add eval flag

README.md

@@ -10,6 +10,16 @@ Where `cc` is our C compiler e.g. `gcc`, `clang`, `zig cc`.
 
 ## How to Use
 
+### Single Eval
+
+Run the executable with `-e` flag to evaluate a single expression and exit.
+
+```sh
+❯ ./calc -e "3.14**2"
+9.8596
+```
+
+### REPL
 Run the executable `calc` to be greeted with the following prompt:
 
 `calc>`

main.c

@@ -25,11 +25,22 @@
 
 #define INPUT_MAX 1024
 
-int main(void)
+int main(int argc, char **argv)
 {
     char buf[INPUT_MAX];
     calc_FormatKind fmt = CALC_FORMAT_DECIMAL;
 
+    // -e "expr": evaluate a single expression and exit
+    if (argc == 3 && strcmp(argv[1], "-e") == 0) {
+        double result = calc_eval(argv[2]);
+        if (isnan(result)) {
+            fprintf(stderr, "error: invalid expression\n");
+            return 1;
+        }
+        printf("%s\n", calc_format_result(result, fmt));
+        return 0;
+    }
+
     printf("calc> ");
     fflush(stdout);