Thread: postfix calculator having trouble with segfaults

i'm still beginning to learn c so please bear with me. i'm trying to create a postfix calculator using linked-list. when i compile, it doesn't show any errors but when it's executed it would show Segmentation Fault. i don't know how to deal with this, please help. here's my code:

Code:

Code:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <ctype.h>


    typedef struct node	{
	    int num;
	    struct node *next;
    } node;


    void push(int num, node **head);
    int pop(node **head);
    void display(node **head);
    int is_empty();
    int evaluatePostfix();
    
    struct node *head;


    int main()	{
	    head = NULL;
        char exp[1000]; // = "5 1 2 + 4 * + 3 -";
	    printf("Input expression:\t");
    	fgets(exp, 1000, stdin);
    	for(int i = 1; i <= strlen(exp); i++) {
    		if(exp[i] == '\n') {
    			exp[i] = '\0';
    		}
    		else if (exp[0] == '\n') {
				printf("stack is empty\n");
				exit(0);
	    	}
    	}
	    printf("%s = %d\n", exp, evaluatePostfix(exp));
    
    	return 0;
}




    int evaluatePostfix(char* exp)	{
	    char * token;
    	int counter = 0;
    	char temp[256][256];
    	token = strtok(exp, " ");
    	while(token != NULL)	{
    		strcpy(temp[counter], token);
    		token = strtok(NULL, " ");	
			counter++;
	    }
		
    	for (int i = 0; i < counter; i++)	{
    		if (isdigit(*(temp[i]))) {
    			int val = atoi(temp[i]);
                push(val, &head);
	    	}
            else   {
    			int val1 = pop(&head);
                int val2 = pop(&head);
                switch (exp[i])	{
	    			case '+':    push(val2 + val1, &head);
	    			    	     printf("%d\n", (*head).num);
				                 break;
			    	case '-':    push(val2 - val1, &head);    break;
			    	case '*':    push(val2 * val1, &head);    break;
			    	case '/':    push(val1 / val2, &head);    break;
			    }
		    }
        }
        return pop(&head);
    }




    void push (int item, node **head) {
    	node *temp;
    	node * get_node(int);
    	temp = get_node(item);
    	temp->next = *head;
    	*head = temp;
    }


    node *get_node(int item) {
    	node *temp;
    	temp = (node*)malloc(sizeof(node));
    	if (temp == NULL)
    		printf("\nMemory cannot be allocated");
    	temp->num = item;
	    temp->next = NULL;
    	return(temp);
    }




    int pop(node **head)	{
    	int item;
    	node *temp;
    	item = (*head)->num;
        temp = *head;
    	*head = (*head)->next;
    	free(temp);
    	return(item);
    }
	
    int is_empty(node *temp) {
    	if (temp == NULL)
    		return 1;
    	else
	    	return 0;
    }




    void display(node **head)	{
    	node *temp;
    	temp = *head;
    	if(head == NULL)	{
    		printf("stack is empty\n");
    		return;
    	}
    	printf("\n");
	    printf("=========\n");
    	while(temp!=NULL)	{
    		printf("%d\n", (*temp).num);
    		temp = (*temp).next;
    	}
    	printf("=========\n");
    }

i think the problem is with

Code:

 evaluatePostfix(char* exp)

but im not sure how to fix it. i appreciate the help

i tried gdb and it showed this. i have no idea how to fix it. what do you suggest?

Code:

 Program received signal EXC_BAD_ACCESS, Could not access memory.Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x0000000100000ba5 in pop (head=0x1000010e8) at ask.c:104
104           item = (*head)->num;

The switch condition in evaluatePostFix() uses the original array instead of the tokenised one.

A default case is often a good idea, e.g. one that prints "Error: unknown operator".

btw, all identifiers for functions in the Standard Library are reserved.

There is a function exp() in math.h, so exp falls into that category.

Technically, using a reserved identifier in your code results in undefined behaviour.

(If you look at your posted code, exp appears in red just like the other standard functions used in the code.)

changing exp to another variable name did the trick. here's the working code so far:

Code:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <ctype.h>


    typedef struct node    {
        int num;
        struct node *next;
    } node;


    void push(int num, node **head);
    int pop(node **head);
    void display(node **head);
    int is_empty(node *temp);
    int evaluate(char *expression);


    struct node *head;


    int main()    {
        head = NULL;
        char expression[1000]; // = "5 1 2 + 4 * + 3 -";
        printf("Input expression:\t");
        fgets(expression, 1000, stdin);
        for(int i = 1; i <= strlen(expression); i++) {
            if(expression[i] == '\n') {
                expression[i] = '\0';
            }
            else if (expression[0] == '\n') {
                printf("empty stack\n");
                exit(0);
            }
        }


        printf("Answer: %d\n", evaluate(expression) );
        return 0;
}




    int evaluate(char* expression)    {
        char * token;
        int counter = 0;
        char temp[256][256];
        token = strtok(expression, " ");
        while(token != NULL)    {
            strcpy(temp[counter], token);
            token = strtok(NULL, " ");    
            counter++;
        }
        
        for (int i = 0; i < counter; i++)    {
            if (isdigit(*(temp[i]))) {
                int val = atoi(temp[i]);
                push(val, &head);
            }
            else   {
                int val1 = pop(&head);
                int val2 = pop(&head);
                switch (*(temp[i]))    {
                    case '+':    push(val2 + val1, &head);    break;
                    case '-':    push(val2 - val1, &head);    break;
                    case '*':    push(val2 * val1, &head);    break;
                    case '/':    push(val1 / val2, &head);    break;
                }
            }
        }
        return pop(&head);
    }




    void push (int item, node **head) {
        node *temp;
        node * get_node(int);
        temp = get_node(item);
        temp->next = *head;
        *head = temp;
    }


    node *get_node(int item) {
        node *temp;
        temp = (node*)malloc(sizeof(node));
        if (temp == NULL)
            printf("\nMemory cannot be allocated");
        temp->num = item;
        temp->next = NULL;
        return(temp);
    }




    int pop(node **head)    {
        int item;
        node *temp;
        item = (*head)->num;
        temp = *head;
        *head = (*head)->next;
        free(temp);
        return(item);
    }
    
    int is_empty(node *temp) {
        if (temp == NULL)
            return 1;
        else
            return 0;
    }




    void display(node **head)    {
        node *temp;
        temp = *head;
        if(head == NULL)    {
            printf("stack is empty\n");
            return;
        }
        printf("\n");
        printf("=========\n");
        while(temp!=NULL)    {
            printf("%d\n", (*temp).num);
            temp = (*temp).next;
        }
        printf("=========\n");
    }

pcyap1It totally sounds a good exercise for a new programmers to improve their skills, may you explain what your code does by an example?
I got that if you input a string "4+5+6-7", the output should be 6?(printing the number before the least one)

Originally Posted by Romyo2

pcyap1It totally sounds a good exercise for a new programmers to improve their skills, may you explain what your code does by an example?
I got that if you input a string "4+5+6-7", the output should be 6?(printing the number before the least one)

it's a postfix calculator. so if i input "5 1 2 + 4 * + 3 -" the answer would be 14