Fast atoi() in C and C++

2022-01-20 17:10:10 EET | back | home | git

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std::atoi (in C++) is fast on its own but can be slow so for stuff you need speed you can use this:

• Option one

int fast_atoi(const char *int_string) {
    int value = 0;

    while (*int_string && *int_string >= '0' && *int_string <= '9')
        value = value * 10 + (*int_string++ - '0');

    return value;
}

This requires no headers, just pure C (also valid in C++), you can put this into your code and it will just work!

• Option two

#include <assert.h>

int fast_atoi(char *int_string) {
    int value = 0;

    while (*int_string) {
        if (!(*int_string >= '0' && *int_string <= '9'))
            assert(0 && "atoi(): invalid int_string");

        value = value * 10 + (*int_string++ - '0');
    }

    return value;
}

This option is throws an assertion error if it encounters an invalid string but this requires assert.h as a dependency.

In C++ assert.h should be replaced with cassert.