爲什麼當我將其聲明爲全局變量時嘗試在python中編寫堆棧時，出現局部變量錯誤？

Question

我不斷收到以下錯誤信息：

if stack_pointer < max_length: UnboundLocalError: local variable 'stack_pointer' referenced before assignment

我想編寫Python中的堆棧數據結構。

這裏是我的代碼：

stack_pointer = -1 
stack =[] 
max_length = 10 

def view(): 
     for x in range (len(stack)): 
      print(stack[x]) 
def push(): 
    if stack_pointer < max_length: 
    item = input("Please enter the item you wishto add to the stack: ") 
    stack[stack_pointer].append(item) 
    stack_pointer = stack_pointer + 1 
else: 
    print("Maximum stack length reached!") 

def pop(): 
    if stack_pointer < 0: 
     print ("stack is empty!") 
    else: 
     item = stack[stack_pointer].pop(-1) 
     stack_pointer = stackpointer - 1 
     print ("you just popped out: ", item) 

while True: 
print ("") 
print("Python implementation of a stack") 
print("********************************") 
print("1. view Stack") 
print("2. Push onto Stack") 
print("3. Pop out of Stack") 
print("********************************") 
print("") 
menu_choice = int (input("Please enter your menu choice: ")) 
print ("") 
print ("") 

if menu_choice == 1: 
    view() 
elif menu_choice == 2: 
    push() 
elif menu_choice == 3: 
    pop() 

Answer 1

你忘了聲明變量全球在你改變它的功能。

例如：

def pop(): 
    global stack_pointer # this is what you forgot 
    if stack_pointer < 0: 
     print ("stack is empty!") 
    else: 
     item = stack[stack_pointer].pop(-1) 
     stack_pointer = stack_pointer - 1 # was a missing underscore here 
     print ("you just popped out: ", item) 

變量的函數，如果分配給他們，被認爲是當地的，除非聲明的全局（或在Python 3外地）。