Try this code:
char_list = ['A', 'B', 'D', 'B', 'C', 'A', 'B']
count = 0
for index in range(0, len(char_list)-1):
if char_list[index] == 'A' and char_list[index+1] == 'B':
count += 1
print('count :', count)
Try this new one:
char_list = ['A', 'B', 'D', 'B', 'C', 'A', 'B', 'A', 'B']
prev_char = ''
count = 0
for char in char_list:
if char == 'B' and prev_char == 'A':
count += 1
prev_char = char
print('count :', count)
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