如何正确舍入十进制数字的一半？

Question

I am trying to round half down a number, but my program doesn't want to round half down but up.

For instance, I have 0.0498512222 I want to get 0.049, but the program round half up and give me 0.050

Code

#!/usr/bin/python
import sys, signal, json, time
import random
import decimal
from decimal import Decimal, ROUND_HALF_DOWN
num = 0.049852124
num = Decimal(num)
numCoins = Decimal(num.quantize(Decimal('0.001'), rounding=ROUND_HALF_DOWN))
numCoins = float(numCoins)
print numCoins

I don't know how to resolve this error, because it's for a cryptocurrency bot, and the numCoins is the number of a coin that I have. If the program round half up I'll get an error like 'Account has insufficient balance for requested action' because I can't sell more than I have.

Answer 1

ROUND_HALF_DOWN is round half down, as in when a number is exactly halfway between two options of what to round to, it rounds down. Everything else rounds to nearest. You are significantly above the halfway point between 0.049 and 0.050, so you round to nearest.

If you want to round everything down, that's ROUND_DOWN, not ROUND_HALF_DOWN.

Also, if you want decimal arithmetic, using floats is a mistake from the start. Instead of num = 0.049852124 , you should use avoid floats entirely with something like num = Decimal('0.049852124') , and you should avoid producing a float at the end.

Answer 2

Something like this?

import sys, signal, json, time
import random
import math
num = 0.049852124 
# where 1 is in the position you want to truncate
numCoins = num - math.fmod(num, 0.001)
print numCoins # 0.049