The other day I was doing some Python benchmarking and I came across something interesting. Below are two loops that do more or less the same thing. Loop 1 takes about twice as long as loop 2 to execute.
Loop 1:
i = 0
while i < 100000000:
i += 1
Loop 2:
for n in range(0,100000000):
pass
Why is the first loop so much slower? I know it's a trivial example but it's piqued my interest. Is there something special about the range() function that makes it more efficient than incrementing a variable the same way?
see the disassembly of python byte code, you may get a more concrete idea
use while loop:
1 0 LOAD_CONST 0 (0)
3 STORE_NAME 0 (i)
2 6 SETUP_LOOP 28 (to 37)
>> 9 LOAD_NAME 0 (i) # <-
12 LOAD_CONST 1 (100000000) # <-
15 COMPARE_OP 0 (<) # <-
18 JUMP_IF_FALSE 14 (to 35) # <-
21 POP_TOP # <-
3 22 LOAD_NAME 0 (i) # <-
25 LOAD_CONST 2 (1) # <-
28 INPLACE_ADD # <-
29 STORE_NAME 0 (i) # <-
32 JUMP_ABSOLUTE 9 # <-
>> 35 POP_TOP
36 POP_BLOCK
The loop body has 10 op
use range:
1 0 SETUP_LOOP 23 (to 26)
3 LOAD_NAME 0 (range)
6 LOAD_CONST 0 (0)
9 LOAD_CONST 1 (100000000)
12 CALL_FUNCTION 2
15 GET_ITER
>> 16 FOR_ITER 6 (to 25) # <-
19 STORE_NAME 1 (n) # <-
2 22 JUMP_ABSOLUTE 16 # <-
>> 25 POP_BLOCK
>> 26 LOAD_CONST 2 (None)
29 RETURN_VALUE
The loop body has 3 op
The time to run C code is much shorter than intepretor and can be ignored.