為什麼要寫得更有效率 I

使用 pandas 撰寫高效程式碼

Leonidas Souliotis

PhD Researcher

怎麼量測時間？

time.time(): 回傳自 1970 年 1 月 1 日 12:00am 起的秒數

import time

# 執行前記錄時間
start_time = time.time()

# 執行運算
result = 5 + 2

# 執行後記錄時間
end_time = time.time()

print("Result calculated in {} sec".format(end_time - start_time))

Result calculated in 9.48905944824e-05 sec

for 迴圈 vs 串列生成式

串列生成式：

list_comp_start_time = time.time()
result = [i*i for i in range(0,1000000)]
list_comp_end_time = time.time()
print("Time using the list_comprehension: {} sec".format(list_comp_end_time - 
list_comp_start_time))

for 迴圈：

for_loop_start_time= time.time()
result=[]
for i in range(0,1000000):
  result.append(i*i)
for_loop_end_time= time.time()
print("Time using the for loop: {} sec".format(for_loop_end_time - for_loop_start_time))

for 迴圈 vs 串列生成式 II

Time using the list comprehension: 0.11042404174804688 sec

Time using the for loop: 0.2071230411529541 sec

list_comp_time = list_comp_end_time - list_comp_start_time
for_loop_time = for_loop_end_time - for_loop_start_time
print("Difference in time: {} %".format((for_loop_time - list_comp_time)/
list_comp_time*100))

Difference in time: 87.55527367398622 %

時間在哪裡重要 I

計算 $1+2+...+1000000$。

逐一相加：

def sum_brute_force(N):
    res = 0
    for i in range(1,N+1):
        res+=i
    return res

使用 $\footnotesize 1 + 2 +... + N = \dfrac{N\cdot(N+1)}{2}$

def sum_formula(N):
    return N*(N+1)/2

時間在哪裡重要 II

使用公式：

# Using the formula
formula_start_time = time.time()
formula_result = formula(1000000)
formula_end_time = time.time()

print("Time using the formula: {} 
sec".format(formula_end_time - formula_start_time))

Using the formula: 0.000108957290649 sec

使用暴力法：

# Using brute force
bf_start_time = time.time()
bf_result = sum_brute_force(1000000)
bf_end_time = time.time()

print("Time using brute force: {} 
sec".format(bf_end_time - start_time))

Time using brute force: 0.174870967865 sec

Difference in speed: 160,394.967179%

開始動手！

使用 pandas 撰寫高效程式碼