How to express this meaning?

Round to the nearest integer.

eg.

X = Round(X)

If I don't round to the nearest integer, but to a real number, which is the approximate value of the true value.

I suppose you can truncate the value to whatever number of decimal places you desire.

eg. truncate the value of ?
3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848117450284102701938521105559644622948954930381964428810975665933446128475648233786783165271201909145648566923460348610

Yes, I know I can do that in MATLAB. But I really want to get is the expression like "58.3 is nearly 60" and "107 times 56 is a bit more more than 100 times 50, which is 5,000".
It shows us how two numbers are approximately equal.

I think you can just say, ‘X is approximately Y’ or ‘X is approximately equal to Y’.

Li HangIt shows us how two numbers are approximately equal.

So you are not looking for a mathematical expression, but a layman's expression.

You have already used the most common ways.
We can say, "58.4 is in the ballpark of 60, so let's use 60."
In round numbers, 958 is close enough to 1000 to give us a result to the first order of magnitude.

OK. Thank you!

The expression is "rounded to the nearest integer."

‘X is approximately Y’

'approximately' is a adverb and 'Y' should be a noun. Why a adverb can modify a noun ?

"How can an adverb modify a noun?"

It can't. "Approximately" modifies the verb "is," telling us in what manner X exists.