# python – Leetcode atoi (string to integer)

link here I’ll include a solution in python and c++ and you can review one, I’m mostly interested in reviewing the c++ code which is a thing I recently started learning, those who don’t know c++ can review python code.

Problem statement

Implement atoi which converts a string to an integer. The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value. The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function. If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed. If no valid conversion could be performed, a zero value is returned.

Note:

Only the space character ‘ ‘ is considered a whitespace character.
Assume we are dealing with an environment that could only store integers within the 32-bit signed integer range: (−231, 231 − 1). If the numerical value is out of the range of representable values, 231 − 1 or −231 is returned.

Example 1:

``````Input: str = "42"
Output: 42
``````

Example 2:

``````Input: str = "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign. Then take as many numerical digits as possible, which gets 42.
``````

Example 3:

``````Input: str = "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
``````

Example 4:

``````Input: str = "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical digit or a +/- sign. Therefore no valid conversion could be performed.
``````

Example 5:

``````Input: str = "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer. Thefore INT_MIN (−231) is returned.
``````

`str_int.py`

``````def convert(s):
chars = (c for c in s)
ss = ()
while True:
try:
current = next(chars)
if (space := current.isspace()) and ss:
break
if (pm := current in '+-') and ss:
break
if not current.isnumeric() and not pm and not space:
break
if not space:
ss.append(current)
except StopIteration:
break
try:
number = int(''.join(ss).strip())
if number < 0:
return max(-2 ** 31, number)
return min(2 ** 31 - 1, number)
except ValueError:
return 0

if __name__ == '__main__':
print(convert("    48-"))
``````

There is an overloaded function I call `atoi_impl()` one version accepts 3 parameters `s`, `start_idx` and `end_idx` and uses `std::stoi()` to return the result and another that is the proper implementation of atoi and accepts only `s` but for some reason, it causes a runtime error on leetcode platform only (works perfectly fine on my mbp) so as long as it achieves what it’s meant to do, I might consider it better / more readable.

`str_int.h`

``````#ifndef LEETCODE_STR_TO_INT_H
#define LEETCODE_STR_TO_INT_H
#include <string>
int atoi_impl(const std::string& s);
int atoi_impl(const std::string& s, size_t start_idx, size_t end_idx);
int convert_str(const std::string &s);
#endif //LEETCODE_STR_TO_INT_H
``````

`str_int.cpp`

``````#include <string>
#include <iostream>

int atoi_impl(const std::string& s) {
int result = 0;
size_t i;
for ((s(0) == '-') ? i = 1: i = 0; s(i) != ''; ++i) {
auto update = result * 10 + s(i) - '0';
if (update / 10 != result) {
result = (s(0) == '-') ? INT32_MIN: INT32_MAX;
return result;
}
result = result * 10 + s(i) - '0';
}
result = (s(0) == '-') ? -result: result;
return result;
}

int atoi_impl(const std::string& s, size_t start_idx, size_t end_idx) {
try {
return std::stoi(s.substr(start_idx, end_idx));
}
catch (const std::out_of_range &e) {
return (s(start_idx) == '-') ? INT32_MIN : INT32_MAX;
}
catch (const std::invalid_argument &e) {
return 0;
}
}

int convert_str(const std::string &s) {
size_t start_idx = 0;
size_t end_idx = s.size();
for (size_t i = 0; i < s.size(); ++i) {
bool digit = std::isdigit(s(i));
bool pm = s(i) == '+' || s(i) == '-';
bool space = std::isspace(s(i));
if (i == start_idx && !space && !digit && !pm)
return 0;
if ((space || !digit) && i != start_idx) {
end_idx = i;
break;
}
if (space)
start_idx++;
}
if (start_idx != end_idx)
return atoi_impl(s, start_idx, end_idx);
return 0;
}

int main() {
std::cout << "result1: " << convert_str(" -912332") << "n";
}
``````