Write a program to analyze the frequency of a~z letters appearing in a given string, ignore case, and output in descending order.
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txt=input("请输入一段英文文本：")
txt=txt.lower()
counts={}
for i in txt:
if i in "abcdefghijklmnopqrstuvwxyz":
counts[i]=counts.get(i,0)+1
items=list(counts.items())
items.sort(key=lambda x:x[1],reverse=True)
for i in range(len(items)):
word,count=items[i]
print("{0:<10}{1:>5}".format(word,count))
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