num)

方法3：直接轉成字串再擷取

format long

a = input("please give a number which will be changed：");

n = input("how many decimal digits will you keep: ");

num = floor(a);

str = num2str(num);

len = length(str);

str1 = num2str(a);

need_str = str1(1:(len+n+1)) %%%取需要的長度，這裡需要加上整數長和小數點1位

方法4：fprintf方法

a = input("please give a number which will be changed：");

%n = input("how many decimal digits will you keep: ");

fprintf("a = %3.2f",a) %%%%3.2f中.2就是小數位數