Add postfix evaluation

evaluate_postfix.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+typedef struct Node {
+    char data;
+    struct Node* before;
+} Node;
+
+Node* push(Node* node, char data) {
+    Node* new = (Node*)malloc(sizeof(Node));
+    new->data = data;
+    new->before = node;
+    
+    return new;
+}
+
+Node* pop(Node* node) {
+    if(node == NULL) {
+        return node;
+    }
+    Node* ptr = node;
+    node = node->before;
+    free(ptr);
+    return node;
+}
+
+float eval(char a, char b, char operator) {
+    int operand1, operand2;
+    operand1 = ((int)a)-48;
+    operand2 = ((int)b)-48;
+
+    if(operator == '+') return operand1+operand2;
+    else if(operator == '-') return operand1-operand2;
+    else if(operator == '/') return operand1/operand2;
+    else if(operator == '*') return operand1*operand2;
+    
+}
+
+int main() {
+    char exp[1000];
+    printf("Enter a postfix expression: ");
+    gets(exp);
+    puts(exp);
+    exp[strlen(exp)] = ')';
+    exp[strlen(exp)+1] = 0;
+    Node* stack = NULL;
+
+    float hasil = 0;
+    
+    int idx = 0;
+    while(exp[idx] != ')') {
+        char now = exp[idx++];
+        if((int)now <= 57 && (int)now >=48) {
+            stack = push(stack, now);
+        }
+        else {
+            if(now == ' ') continue;
+            // now adalah oeprator
+            char a = stack->data;
+            stack = pop(stack);
+            char b = stack->data;
+            stack = pop(stack);
+            hasil = eval(b, a, now);
+            stack = push(stack, hasil+48);
+        }
+    }
+
+    printf("%.2f\n", hasil);
+
+
+
+
+    return 0;
+}
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