Hi, I've been testing about converting infix to postfix expression. I've made the code and it compiles. My problem is it stops working when I run it. This involves the use of stack and arrays. I just call the method polish to get the strings infix and postfix, and to run the other methods, like findRank to know if it is a well formed postfix exp'n and evaluate it in method evaluate(). Please give me some ideas on how to fix this. This is the code...

/* From Infix Notation to Postfix form, 
Knowing if it is Well-Formed, 
and Evaluation of its Postfix Expression
*/

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

typedef char StackElemType;
typedef struct stacknode StackNode;

struct stacknode {
    StackElemType info;
    StackNode *link;
};
struct stack {
    StackNode *top;
};
typedef struct stack Stack;

void initStack(Stack *S) {
    S->top = NULL;
}

int isEmptyStack(Stack *S) {
    return (S->top == NULL);
}

void StackOverflow(void) {
    printf("Stack overflow detected.\n");
    exit(1);
}

void StackUnderflow(void) {
    printf("Stack underflow detected.\n");
    exit(1);
}

void push(Stack *S, StackElemType *x) {
    StackNode *alpha;
    alpha = (StackNode *) malloc(sizeof(StackNode));
    if(alpha == NULL)
        StackOverflow();
    else {
        alpha->info = *x;
        alpha->link = S->top;
        S->top = alpha;
    }
}

void pop(Stack *S, StackElemType *x) {
    StackNode *alpha;
    if(S->top == NULL)
        StackUnderflow();
    else {
        alpha = S->top;
        *x = S->top->info;
        S->top = S->top->link;
        free(alpha);
    }
}

int ICP(char x) {
    if((x == '+') || (x == '-'))
        return 1;
    else if((x == '*') || (x == '/'))
        return 3;
    else if(x == '^')
        return 6;
}

int ISP(char x) {
    if((x == '+') || (x == '-'))
        return 2;
    else if((x == '*') || (x == '/'))
        return 4;
    else if(x == '^')
        return 5;
    else 
        return 0;
}

int rank(char x) {
    if ((isalpha(x)) || (isdigit(x)))
        return 1;
    else
        return -1;
}

int IsOpnd(char x) {
    if ((isalpha(x)) || (isdigit(x)));
        return 1;
    return 0;
}

double operate(double first, double sec, char opnd) {
    int i = 0;
    if (opnd == '+') 
        return (first + sec);
    else if (opnd == '-')
        return (first - sec);
    else if (opnd == '*')
        return (first * sec);
    else if (opnd == '/')
        return (first / sec);
    else if (opnd == '^') {
        for(i = 1; i < sec; i++) {
            first *=  first; }
        return first;
    }
}

double value (char var) {
    double num = var - '0';
    if(num > 16.0 && num < 43.0)
        return (num - 16.0);
    else if(num > 48.0 && num < 75.0) 
        return ((49.0 - num) - 1.0); 
    else 
        return num;
}

//Start of methods for Evaluating the Postfix Expression

typedef double StackEvalType;
typedef struct stackEvalnode StackEvalNode;

struct stackEvalnode {
    StackEvalType info;
    StackEvalNode *link;
};

struct stackEval {
    StackEvalNode *top;
};
typedef struct stackEval StackEval;

void initStackEval(StackEval *S) {
    S-> top = NULL;
}

int isEmptyStackEval(StackEval *S) {
    return (S->top == NULL);
}

void pushEval(StackEval *S, StackEvalType *x) {
    //like in the push()
}

void popEval(StackEval *S, StackEvalType *x) {
    //like in pop
}
//If it is well formed in findRank(), do method evaluate
double evaluate(char postfix[], double answer) {
    Stack T;
    initStack(&T);
    StackEval S;
    initStackEval(&S);
    int i;
    answer = 0.0;
    char x, first, sec;
    for(i =0; i < strlen(postfix); i++) {
        x = postfix[i];
        if(IsOpnd(x)) {
            push(&T, &x);
        }
        else {
            pop(&T, &first);
            pop(&T, &sec);
            answer = operate(value(first), value(sec), x);
            pushEval(&S, &answer);
        }
    }
    popEval(&S, &answer);
    return answer;
}

void findRank(char postfix[], int r) {
    char x;
    r = 0;
    int i;
    double ans;
    for(i = 0; postfix[i] != '\0'; i++) {
        x = postfix[i];
        r += rank(x);
        if (r < 1) {
            printf("Postfix expression is not well-formed. Rank is %d.  \n", r);
            break;
        }
    }
    if(r == 1) {
        printf("Postfix expression is well-formed. Total rank is %d. \n", r);
        printf("Evaluated value is %f", evaluate(postfix, ans));
    }
}

//postfix[] needs size so that there are no gibberish things in its value
void polish(char infix[], char postfix[], int r) {
    Stack S;
    initStack(&S);
    postfix = "";
    int i;
    char x, xtop;
    for(i =0; i< strlen(infix); i++) {
        x = infix[i];

        if(IsOpnd(x)) { 
            postfix[strlen(postfix)] = x;
            postfix[strlen(postfix) + 1] = 0;
            }
        else if(x == '(')
            push(&S, &x);
        else if(x == ')') {
            while(!isEmptyStack(&S)) { 
                pop(&S, &xtop);
                if(xtop != '(') { //concatenate
                    postfix[strlen(postfix)] = xtop;
                    postfix[strlen(postfix) + 1] = 0;
                }
                else if(isEmptyStack(&S)) {
                    printf("The postfix expression is %s", postfix);
                    findRank(postfix, r);
                }
            }
        }
        else {
            while(!isEmptyStack(&S)) {
                pop(&S, &xtop);
                if(ICP(x) < ISP(xtop)){ //concatenate
                    postfix[strlen(postfix)] = xtop;
                    postfix[strlen(postfix) + 1] = 0;
                }
                else {
                    push(&S, &xtop);
                    push(&S, &x);
                }
            }
        }
    }
    r = 0;
    //End of polish
}
int main() {
    char infix [200] = "((A+B)^2)";
    char postfix[200];
    int r = 0;
    polish(infix, postfix, r);

    return 0;
}

(1) Use your compiler's debugger so that you can step through the execution of the program and see where it stops.

(2) or scatter some print statements around.

how can I use the debugger of a gcc compiler? thanks

The debugger is called gdb. A quick Google search for "gdb tutorial" will get you started.