有关if的简单python代码

Question

Under python 2

    for i in range(6):
        for j in range(i):print i,j

    for i in range(6):
        for j in range(i):
            if j:print i,j

I notice the difference of those two results but I still don't understand what does if j mean.

Answer 1

for i in range(6):
    for j in range(i):
        if j:print i,j

The above if statements makes sure that i and j are not printed when the value of j is zero. Most datatypes have intrinsic boolean property in Python. for numbers any non-zero values translates to True while zero translates to False

Answer 2

Python Documentation on truth testing:

http://docs.python.org/2/library/stdtypes.html#truth-value-testing

Specifically:

Any object can be tested for truth value, for use in an if or while condition. The following values are considered false:

zero of any numeric type, for example, 0, 0L, 0.0

All other values are considered true

if j: will trigger when j is non-zero and do nothing when j is 0.

Answer 3

To understand what happened in if i: , try this.

for i in range(-6,6):
    if i:
        print "%2d is evaluated as True"%i
    else:
        print "%2d is evaluated as False"%i

output:

-6 is evaluated as True
-5 is evaluated as True
-4 is evaluated as True
-3 is evaluated as True
-2 is evaluated as True
-1 is evaluated as True
 0 is evaluated as False    <----- Just "if 0:" is evaluated as "if False:"
 1 is evaluated as True
 2 is evaluated as True
 3 is evaluated as True
 4 is evaluated as True
 5 is evaluated as True

Answer 4

It checks if j is True. Numerical value of 0 is interpreted as False and every other as True.

Answer 5

第二个循环以j为条件，如果j不为假，即j <> 0，则仅打印j不等于0的值