c - Declaringdefining an unused variable changes the output from an unrelated variable - Stack Overflow

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Scenario

I have an odd bug where when I add a random variable to the code it changes the output. I figured out what was causing the issue, however I am not entirely sure why it is causing the issue.

The code is intended to validate credit card numbers using the Luhns Algorithm. It is part of the edX CS50 course, however I decided to try it without their custom-built libraries in an attempt to learn as much as possible.

The issue I am encountering is, if I run the code below as-is, then the answer comes out wrong (i.e., a '9' appended to the integer output). Now if I uncomment either int random_integer = 0; or char random_character = 'M';, then the output works entirely as anticipated.

• Wrong output (leaving those random variables commented): 209
• Correct output (uncommenting one (or both) of those random variables): 20

Oddly enough, I noticed that if I change char sum_to_str[3]; to char sum_to_str[10];, the problem goes away entirely.

Please let me know your thoughts as I am newer to C and am interested in understanding the nuances.

Code

#include <stdio.h> // For Standard Input/Output
#include <stdlib.h> // For the atoi() function
#include <string.h> // For the strchr() and strlen() functions

// This value is the length plus one
#define MAX_INPUT_LENGTH 255

int main(void) {
    char user_input[MAX_INPUT_LENGTH];
    char *p;

    // Output CLI instructions
    printf("Welcome to the Credit Card Validator!!\n");
    printf("INSTRUCTIONS: At the prompt, please provide a CC number.\n");
    printf("NOTES ON LENGTH: Visa -> 13 || 16, AmEx -> 15 and MC -> 16\n");

    // Algorithm
    char example_card_num[] = "4003600000000014";

    // Check the length
    int card_num_length = strlen(example_card_num);

    int skip_flag = 0;
    int sum_of_values = 0;
    char value_at_index;
    char str_of_evens[20];
    for (int i = card_num_length - 1; i >= 0; i--) {
        char sum_to_str[3];
        switch (skip_flag) {
            case 0:
                // Add 'odd' values together
                value_at_index = example_card_num[i];
                sum_of_values = sum_of_values + atoi(&value_at_index);

                // Toggle flag
                skip_flag = 1;
                break;
            case 1:
                // Add 'even' values together (with multiplier)
                value_at_index = example_card_num[i];

                // 1. Convert each str to int
                // 2. Multiply by two
                // 3. Convert back to str in new variable
                sprintf(sum_to_str, "%d", (atoi(&value_at_index) *2));

                // Concatenate each substring to a new string
                strcat(str_of_evens, sum_to_str);
                 
                // Toggle flag
                skip_flag = 0;
                break;
        }
    }

    // int random_integer = 0;
    // char random_character = 'M';

    char value_at_index_two;
    for (int i = 0; i < strlen(str_of_evens); i++) {
        value_at_index_two = str_of_evens[i];
        sum_of_values = sum_of_values + atoi(&value_at_index_two);
    }

    printf("~~~~~~~~~~~\n");
    printf("Sum of Values 01: %d\n", sum_of_values);

    // Terminate the program
    return 0;
}

I tried the following:

• Commenting/uncommenting the int random_integer = 0; then recompiling
• Adding the char random_character = 'M'; then toggling it as a comment and recompiling
• Modified the char sum_to_str[3]; to char sum_to_str[3];

I also tried to change the input (i.e., example_card_num[]) slightly to determine if there was something throwing an arbitrary "9" or if the appended number changed, which it did.

Scenario

I have an odd bug where when I add a random variable to the code it changes the output. I figured out what was causing the issue, however I am not entirely sure why it is causing the issue.

The code is intended to validate credit card numbers using the Luhns Algorithm. It is part of the edX CS50 course, however I decided to try it without their custom-built libraries in an attempt to learn as much as possible.

The issue I am encountering is, if I run the code below as-is, then the answer comes out wrong (i.e., a '9' appended to the integer output). Now if I uncomment either int random_integer = 0; or char random_character = 'M';, then the output works entirely as anticipated.

• Wrong output (leaving those random variables commented): 209
• Correct output (uncommenting one (or both) of those random variables): 20

Oddly enough, I noticed that if I change char sum_to_str[3]; to char sum_to_str[10];, the problem goes away entirely.

Please let me know your thoughts as I am newer to C and am interested in understanding the nuances.

Code

#include <stdio.h> // For Standard Input/Output
#include <stdlib.h> // For the atoi() function
#include <string.h> // For the strchr() and strlen() functions

// This value is the length plus one
#define MAX_INPUT_LENGTH 255

int main(void) {
    char user_input[MAX_INPUT_LENGTH];
    char *p;

    // Output CLI instructions
    printf("Welcome to the Credit Card Validator!!\n");
    printf("INSTRUCTIONS: At the prompt, please provide a CC number.\n");
    printf("NOTES ON LENGTH: Visa -> 13 || 16, AmEx -> 15 and MC -> 16\n");

    // Algorithm
    char example_card_num[] = "4003600000000014";

    // Check the length
    int card_num_length = strlen(example_card_num);

    int skip_flag = 0;
    int sum_of_values = 0;
    char value_at_index;
    char str_of_evens[20];
    for (int i = card_num_length - 1; i >= 0; i--) {
        char sum_to_str[3];
        switch (skip_flag) {
            case 0:
                // Add 'odd' values together
                value_at_index = example_card_num[i];
                sum_of_values = sum_of_values + atoi(&value_at_index);

                // Toggle flag
                skip_flag = 1;
                break;
            case 1:
                // Add 'even' values together (with multiplier)
                value_at_index = example_card_num[i];

                // 1. Convert each str to int
                // 2. Multiply by two
                // 3. Convert back to str in new variable
                sprintf(sum_to_str, "%d", (atoi(&value_at_index) *2));

                // Concatenate each substring to a new string
                strcat(str_of_evens, sum_to_str);
                 
                // Toggle flag
                skip_flag = 0;
                break;
        }
    }

    // int random_integer = 0;
    // char random_character = 'M';

    char value_at_index_two;
    for (int i = 0; i < strlen(str_of_evens); i++) {
        value_at_index_two = str_of_evens[i];
        sum_of_values = sum_of_values + atoi(&value_at_index_two);
    }

    printf("~~~~~~~~~~~\n");
    printf("Sum of Values 01: %d\n", sum_of_values);

    // Terminate the program
    return 0;
}

I tried the following:

• Commenting/uncommenting the int random_integer = 0; then recompiling
• Adding the char random_character = 'M'; then toggling it as a comment and recompiling
• Modified the char sum_to_str[3]; to char sum_to_str[3];

I also tried to change the input (i.e., example_card_num[]) slightly to determine if there was something throwing an arbitrary "9" or if the appended number changed, which it did.

• c
• clang

Share Improve this question Follow edited Mar 8 at 18:51 Uwe Keim 40.8k6161 gold badges190190 silver badges304304 bronze badges asked Mar 8 at 18:29 Mike FilesMike Files 3311 silver badge44 bronze badges 2

• 3 "when I add a random variable to the code it changes the output" -- This is a classic symptom of undefined behavior. One obvious problem: atoi takes a pointer to a string as input, not a pointer to char. – ad absurdum Commented Mar 8 at 18:42
• 1 chars denoting digits happens to be in sequence, so '0' + 5 gets you a '5'. And, for the reverse, '5' - '0' gives you the value 5. Now use that on value_at_index. – BoP Commented Mar 8 at 18:57

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1 Answer 1

Sorted by: Reset to default 4

You should not use atoi on a non string array char variable because there is no guarantee of it to be null terminated, so atoi would yield unpredictable results.

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