function.CalculateExpression.h

// make strOut = strIn[start] to strIn[end]
void makeStrStartEnd(char* strIn, char* strOut, int start, int end){
	int i;
	for (i = start; i <= end; i++) strOut[i - start] = strIn[i];
	strOut[end - start + 1] = '\0';
}

// calculate expression
void calExpression(char* strIn, char* strOut, char* ans){
	
	// case \0
	if (strlen(strIn) == 0){
		strcpy(strOut, "SYNTAX ERROR");
		return;
	}
	
	// initialize
	int length = strlen(strIn);
	int countBracket = 0; // work as a stack, +1 for '(', -1 for ')'
	int i;
	
	// case + -
	// divide strIn into 2 parts: left and right of the operator, then do the math
	// i.g. calExpression("2*3+4-3") = addRealNum(calExpression("2*3"), calExpression("4-3"));
	for (i = length - 1; i >= 0; i--){
		
		// count bracket
		if (strIn[i] == ')') countBracket++;
		if (strIn[i] == '(') countBracket--;
		
		// check + -
		if (((strIn[i] == '+') || (strIn[i] == '-')) && (countBracket == 0)){
			
			// divide strIn to 2 parts: strIn1 and strIn2
			char strIn1[MAX_LENGTH], strIn2[MAX_LENGTH], strOut1[MAX_LENGTH], strOut2[MAX_LENGTH];
			makeStrStartEnd(strIn, strIn1, 0, i - 1);
			makeStrStartEnd(strIn, strIn2, i + 1, length - 1);
			
			// calculate strIn1 and strIn2
			calExpression(strIn1, strOut1, ans); if(strIn1[0] == '\0') strcpy(strOut1, "0");
			calExpression(strIn2, strOut2, ans); // strOut1 and strOut2 are numbers
			
			// check for MATH ERROR or SYNTAX ERROR
			if ((strOut1[0] == 'M') || (strOut2[0] == 'M')){
				strcpy(strOut, "MATH ERROR!");
				return;
			}
			if ((strOut1[0] == 'S') || (strOut2[0] == 'S')){
				strcpy(strOut, "SYNTAX ERROR!");
				return;
			}

			// calculate strIn
			if (strIn[i] == '+') addRealNum(strOut1, strOut2, strOut);
			else subtractRealNum(strOut1, strOut2, strOut);
			
			// exit
			return;
		}
	}
	
	// case x /
	for (i = length - 1; i >= 0; i--){
		
		// count bracket
		if (strIn[i] == ')') countBracket++;
		if (strIn[i] == '(') countBracket--;
		
		// check x /
		if (((strIn[i] == 'x') || (strIn[i] == '/')) && (countBracket == 0)){
			
			// divide strIn into 2 parts: strIn1 and strIn2
			char strIn1[MAX_LENGTH], strIn2[MAX_LENGTH], strOut1[MAX_LENGTH], strOut2[MAX_LENGTH];
			makeStrStartEnd(strIn, strIn1, 0, i - 1);
			makeStrStartEnd(strIn, strIn2, i + 1, length - 1);
			
			// calculate strIn1 and strIn2
			calExpression(strIn1, strOut1, ans);
			calExpression(strIn2, strOut2, ans);
			
			// check for MATH ERROR or SYNTAX ERROR
			if ((strOut1[0] == 'M') || (strOut2[0] == 'M')){
				strcpy(strOut, "MATH ERROR!");
				return;
			}
			if ((strOut1[0] == 'S') || (strOut2[0] == 'S')){
				strcpy(strOut, "SYNTAX ERROR!");
				return;
			}
			
			// calculate strIn
			if (strIn[i] == 'x') multiplyRealNum(strOut1, strOut2, strOut);
			else divideRealNum(strOut1, strOut2, strOut);
			
			// exit
			return;
		}
	}
	
	// case ^
	for (i = 0; i <= length - 1; i++){
		
		// count bracket
		if (strIn[i] == '(') countBracket++;
		if (strIn[i] == ')') countBracket--;
		
		// check ^
		if ((strIn[i] == '^') && (countBracket == 0)){
			
			// divide strIn to 2 parts: strIn1 and strIn2
			char strIn1[MAX_LENGTH], strIn2[MAX_LENGTH], strOut1[MAX_LENGTH], strOut2[MAX_LENGTH];
			makeStrStartEnd(strIn, strIn1, 0, i - 1);
			makeStrStartEnd(strIn, strIn2, i + 1, length - 1);
			
			// calculate strIn1 and strIn2
			calExpression(strIn1, strOut1, ans);
			calExpression(strIn2, strOut2, ans);
			
			// check for MATH ERROR or SYNTAX ERROR
			if ((strOut1[0] == 'M') || (strOut2[0] == 'M')){
				strcpy(strOut, "MATH ERROR!");
				return;
			}
			if ((strOut1[0] == 'S') || (strOut2[0] == 'S')){
				strcpy(strOut, "SYNTAX ERROR!");
				return;
			}
			
			// calculate strIn
			powerRealNum(strOut1, strOut2, strOut);
			
