How can I get the first two digits of a number

Question

I want to check the first two digits of a number in Python  Something like this   for i in range 1000        if first two digits of i    15           print  15        elif first two digits of i    16           print  16     Is there a command to check the first two digits of a number  I d like to avoid commands like if i gt 149 and i lt 160

User · Answer

Both of the previous 2 answers have at least O(n) time complexity and the string conversion has O(n) space complexity too. Here's a solution for constant time and space:

num // 10 ** (int(math.log(num, 10)) - 1)

Function:

import math

def first_n_digits(num, n):
    return num // 10 ** (int(math.log(num, 10)) - n + 1)

Output:

>>> first_n_digits(123456, 1)
1
>>> first_n_digits(123456, 2)
12
>>> first_n_digits(123456, 3)
123
>>> first_n_digits(123456, 4)
1234
>>> first_n_digits(123456, 5)
12345
>>> first_n_digits(123456, 6)
123456

You will need to add some checks if it's possible that your input number has less digits than you want.

User · Answer

Comparing the O n  time solution with the  constant time  O 1  solution provided in other answers goes to show that if the O n  algorithm is fast enough  n may have to get very large before it is slower than a slow O 1     The strings version is approx  60  faster than the  maths  version for numbers of 20 or fewer digits  They become closer only when then number of digits approaches 200 digits    the  maths  version import math  def first n digits1 num  n       return num    10     int math log num  10   - n   1    timeit first n digits1 34523452452  2  1 21   s    75 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit first n digits1 34523452452  8  1 24   s    47 5 ns per loop  mean    std  dev  of 7 runs  1000000 loops each     22 digits  timeit first n digits1 3423234239472523452452  2  1 33   s    59 4 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit first n digits1 3423234239472523452452  15  1 23   s    61 2 ns per loop  mean    std  dev  of 7 runs  1000000 loops each     196 digits  timeit first n digits1 3423234239472523409283475908723908723409872390871243908172340987123409871234012089172340987734507612340981344509873123401234670350981234098123140987314509812734091823509871345109871234098172340987125988123452452  39  1 86   s    21 8 ns per loop  mean    std  dev  of 7 runs  100000 loops each     The  string  verions def first n digits2 num  n       return int str num   n     timeit first n digits2 34523452452  2  744 ns    28 1 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit first n digits2 34523452452  8  768 ns    42 7 ns per loop  mean    std  dev  of 7 runs  1000000 loops each     22 digits  timeit first n digits2 3423234239472523452452  2  767 ns    33 6 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit first n digits2 3423234239472523452452  15  830 ns    55 1 ns per loop  mean    std  dev  of 7 runs  1000000 loops each     196 digits  timeit first n digits2 3423234239472523409283475908723908723409872390871243908098712340987123401208917234098773450761234098134450987312340123467035098123409812314098734091823509871345109871234098172340987125988123452452  39  1 87   s    140 ns per loop  mean    std  dev  of 7 runs  100000 loops each

User · Answer

You can use a regular expression to test for a match and capture the first two digits   import re  for i in range 1000       match   re match r  1 56     str i        if match          print i   begins with   match group 1     The regular expression  1 56   matches a 1 followed by either a 5 or a 6 and stores the result in the first capturing group   Output   15 begins with 15 16 begins with 16 150 begins with 15 151 begins with 15 152 begins with 15 153 begins with 15 154 begins with 15 155 begins with 15 156 begins with 15 157 begins with 15 158 begins with 15 159 begins with 15 160 begins with 16 161 begins with 16 162 begins with 16 163 begins with 16 164 begins with 16 165 begins with 16 166 begins with 16 167 begins with 16 168 begins with 16 169 begins with 16

User · Answer

You can convert your number to string and use list slicing like this   int str number   2     Output    gt  gt  gt  number   1520  gt  gt  gt  int str number   2   15

[python] How can I get the first two digits of a number?

Function:

Output:

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