eval.c

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

#define MAX_SIZE 100

typedef struct Stack {
  int arr[MAX_SIZE];
  int top;
} Stack;

Stack* init() {
  Stack *s = (Stack*)malloc(sizeof(Stack));
  s->top = -1;
  return s;
}
void push(Stack *s, int data) {
    s->arr[++(s->top)] = data;
}
int pop(Stack *s) {
    return s->top == -1 ? -1 : s->arr[s->top--];
}
int peak(Stack *s) {
    return s->top == -1 ? -1 : s->arr[s->top];
}

int main() {
  char buff[50];
  scanf("%s", buff);
  Stack *s = init();

  for (size_t i = 0; buff[i] != '\0'; i++) {
    if (isdigit(buff[i])) {
      push(s, buff[i] - '0');
    } else {
      int o2 = pop(s), o1 = pop(s);
      switch(buff[i]) {
        case '+': push(s, o1 + o2); break;
        case '-': push(s, o1 - o2); break;
        case '*': push(s, o1 * o2); break;
        case '/': push(s, o1 / o2); break;
        case '^': push(s, pow(o1, o2)); break;

        default: return 1;
      }
    }
  }

  printf("Result = %d\n", peak(s));
}

infix-postfix.c

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

#define MAX_SIZE 100

typedef struct Stack {
  char arr[MAX_SIZE];
  int top;
} Stack;

Stack* init() {
  Stack *s = (Stack*)malloc(sizeof(Stack));
  s->top = -1;
  return s;
}
void push(Stack *s, int data) {
    s->arr[++(s->top)] = data;
}
int pop(Stack *s) {
    return s->top == -1 ? -1 : s->arr[s->top--];
}
int peak(Stack *s) {
    return s->top == -1 ? -1 : s->arr[s->top];
}

int priority(char c) {
  switch (c) {
    case '+':
    case '-':
      return 1;
    case '*':
    case '/':
      return 2;
    case '^':
      return 3;

    default:
      return 0;
  }
}

int is_operand(char c) {
  if (
    (c >= 'A' && c <= 'Z') ||
    (c >= 'a' && c <= 'z') ||
    (c >= '0' && c <= '9')
  ) {
    return 1;
  }

  return 0;
}

int main() {
  char buff[50];
  scanf("%49[^\n]s", buff);

  char ans[50];
  size_t j = 0;

  Stack *s = init();
  for (size_t i = 0; buff[i] != '\0'; i++) {
    if (is_operand(buff[i])) {
      ans[j++] = buff[i];
    } else if (buff[i] == ')') {
      while (s->top != -1 && peak(s) != '(') {
        ans[j++] = peak(s);
        pop(s);
      }
        
      pop(s);
    } else if (buff[i] == '(') {
      push(s, '(');
    } else {
      while (s->top != -1 && peak(s) != '(' && priority(peak(s)) >= priority(buff[i])) {
          ans[j++] = peak(s);
          pop(s);
      }
      push(s, buff[i]);
    }
  }

  while (s->top != -1) {
    ans[j++] = peak(s);
    pop(s);
  }

  ans[j] = '\0';
  printf("%s\n", ans);
}