			// exit
			return;
		}
	}
	
	// case !
	if (strIn[length - 1] == '!'){
		
		// make temporary string
		char str[MAX_LENGTH], strOut1[MAX_LENGTH];
		makeStrStartEnd(strIn, str, 0, length - 2);
		
		// calculate the number to calculate factorial
		calExpression(str, strOut1, ans);
		
		// check for MATH ERROR or SYNTAX ERROR
		if (strOut1[0] == 'M'){
			strcpy(strOut, "MATH ERROR!");
			return;
		}
		if (strOut1[0] == 'S'){
			strcpy(strOut, "SYNTAX ERROR!");
			return;
		}
			
		// calculate strIn
		getFactorial(strOut1, strOut);
		
		// exit
		return;
	}
	
	// case sqrt()
	if (strIn[0] == -5){
		
		// make temporary string
		char str[MAX_LENGTH], strOut1[MAX_LENGTH], strOut2[MAX_LENGTH];
		makeStrStartEnd(strIn, str, 1, length - 1);
		
		// calculate the number to calculate square root
		calExpression(str, strOut1, ans);
		strcpy(strOut2, "0.5");
		
		// check for MATH ERROR or SYNTAX ERROR
		if (strOut1[0] == 'M'){
			strcpy(strOut, "MATH ERROR!");
			return;
		}
		if (strOut1[0] == 'S'){
			strcpy(strOut, "SYNTAX ERROR!");
			return;
		}
			
		// calculate strIn
		powerRealNum(strOut1, strOut2, strOut);
		
		// exit
		return;
	}
	
	// case sin() cos() tan() cot() ln() log() abs()
	int sincos = 0;
	if (strstr(strIn, "sin") == strIn) sincos = 1;
	if (strstr(strIn, "cos") == strIn) sincos = 2;
	if (strstr(strIn, "tan") == strIn) sincos = 3;
	if (strstr(strIn, "cot") == strIn) sincos = 4;
	if (strstr(strIn, "ln") == strIn) sincos = 5;
	if (strstr(strIn, "log") == strIn) sincos = 6;
	if (strstr(strIn, "abs") == strIn) sincos = 7;
	if (sincos != 0){
		
		// make temporary string
		char str[MAX_LENGTH], strOut1[MAX_LENGTH];
		makeStrStartEnd(strIn, str, (sincos == 5) ? 2 : 3, length - 1);
		
		// calculate the number to calculate the function
		calExpression(str, strOut1, ans);
		
		// check for MATH ERROR or SYNTAX ERROR
		if (strOut1[0] == 'M'){
			strcpy(strOut, "MATH ERROR!");
			return;
		}
		if (strOut1[0] == 'S'){
			strcpy(strOut, "SYNTAX ERROR!");
			return;
		}
			
		// calculate strIn
		switch (sincos){
			case 1: getSinRadian(strOut1, strOut); break;
			case 2: getCosRadian(strOut1, strOut); break;
			case 3: getTanRadian(strOut1, strOut); break;
			case 4: getCotRadian(strOut1, strOut); break;
			case 5: getNaturalLog(strOut1, strOut); break;
			case 6: getLog10(strOut1, strOut); break;
			case 7:
				strcpy(strOut, strOut1);
				getAbs(strOut);
				break;
			default: break;
		}
		
		// exit
		return;
	}
	
	// case ( )
	if ((strIn[0] == '(') && (strIn[length - 1] == ')')){
			
		// make temporary string
		char str[MAX_LENGTH];
		makeStrStartEnd(strIn, str, 1, length - 2);
		
		// proceed calculating
		calExpression(str, strOut, ans);
		
		// exit
		return;
	}
	
	
	// case e
	if (strcmp(strIn, "e") == 0){
		strcpy(strOut, "2.7182818284590452353602874713527");
		return;
	}
	
	// case pi
	if ((strIn[0] == -29) && (length == 1)){
		strcpy(strOut, "3.1415926535897932384626433832795");
		return;
	}
	
	// case ans
	if (strcmp(strIn, "ans") == 0){
		strcpy(strOut, ans);
		return;
	}
	
	// case number
	if ((strIn[0] == '-') || ((strIn[0] >= '0') && (strIn[0] <= '9'))){
	
		// check '-'
		int minus = 0;
		if (strIn[0] == '-') minus = 1;
		for (i = 1; i <= length - 1; i++) if (strIn[i] == '-') goto syntaxError;
	
		// check '.'
		int dot = 0;
		for (i = minus; i <= length - 1; i++)
			if (strIn[i] == '.'){
				if ((i == minus) || (i == length - 1) || (dot == 1)) goto syntaxError;
				dot++;
			}
	
		// check ['0', '9']
		for (i = 0; i <= length - 1; i++)
		if (!(((strIn[i] >= '0') && (strIn[i] <= '9')) || (strIn[i] == '.') || (strIn[i] == '-'))) goto syntaxError;
		
		// return number
		strcpy(strOut, strIn);
		
		return;
	}
	
	// SYNTAX ERROR
	syntaxError:
	strcpy(strOut, "SYNTAX ERROR!");
}