格式化字符串包含普通文本和特殊的格式化序列 “%d”=整数,"%s"=字符串,"%f"=浮点数
format（）函数可以单独格式化每个部分
principal =100 #初始金额
rate = 0.05 #利率
numyears =5 #年数
year =1
while year <=numyears:
principal = principal * (1+rate)
#print (year, principal)
#print("%4d %0.2f" %(year, principal))
#print(format(year, "3d"),format(principal,"0.2f"))
print( "{0:3d}" "{1:0.2f}".format(year, principal))
year +=1