How do you chop a number to a specified number of significant digits, not round

Is there a way to chop a number in matlab? I dont want to round it, just chop it off after a number of significants.

like e = 2.71828 eRound=2.72 eChop=2.71

[KSSV]

x = exp(1) ;
iwant = str2num(sprintf('%0.2f\n',x)) ;

Answer 1

[Bruno Luong]

2 digits

fix(x*100)/100

[Stephen23]

+1 The simple, traditional way, which can also be used with round and ceil.

[Star Strider]

‘How do you chop a number to a specified number of significant digits, not round’

Note that by definition, fix rounds toward zero.

It does not truncate.

And that is the reason I did not use this approach.

[Stephen23]

"Note that by definition, fix rounds toward zero. It does not truncate."

What is the difference? Disregarding any possible floating-point effects, I do not see how these are different. According to Wikipedia these are the same: "round towards zero (or truncate, ..."

source: https://en.wikipedia.org/wiki/Rounding#Rounding_towards_zero

Indeed the standard definition of truncation is

source: https://en.wikipedia.org/wiki/Truncation

which is exactly what Bruno Luong's answer is doing.

I note that you highlighted the term "rounds" but did not define it: rounding does not only include rounding to integer and rounding half up, but is really a family of functions (which includes fix and floor and ceil). Without defining exactly which one you are referring to by "rounds", your critique is entirely lost on me...

[Bruno Luong]

I also fail to see why the fix method is criticized. I run this code, it does not look like I get different result than Star's solution

a = 1000*randn(1,1e6);
for x=a
    r1=fix(x*100)/100;
    s = sprintf('%.15f',x);
    i = find(s=='.');
    r2 = str2num(s(1:i+2)); 
    if r1~=r2
        keyboard
    end
end

Answer 2

[Star Strider]

The print functions (such as sprintf and fprintf) round the result, so it is necessary to create more than the necessary number of decimal places, then use indexing to return only the number of digits desired.

Example:

x = exp(1);
s = sprintf('%.15f',x)                              % Generate 15 Decimal Places
n = str2num(s(1:4))                                 % Choose First 4 Characters

produces:

n =
         2.71

Note that:

p = str2num(sprintf('%.2f',x))

rounds to produce:

p =
         2.72

Experiment to get the result you want.