I am getting a uninitialized local variable error on line 210 and 135 in this program for converting infix into postfix. Can you tell what I'm doing wrong?
/*
1)
infix: A*B+C
postfix: AB*C+
2)
infix: A+B*C
postfix: ABC*+
3)
infix: A*B+C*D
postfix: AB*CD*+
4)
infix: A*B^C+D
postfix: ABC^*D+
5)
infix: A*(B+C*D)+E
postfix: ABCD*+*E+
6)
infix: A+(B*C-(D/E^F)*G)*H
postfix: ABC*DEF^/G*-H*+
7)
infix: (A+B)*C+D/(E+F*G)-H
postfix: AB+C*DEFG*+/+H-
8)
infix: A-B-C*(D+E/F-G)-H
postfix: AB-CDEF/+G-*-H-
*/
#include<stdio.h>
#include<stdlib.h>
//*******************************************
//** STACK
#define size 10
struct stack {
int count;
char stack[size];
} s;
void stack_push(char c) {
if (s.count < size) {
s.stack[s.count] = c;
s.count = s.count + 1;
}
}
char stack_pop() {
char item;
if (s.count > 0) {
s.count = s.count - 1;
item = s.stack[s.count];
}
return item;
}
int stack_isEmpty() {
return s.count == 0;
}
char stack_topChar() {
return s.stack[s.count - 1];
}
//*******************************************
//** Aux operations
int isOperand(char c) {
return c >= 'A' && c <= 'Z';
}
int isOperator(char c) {
char* operators = "+-*/^\0";
int result = 0;
for (int i = 0; operators[i] != '\0'; i++) {
if (operators[i] == c) {
result = 1;
break;
}
}
return result;
}
int getPrecedence(char c) {
int result = 0;
switch (c) {
case '^': result++;
case '/':
case '*': result++;
case '-':
case '+': result++;
}
return result;
}
//*******************************************
//** to Postfix
void toPostfix(char* expression) {
char* result;
int idx = 0;
for (int i = 0; expression[i] != '\0'; i++) {
char c = expression[i];
if (isOperand(c)) {
result[idx++] = c;
}
else if (isOperator(c)) {
char topChar;
while (1) {
topChar = stack_topChar();
if (stack_isEmpty() || topChar == '(') {
stack_push(c);
break;
}
else {
int precedenceC = getPrecedence(c);
int precedenceTC = getPrecedence(topChar);
if (precedenceC > precedenceTC) {
stack_push(c);
break;
}
else {
char cpop = stack_pop();
result[idx++] = cpop;
}
}
}
}
else if (c == '(') {
stack_push(c);
}
else if (c == ')') {
char cpop = stack_pop();
while (cpop != '(') {
result[idx++] = cpop;
cpop = stack_pop();
}
}
}
while (!stack_isEmpty()) {
char c = stack_pop();
result[idx++] = c;
}
result[idx] = '\0';
printf("%s", result);
}
//*******************************************
//** main
int main() {
printf("Insert expression: ");
char* expression;
char c;
int idx = 0;
do {
c = getchar();
if (c == '\n' || c == EOF)
c = '\0';
expression[idx++] = c;
} while (c != '\0');
toPostfix(expression);
return 0;
}
I've tried looking everywhere on Google for solutions and have not found any so I was hoping you would help me here.
The error I am getting can be found here: https://i.sstatic.net/l1bQK.jpg
This:
char *result;
will create an uninitialized pointer that points to an indeterminate location. It does not create a string. Therefore, accessing it with result[idx++] invokes undefined behaviour.
In your cases, where you just want to use the string locally, it is best to create a char buffer of a fixed size:
char result[80];
And you must make sure that you never write beyond the 80 characters, leaving space for the null character at the end of the string.
That's only a quick fix for your immediate problem. You should look into arrays, C strings, pointers and memory allocation to learn more about how these things work in C.