I'm trying to implement add2strings, sub2strings, mult2strings functions in Python. They're all very easy if you just do int(string), but I want to do them without that and without importing another cheating thing like Decimal. My current idea is to use bytes.
Is there another way to do this?
Refer to a basic atoi in C:
int myAtoi(char *str)
{
int res = 0; // Initialize result
// Iterate through all characters of input string and update result
for (int i = 0; str[i] != '\0'; ++i)
res = res*10 + str[i] - '0';
// return result.
return res;
}
Which translates into the Python:
def atoi(s):
rtr=0
for c in s:
rtr=rtr*10 + ord(c) - ord('0')
return rtr
Test it:
>>> atoi('123456789')
123456789
If you want to accommodate an optional sign and whitespace the way that int does:
def atoi(s):
rtr, sign=0, 1
s=s.strip()
if s[0] in '+-':
sc, s=s[0], s[1:]
if sc=='-':
sign=-1
for c in s:
rtr=rtr*10 + ord(c) - ord('0')
return sign*rtr
Now add exceptions and you are there!
This is really inefficient but:
>>> zero = ord("0")
>>> s = "1234"
>>> sum([x * 10**i for i, x in enumerate(map(lambda x: x - zero, map(ord, s))[::-1])])
1234
This is slightly better:
>>>> sum([x * 10**i for i, x in enumerate([ord(x) - zero for x in s[::-1]])])
1234
>>> atoi = lambda s: sum([x * 10**i for i, x in enumerate([ord(x) - zero for x in s[::-1]])])
>>> atoi("1234")
1234
My solution:
def str_to_int(input_str):
if input_str[0] == "-":
is_negative = True
input_str = input_str[1:]
else:
is_negative = False
output_int = 0
for i in input_str:
output_int = output_int * 10 + ord(i) - ord("0")
if is_negative:
output_int *= -1
return output_int
print(str_to_int("123"))
print(str_to_int("-123"))
output:
123
-123
import exceptions
MAX_INT = 1000
MIN_INT = -1000
def str2int(s):
for i in range(MIN_INT,MAX_INT):
if s == str(i):
return i
raise exceptions.OverflowError
def add2strings(s,t):
return str(str2int(s)+str2int(t))
print add2strings("170","-300")
print add2strings("170","-1001")
This gives:
"-170"
Traceback (most recent call last):
Line 15, in <module>
print add2strings("170","-1001")
Line 12, in add2strings
return str(str2int(s)+str2int(t))
Line 9, in str2int
raise exceptions.OverflowError
OverflowError
"0"to get digit0, etc.