C Snippets

Cast Strings to Numbers

The atoi() function in stdlib is implemented similarly to the one below.

ASCII encodes numbers in order, after special characters.

The encoded value for '0' is 48, so subtracting any numeric char by 48 outputs its real numerical value.

char number = '7';
int result = number - 48;
int same_result = number - '0';

Algorithm to convert strings to numbers:

int str_to_int(char *str) {
	int result = 0;
	for (int i = 0; str[i] != '\0'; i++) {
		if (str[i] < '0' && str[1] > '9') return -1; // Error if NaN
		result = (result * 10) + (str[i] - '0');
	return result;
}

(result * 10) is shifting the previous number to the left, as it is an order of magnitude above the following digit.