added calculator

calculator/README.md

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+# Simple calculator application
+This application asks the user to enter an operator and to two operands. Example:
+```
+> Enter operator: +
+> Enter operand 1: 67
+> Enter operand 2: 8
+```
+It then preceeds to print the result of the calculation:
+```
+The Result is: 75
+```

calculator/main.c

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+#include <stdio.h>
+#include <math.h>
+#include <stdlib.h>
+
+// ansi escape codes for colors
+#define GREEN "\033[32m"
+#define RED "\033[31m"
+#define RESET "\033[0m"
+
+int main() {
+    do {
+        // allocate memory
+        double a;
+        double b;
+        char op;
+
+        printf(GREEN"Enter operator: "RESET);
+        scanf(" %c", &op); // read character and ignore whitespace
+        // read operands
+        printf(GREEN"Enter operand 1: "RESET);
+        scanf("%lf", &a);
+        printf(GREEN"Enter operand 2: "RESET);
+        scanf("%lf", &b);
+
+        // result is by default not a number
+        double res = NAN;
+        
+        switch (op)
+        {
+        case '+':
+            res = a + b;
+            break;
+        case '-':
+            res = a - b;
+            break;
+        case '*':
+            res = a * b;
+            break;
+        case '/':
+            res = a / b;
+            break;
+        
+        default:
+            printf(RED"invalid operation specified\n"RESET);
+            break;
+        }
+
+        if (!isnan(res))
+        {
+            printf("\nTHE RESULT IS:\n");
+
+            char buf[64] = {0};
+            sprintf(buf, "figlet %lf | lolcat", res);
+            system(buf);
+        }
+
+    } while(1);
+
+    return 0;
+}