Python程序在列表中的第一个 j 之前出现 i

给定一个列表，任务是编写一个Python程序来计算第 i^个元素在第 j^个元素第一次出现之前的出现次数。

例子：

Input : test_list = [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9], i, j = 4, 8 
Output : 3 
Explanation : 4 occurrs 3 times before 8 occurs.

Input : test_list = [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9], i, j = 5, 8 
Output : 1 
Explanation : 5 occurrs 1 time before 8 occurs. 
 

方法#1：使用循环。

在这种情况下，我们在遇到 i 时增加 counter 并在发生任何 j 时停止，即从循环中中断。

Python3

# Python3 code to demonstrate working of
# Occurrences of i before first j
# Using loop
  
# initializing Matrix
test_list = [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9]
               
# printing original list
print("The original list is : " + str(test_list))
  
# initializing i, j 
i, j = 4, 8
  
res = 0
for ele in test_list:
      
    # breaking on 1st j
    if ele == j:
        break
      
    # counting i till 1st j
    if ele == i:
        res += 1
  
# printing result
print("Number of i's till 1st j : " + str(res))

输出：

The original list is : [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9]
Number of i's till 1st j : 3

方法#2：使用index() +切片+ count()

在此，我们执行获取 j 的索引的任务，然后将列表切片到那里，count() 用于获取切片列表中 i 的计数以获得所需的结果。

蟒蛇3

# Python3 code to demonstrate working of
# Occurrences of i before first j
# Using index() + slicing + count()
  
# initializing Matrix
test_list = [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9]
               
# printing original list
print("The original list is : " + str(test_list))
  
# initializing i, j 
i, j = 4, 8
  
# getting index
jidx = test_list.index(j)
  
# slicing list 
temp = test_list[:jidx]
  
# getting count 
res = temp.count(i)
  
# printing result
print("Number of i's till 1st j : " + str(res))

输出：

The original list is : [4, 5, 6, 4, 1, 4, 8, 5, 4, 3, 4, 9]
Number of i's till 1st j : 3