Create a function that takes in an enum variable and returns a string pointer

Question

I need to create a function that takes in an enum variable representing the day of the week as a parameter and returns a string pointer with the name of the day.

This is my code:

char *week(enum DAY x);

enum DAY {Monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday};

void main()
{
char *date;
date = week(Monday);

printf("%s",date);
}


char *week(enum DAY x) {

char date[20];

switch (x)
{
case Monday:
    strcpy(date,"Monday");
    break;
case Tuesday:
    strcpy(date,"Tuesday");
    break;
case Wednesday:
    strcpy(date,"Wednesday");
    break;
case Thursday:
    strcpy(date,"Thursday");
    break;
case Friday:
    strcpy(date,"Friday");
    break;
case Saturday:
    strcpy(date,"Friday");
    break;
case Sunday:
    strcpy(date,"Friday");
    break;
}
return date;
}

When I run it,the console prints symbols. Assistance much appreciated.

Also, is it possible to have an enum variable as a user input instead of directly through the compiler ?

Way more work than needed. Create a global array of pointers to string literals holding the days, e.g. `const char *days[] = { "Monday", "Tuesday", ... }`, then just `return days[x];`. (but note, the date strings cannot be modified. — David C. Rankin, Apr 14 '18 at 18:11

score 3 · Answer 1 · answered Apr 14 '18 at 18:13

3

The basic problem is that your function is returning a character array that is allocated on the stack. This memory gets over-written by other operations. Either allocate the buffer in main and then pass it to the function or simply return the string constants.

In answer to your second question, no. Enums are really just integers with a pile of semantic checking at compile time.

answered Apr 14 '18 at 18:13

DrC

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I added a global char pointer array with the names of the days and changed the switch to return pointers to it instead.It worked just fine.Thank you for your help. – Mohamad Moustafa Apr 14 '18 at 18:28

score 1 · Answer 2 · answered Apr 15 '18 at 16:11

There are some good answers already, but no one's implemented a const char* array[]. As David C. Rankin said in his comment:

Create a global array of pointers to string literals holding the days, e.g. const char *days[] = { "Monday", "Tuesday", ... }, then just return days[x];

I'm surprised this wasn't the "go to" implementation, and I'm curious to hear why from everyone who disagrees. In the mean time, this is what this implementation looks like:

const char* daysOfTheWeek[] = { 
    "Monday", 
    "Tuesday", 
    "Wednesday", 
    "Thursday", 
    "Friday", 
    "Saturday", 
    "Sunday"
};

Implementing a function to return the specified day of the week is then trivial:

void PrintDayOfTheWeek(int n) {
    if ((n < 0) || (n > 6)) {
        // TODO: Handle error here somehow
        // Currently ignores call with invalid parameter

        return;
    }

    printf("Day %d: %s\n", n+1, daysOfTheWeek[n]);
}

To test this, you can iterate through a simple for loop from 0 to 6.

int main()
{
    for (int i = 0; i < 7; ++i) {
        PrintDayOfTheWeek(i);
    }

    return EXIT_SUCCESS;
}

Output:

Day 1: Monday
Day 2: Tuesday
Day 3: Wednesday
Day 4: Thursday
Day 5: Friday
Day 6: Saturday
Day 7: Sunday

Now, in your question, you specifically asked for a function that takes in an enum [value] and returns a string pointer. Implementing this is pretty similar to the previous example. We begin by declaring the enum DAYS:

typedef enum DAYS {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
} Day;

Conveniently, enums assign a value of zero (0) for their first value unless otherwise assigned, so this enum already works as expected with our PrintDayOfTheWeek(int n) function.

So, for example, rather than calling it with an number directly, we can now define a new function that takes in the day we want and returns the string, as you requested. So just pass in on of the names of the week defined in the enum DAYS:

const char* dayOfTheWeek(Day day) {
    return daysOfTheWeek[day];
}

And we can print it like this:

printf("%s\n", dayOfTheWeek(SUNDAY));

Output:

Sunday

Here's something cool, too. Enums are best used in a switch statement, obviously, it is technically possible to iterate over an enum, like this:

for (Day day = MONDAY; day <= SUNDAY; ++day) {
    PrintDayOfTheWeek(day);
}

While enums are obviously not meant to be iterated over, I think that example is pretty cool because I think it helps remember that enums are just integral data types. They also give no additional type safety, which is why enum classes were added in to C++.

"There are some good answers already, but no one's implemented a const char* array[]. As David C. Rankin said in his comment:", my answer do it... — Stargateur, Apr 15 '18 at 16:15
Oh, wow, you're right. When I was browsing through the answers I thought you were using lambdas, which obviously makes no sense in C haha. I like your implementation though, especially because you're array isn't global, nice — Jose Fernando Lopez Fernandez, Apr 15 '18 at 16:17

score 0 · Accepted Answer · answered Apr 14 '18 at 18:21

Your main() function should return an int, date is a local array, use it outside week() function is undefined behavior. You must change your way to return the result with dynamic allocation or with an string with static life time.

As your question is also about good practice, I will implement your feature as follow:

#include <stdio.h>

enum DAY {
    Monday = 0,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
};

char const *week(enum DAY x) {
    return (char const *[]) {
        "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"
    }[x];
}

int main(void)
{
    char const *date = week(Monday);
    printf("%s", date);
}

I added a global char pointer array with the names of the days and changed the switch to return pointers to it instead.Your method is new to me and Ill definitely try it out.Thank you for your help. — Mohamad Moustafa, Apr 14 '18 at 18:29

Create a function that takes in an enum variable and returns a string pointer

3 Answers3