python difference between round and int

Question

I've been playing around with python for a bit now and i've noticed a strange behavior that makes me curious: what is the difference between float(int(n)) and round(n)?

When should I use one, another or neither of them?

Solution 1

For the sake of completeness, let me add two more functions to your question and explain the differences between float(int(x)), math.floor(x), round(x) and math.ceil(x).

Let's start with a question: "What integer represents best the number 1.6?" We have two possible answers (1 and 2) but many different reasons why one answer may be better than the other one:

• int(1.6)==1: This is what you get when you cut off the decimals.
• math.floor(1.6)==1: Its less than 2. Incomplete pieces don't count.
• round(1.6)==2: Because 2 is closer than 1.
• math.ceil(1.6)==2: Its more than 1. When you start a part, you have to pay the full price.

Let's ask python to print a nice table of the results you get with different values of x:

from math import floor, ceil
tab='\t' 

print 'x \tint\tfloor\tround\tceil'
for x in (1.0, 1.1, 1.5, 1.9, -1.1, -1.5, -1.9):
    print x, tab, int(x), tab, floor(x), tab, round(x), tab, ceil(x)

Here is the output:

x       int floor   round   ceil
1.0     1   1.0     1.0     1.0
1.1     1   1.0     1.0     2.0
1.5     1   1.0     2.0     2.0
1.9     1   1.0     2.0     2.0
-1.1    -1  -2.0    -1.0    -1.0
-1.5    -1  -2.0    -2.0    -1.0
-1.9    -1  -2.0    -2.0    -1.0

You see that no of these four functions are equal.

• floor rounds towards minus infinity: It chooses always the lowest possible answer: floor(1.99)==1 and floor(-1.01)==-2.
• ceil rounds towards infinity: It chooses always the highest possible answer: ceil(1.01)==2 and ceil(-1.99)=-1.
• int rounds towards zero: For positive x it is like floor, for negative x it is like ceil.
• round rounds to the closest possible solution: round(1.49)=1 and round(1.51)==2. When x is precisely between two numbers, round(x) rounds away from zero: round(1.5)==2 and round(-1.5)==-2. This is the opposite of what int(x) would do in this case.

Note that int(x) always returns an integer --- the other functions return floating point numbers.

Solution 2

round(n) is a function to round a float, int(n) will cast a float to an integer and will get rid of the decimal part by truncating it.

Solution 3

round is a mathematical rounding and int just casts to an integer, essentially truncating the value.

Take 2 variables:

var1 = 5.5
var2 = 5.1

If we round them

round(var1)
round(var2)

We get 6.0 and 5.0 respectively.

However, if we just cast them to an int

int(var1)
int(var2)

We get 5 for both of them.

You can test this out yourself in the python interpreter.