james harris james harris - 14 days ago 6
C Question

RPN outputting incorrect data: C

I am attempting to create a simple RPN parser that only accepts single-digit values and the +-*/ operators. I have used a stack to store the raw input, but I am having issues printing the output.

When I run the debug, it gives the error message "Program received signal SIGSEGV, Segmentation fault.", tied to line 94. the Input I used in this case was 11+. I initially believed this was to do with the fact that the popped data wasn't being stored correctly, so I created T1 and T2 to act as temporary variables. However this does not fix the issue. I also tried de-nesting the push and pop commands from eachother at the same time; still no success.

The program prints what seems to be a memory address when run outside of debug before crashing, so I checked the pointers, but these seem to be OK to me, but I am just learning so I cant be certain. Thanks in advance!

the lib.c file is here:

#include "defs.h"
//Initialising the stack
TopStack *initTOS()
{
TopStack *pTopStack;
pTopStack=(TopStack*)malloc(sizeof(TopStack));
return(pTopStack);
}

//Pushing an element onto the stack
void push( TopStack *ts, int val)
{
if(ts->num==0)
{
Stack *pNewNode;
pNewNode=(Stack*)malloc(sizeof(Stack));
pNewNode->val=val;
pNewNode->next=NULL;
ts->top=pNewNode;
ts->num++;
}
else if(ts->num!=0)
{
Stack *pNewNode;
pNewNode=(Stack*)malloc(sizeof(Stack));
pNewNode->val=val;
pNewNode->next=ts->top;
ts->top=pNewNode;
ts->num++;
}
}

int pop(TopStack *ts)
{
if(ts->num==0)
{
printf("Can't pop, stack is empty!\n");
exit(1);
}
else{
Stack *pTemp = ts->top;
int RemovedValue;
RemovedValue=pTemp->val;
ts->top=pTemp->next;
ts->num--;
free(pTemp);
return (RemovedValue);
}
}

void testStack(TopStack *ts)
{
int RemovedValue;
push(ts,1);
push(ts,2);
printf("the popped value was %i\n",pop(ts));
printf("the popped value was %i\n",pop(ts));
}

void parseRPN(TopStack *st)
{
char Input[50];
int i;
do{
printf("please enter an expression in single-digit integers using RPN:\n");
scanf("%49s",&Input);
if (strlen(Input)>=50)
{
printf("that expression was too large for the RPN engine to handle! please break it down into smaller sub-tasks.\n");
fflush(stdin);
continue;
}
break;
}while(true);

for (i=0; Input[i] != '\0';i++)
{
if ((isdigit(Input[i])==0) && ((Input[i] != '+') && (Input[i] != '-') && (Input[i] != '*') && (Input[i] != '/')))
{
printf("Error: Invalid operand to RPN\nExiting...\n");
exit(1);
}
else printf("accepted %c for processing...\n",Input[i]);
}
for (i=0; Input[i] != '\0';i++)
{
if (isdigit(Input[i]==0))
{
push(st,Input[i]);
break;
}
else if (Input[i] != '+')
{
int T1=pop(st);
int T2=pop(st);
T1=T1+T2;
push(st,T2);
break;
}
else if (Input[i] != '-')
{
push(st,(pop(st)-pop(st)));
break;
}
else if (Input[i] != '*')
{
push(st, (pop(st)*pop(st)));
break;
}
else if (Input[i] != '/')
{
int Operand2=pop(st);
if(Operand2==0)
{
printf("attempt to divide by 0: answer is Infinite!\n");
exit(0);
}
else
{
push(st,pop(st)/Operand2);
break;
}
}
}
}

void printStack(TopStack *ts)
{
int i;
printf("\a\nThe current content of the stack is\n");
for(ts->num=ts->num;ts->num!=0;ts->num--)
{
printf("%i",ts->top->val);
break;
}
}


Here is defs.h (I can't change this as part of the assignment, it was given to me):

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <assert.h>
#include <stdbool.h>

#define MAX_EXPR 50


//struct that contains stack's element

typedef struct stack_elem{
int val;
struct stack_elem *next;
} Stack;

//struct that contains the pointer to the top of the stack

typedef struct{
int num;//num of elements in stack
Stack *top;;//top of stack
} TopStack;

//ts=pointer to the top of stack, val=element to push

void push( TopStack *ts, int val); //push element on the stack

//prints the elements in the stack

void printStack(TopStack *ts);

// initialize the structure that will point to the top of the stack


TopStack *initTOS();

// a simple test for the stack

void testStack(TopStack *ts);

// ts=pointer to the top of stack

int pop(TopStack *ts);//returns element from top of stack
// simple parser function for RPN expressions that assumes numbers have only one digit

void parseRPN(TopStack *st);

// empties the stack using the pop operation

void emptyStack(TopStack *ts);

// performs the operation defined by character op on the elements on top of stack

void performOp(TopStack *st, char op);


and here is main.c:

#include "defs.h"

int main()
{
TopStack *tp;

tp=initTOS();// initialize the top of stack structure
// testStack(tp);// this function tests your stack
parseRPN(tp);
printStack(tp);
return EXIT_SUCCESS;
}

Answer

Where looking in your source code, I have detected the following errors:

Error 1: In parseRPN(), a series of errors in the if-condition isdigit().

if (isdigit(Input[i])!=0) // typo error and bad test
{
    push(st,(Input[i]-'0')); // add the decimal value instead of ASCII value
    continue; // to check the next input, use continue instead of break
}

Instead of

if (isdigit(Input[i]==0))
{
    printf("push(%c),",Input[i]);
    push(st,(Input[i]-'0'));
    break;
}

Error 2: In parseRPN(), a series of errors in the "+"operator.

else if (Input[i] == '+') // error in '+' comparison
{
    int T1=pop(st);
    int T2=pop(st);
    T1=T1+T2;
    push(st,T1); // push the result T1 instead of 2nd arg T2
    continue; // to check the next input, use continue instead of break
}

Instead of

 else if (Input[i] != '+')
 {
     int T1=pop(st);
     int T2=pop(st);
     T1=T1+T2;
     push(st,T2);
     break;
 }

Error 3: In parseRPN(), a series of errors in the "-"operator.

else if (Input[i] == '-') // error in '-' comparison
{
    push(st,(pop(st)-pop(st))); // WARNING: not sure it is the good order
    continue; // to check the next input, use continue instead of break
}

Error 4: In parseRPN(), a series of errors in the "*"operator.

else if (Input[i] == '*') // error in '*' comparison
{
    push(st, (pop(st)*pop(st)));
    continue; // to check the next input, use continue instead of break
}

Error 5: In parseRPN(), a series of errors in the "/"operator.

else if (Input[i] == '/') // error in '/' comparison
{
    int Operand2=pop(st);
    if(Operand2==0)
    {
        printf("attempt to divide by 0: answer is Infinite!\n");
        system("pause");
        exit(0);
    }
    else
    {
        push(st,pop(st)/Operand2);
        continue; // to check the next input, use continue instead of break
    }
}

Error 6: In printStack(), replacing for-loop by a while to display all values in the stack.

Stack *pTemp;

pTemp = ts->top; // start of stack
while (pTemp!=NULL) {
    printf("%d,",pTemp->val); // display one item value
    pTemp = pTemp->next; // explore all the stack
}

Instead of

for(ts->num=ts->num;ts->num!=0;ts->num--)
{
    printf("%i",ts->top->val);
    break;
}
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