线性数据结构（13）——求质数

求质数

start=datetime.datetime.now()
count=0
for i in range(2,1000):
    for j in range(2,i):
        if i%j==0:
            break
    else:
        #print(i)
        count+=1
end=(datetime.datetime.now()-start).total_seconds()
print(count,end)

168 0.015

优化除数

start=datetime.datetime.now()
count=0
for i in range(2,1000):
    for j in range(2,int(i**0.5+1)):
        if i%j==0:
            break
    else:
        #print(i)
        count+=1
end=(datetime.datetime.now()-start).total_seconds()
print(count,end)

168 0.004

排除偶数，除0，2外，所有偶数都是合数

2是偶数又是最小的质数，排除2，count+1

最小值取3，步数为2

start=datetime.datetime.now()
count=1
for i in range(3,1000,2):
    for j in range(3,int(i**0.5+1),2):
        if i%j==0:
            break
    else:
        #print(i)
        count+=1
end=(datetime.datetime.now()-start).total_seconds()
print(count,end)

168 0.002