14. Changing code from one language to another

Why convert code?

• A college lab is in C. A hackathon needs Python.
• An old program is in Java. The new app needs JavaScript for a website.
• You know Python well. You want to learn C by comparing the same program.

How to ask

Give the full program. Say the source language and the target language. Say what must stay the same.

Convert this C program to Python. Keep the same logic, the same input format and the same output format. Add a comment where Python does something differently from C.

#include <stdio.h>
int main() {
    int n, fact = 1;
    printf("Enter n: ");
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) fact *= i;
    printf("Factorial = %d\n", fact);
    return 0;
}

The AI gives:

n = int(input("Enter n: "))
fact = 1
for i in range(1, n + 1):   # range stops before n + 1, so this goes 1 to n
    fact *= i
print("Factorial =", fact)

Things that change between languages

Notice the last row about integers. In C, fact for n = 13 overflows (becomes a wrong number) because an int is only 32 bits. In Python, it does not. So the "same" program can give different answers. This is why you must verify.

How to verify the converted program

1. Run the original program with 3 inputs. Write down the outputs.
2. Run the converted program with the same 3 inputs.
3. Compare. They must match.
4. Try an edge case (0, a negative number, a very big number). If the outputs differ, ask the AI why.

The C version prints -215430144 for n = 13 but the Python version prints 6227020800. Why? Which one is correct?

Good conversion prompts

• "Convert this Python to C. Use only standard C libraries."
• "Convert this to Java and put it in a class called Main so it runs directly."
• "Translate this C code to Python, but write it the way an experienced Python programmer would (Pythonic style)."
• "Show the C and Python versions side by side in a table, line by line."