Question

We’re supposed to implement a program to check whether the braces, brackets and parens in a given expression all match using a stack structure in C++ for my CS class. Unfortunately, I’m kinda stuck on this one, since I keeps telling me something doesn’t match, even when it most decidedly does.
Here’s what I got so far:

#include <stdlib.h>
#include <iostream>
#include <string>
#include <cstring>
using namespace std;

struct cell {int value; cell* next; };
cell* top;
int numElem;

void init()
{
    top = NULL;
    numElem = 0;
}

int pop()
{
    int res;
    if (top != NULL)
    {
        res = top -> value;
        top = top -> next;
        numElem--;
    } else {
        cout << "FAIL: Stack empty!\n";
        res = -1;
    }
    return res;
}

void push(int element)
{
    cell* cat = new cell;
    cat -> value = element;
    cat -> next = top;
    top = cat;
}

void match(char expr[])
{
    bool pass = true;
    char expected;
    char encountered;
    char closing;
    for (int i=0; pass && (i<strlen(expr)); i++)
    {
        if ((i==40)||(i==91)||(i==123))
            push(i);
        else 
        {
            if (i==41)
                expected = 40;
            if (i==93)
                expected = 91;
            if (i==125)
                expected = 123;
            encountered = pop();
            if (expected != encountered)
                closing = i;
                pass = false;
        }
    }
    if (pass)
        cout << "Parentheses match OK!\n";
    else
        cout << encountered << " has opened, but closing " << closing;
        cout << " encountered!\nParentheses do not match\n";
}

int main(int argc, char * argv[])
{
    init();
    match(argv[1]);
    return 0;
}

Since the stack framework existed from a previous exercise and worked fine there, I’m strongly assuming whatever error there is should be in void match

Answer 1

In your match function you should test characters, not their indices:

void match(char expr[])
{
...
    for (int i=0; pass && (i<strlen(expr)); i++)
    {
        char c = expr[i]; // use this c instead of i below!!!!!!!
        if ((c==40)||(c==91)||(c==123))
            push(c);
        else 
        {
            if (c==41)
                expected = 40;
            if (c==93)
                expected = 91;
            if (c==125)
...

Also you did not update counter in your push:

void push(int element)
{
...
   ++numElems; // Was missing!!!!
}