Thread: problem with infix 2 postfix converter & postfix calculator

This is in regards to a class lab project on building a top down parser using flex in Linux as scanner/lexer and parser should convert valid infix expressions to postfix & calculate the result of the expression. Took me a while to comprehend what to do but I managed to almost complete the parser. It may not be optimal but it works. Unfortunately, I ran into some snags along the way, haven't figured it out yet, and my back's starting to ache. So in case I haven't solved it yet later on, hope someone can point out my logic errors.

Here's my problems:

1) infix 2 postfix conversion works 99% -- the left parenthesis still shows up in the postfix expression when it should not.

input: (1-2)*(3+4)
output: 12-34+(*(
expected: 12-34+*

2) calculator doesn't functioning right--it not really calculating. I thought my code is logically correct but maybe not.

Both 1 & 2 were based on algorithms found on net. I'm including snippets of the problem code. If you think the problem is not in those code snippets, let me know & I'll post other portions of the code.

Code:

//this is mostly C except for var++, etc.

int handleTokenData(char *data){
	
	int len = strlen(data);
	//store input for error handling
	inputStack[isIndex] = malloc(sizeof(char)*len);
	strcpy(inputStack[isIndex++],data);
	
	//infix 2 postfix conversion
	if((token == TOKID) || (token == TOKNUM)){
		//put operand into postfix string
		postfixStrStack[pssIndex] = malloc(sizeof(char)*len);
		strcpy(postfixStrStack[pssIndex++],data);
	}else if(token == TOKADDOP){
		//while stack !empty
		while(psIndex != -1){
			//if top of stack is (, skip poping
			if(!strcmp(postfixStack[psIndex],"(")){
				break;
			}
			//else pop top of stack into postfix string
			len = strlen(postfixStack[psIndex]);
			postfixStrStack[pssIndex] = malloc(sizeof(char)*len);
			strcpy(postfixStrStack[pssIndex++],postfixStack[psIndex]);
			free(postfixStack[psIndex--]);
		}
		//then push + or - to stack
		len = strlen(data);
		postfixStack[++psIndex] = malloc(sizeof(char)*len);
		strcpy(postfixStack[psIndex],data);

	}else if(token == TOKMULOP){
		//push * or / to stack
		postfixStack[++psIndex] = malloc(sizeof(char)*len);
		strcpy(postfixStack[psIndex],data);
	}else if(token == TOKLPAREN){
		//push ( to stack
		postfixStack[++psIndex] = malloc(sizeof(char)*len);
		strcpy(postfixStack[psIndex],data);
	}else if(token == TOKRPAREN){
		//pop top off stack into postfix string until find (
		while(strcmp(postfixStack[psIndex],"(") != 0){
			len = strlen(postfixStack[psIndex]);
			postfixStrStack[pssIndex] = malloc(sizeof(char)*len);
			strcpy(postfixStrStack[pssIndex++],postfixStack[psIndex]);
			free(postfixStack[psIndex--]);
		}
	}else{
		//not a valid input, do nothing
	}
	return 1;
}

int printPostfix(){
	//while stack not empty, pop stack top to finalize postfix string
	int len;
	while(psIndex != -1){
		len = strlen(postfixStack[psIndex]);
		postfixStrStack[pssIndex] = malloc(sizeof(char)*len);
		strcpy(postfixStrStack[pssIndex++],postfixStack[psIndex]);
		free(postfixStack[psIndex--]);
	}
	int i;
	printf("Postfix translation: ");
	for(i = 0; i < pssIndex; i++)
		printf("%s",postfixStrStack[i]);
	printf("\n");
	return 1;
}

int calcExpr(){
	//initialize temp stack for postfix expr calc
	double temp = 0;
	double result = 0;
	double calcStack[100];
	int i;
	int csIndex = 0;
	
	//scan thru postfix expr to parse & calculate
	for(i = 0; i < pssIndex; i++){
		//if item is operator: 
		// 1) pop operand in stack top to temp var
		// 2) evaluate temp operator stack top, (poping
		//    stack top in process) & store in result,
		// 3) push result to stack
		if(!strcmp(postfixStrStack[i],"*")){
			temp = calcStack[csIndex--];
			result = temp * calcStack[csIndex--];
			calcStack[csIndex] = result;
			printf("* now ");
		}else if(!strcmp(postfixStrStack[i],"/")){
			temp = calcStack[csIndex--];
			result = temp / calcStack[csIndex--];
			calcStack[csIndex] = result;
			printf("/ now ");
		}else if(!strcmp(postfixStrStack[i],"+")){
			temp = calcStack[csIndex--];
			result = temp + calcStack[csIndex--];
			calcStack[csIndex] = result;
			printf("+ now ");
		}else if(!strcmp(postfixStrStack[i],"-")){
			temp = calcStack[csIndex--];
			result = temp - calcStack[csIndex--];
			calcStack[csIndex] = result;
			printf("- now ");
		}else{	//item is operand: push to stack
			calcStack[csIndex++] = atof(postfixStrStack[i]);
			printf("#id ");
		}
		printf("%s %f %f %f\n",postfixStrStack[i],calcStack[csIndex],temp,result);
	}
	//after scan, pop result from stack
	printf("Calculated value of expression: %f\n",calcStack[csIndex]);
	return 1;
}