Converting between string and real

Wang Renxin edited this page Nov 19, 2017 · 2 revisions

Some compilers may not implement with the standard C library strictly. Some may miss real number processing functions, especially for embedded compiler which target platforms are often lack of real number processing unit. This page contains two compatible implementation for replacement.

The first one is a strtod implementation, to which you can redefine the mb_strtod macro.

// strtod.c
//
// Convert string to double
//
// Copyright (C) 2002 Michael Ringgaard. All rights reserved.
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//    may be used to endorse or promote products derived from this software
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#include <ctype.h>
#include <errno.h>
#include <float.h>
#include <math.h>
#include <stdlib.h>

double _str_to_real/*strtod*/(const char* str, char** endptr) {
	double number;
	int exponent;
	int negative;
	char* p = (char*)str;
	double p10;
	int n;
	int num_digits;
	int num_decimals;

	// Skip leading whitespace
	while(isspace(*p)) p++;

	// Handle optional sign
	negative = 0;
	switch(*p) {
	case '-': negative = 1; // Fall through to increment position
	case '+': p++;
	}

	number = 0.;
	exponent = 0;
	num_digits = 0;
	num_decimals = 0;

	// Process string of digits
	while(isdigit(*p)) {
		number = number * 10. + (*p - '0');
		p++;
		num_digits++;
	}

	// Process decimal part
	if(*p == '.') {
		p++;

		while(isdigit(*p)) {
			number = number * 10. + (*p - '0');
			p++;
			num_digits++;
			num_decimals++;
		}

		exponent -= num_decimals;
	}

	if(num_digits == 0) {
		errno = ERANGE;

		return 0.0;
	}

	// Correct for sign
	if(negative) number = -number;

	// Process an exponent string
	if(*p == 'e' || *p == 'E') {
		// Handle optional sign
		negative = 0;
		switch(*++p) {
		case '-': negative = 1; // Fall through to increment pos
		case '+': p++;
		}

		// Process string of digits
		n = 0;
		while(isdigit(*p)) {
			n = n * 10 + (*p - '0');
			p++;
		}

		if(negative)
			exponent -= n;
		else
			exponent += n;
	}

	if(exponent < DBL_MIN_EXP || exponent > DBL_MAX_EXP) {
		errno = ERANGE;

		return HUGE_VAL;
	}

	// Scale the result
	p10 = 10.;
	n = exponent;
	if(n < 0) n = -n;
	while(n) {
		if(n & 1) {
			if(exponent < 0)
				number /= p10;
			else
				number *= p10;
		}
		n >>= 1;
		p10 *= p10;
	}

	if(number == HUGE_VAL) errno = ERANGE;
	if(endptr) *endptr = p;

	return number;
}

The other is a real to string conversion implementation. It is used for real number formatting with the STR and PRINT statements. I've already added this function to the kernel code, and enabled for Arduino compiling, you may define an MB_MANUAL_REAL_FORMATTING macro to use this function instead of printf("%g", ...), sprintf("%g", ...).

// Convert a real number to string
static void _real_to_str(double r, char* str, size_t size, size_t afterpoint) {
	size_t pos = 0;
	size_t len = 0;
	char curr[4];
	int val = (int)r;
	char dot = 0;

	itoa(val, str, 10);
	if(r < 0) {
		r *= -1;
		val *= -1;
	}
	pos = len = strlen(str);
	while(pos < size - 1) {
		r = r - (double)val;
		if(r == 0.0)
			break;
		if(!dot) {
			dot = 1;
			str[pos++] = '.';
		}
		r *= 10;
		val = (int)r;
		itoa(val, curr, 10);
		str[pos++] = *curr;
		if(--afterpoint == 0)
			break;
	}
	str[pos] = '\0';
}
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