Program doesn't end after parsing "exit"

Question

I am new to C and I am trying to learn, so go easy on me :D

I am currently trying to make a simple bank account console application. I got the following code:

#include <stdio.h>
#include <string.h>

void parseCommand(char cmd[15], bool end) {
    if(cmd != NULL) {
        if(strstr(cmd, "listaccounts") != NULL) {

        } else if(strstr(cmd, "addaccount") != NULL) {

        } else if(strstr(cmd, "removeaccount") != NULL) {

        } else if(strstr(cmd, "withdraw") != NULL) {

        } else if(strstr(cmd, "deposit") != NULL) {

        } else if (strstr(cmd, "exit") != NULL) {
            end = true;
        } else {
            printf("Unknown Command: %s\n", cmd);
        }
    } else {
        printf("cmd is null");
    }
}

int main() {
    printf("Welcome to the Account Management System\n");
    printf("Please use one of the following commands:\n");
    bool end = false;
    while(true) {
        printf("\tlistaccounts\n");
        printf("\taddaccount\n");
        printf("\tremoveaccount\n");
        printf("\twithdraw\n");
        printf("\tdeposit\n");
        printf("\texit\n");
        char cmd[15];
        fgets(cmd,15,stdin);
        parseCommand(cmd, end);
        if(end == true) {
            printf("Shutting down...");
            break;
        }
    }

    return 0;
}

But when I type "exit" the program just starts over in the while loop and asks for new input. What am I doing wrong? My guess would be how I try to compare two strings.

You never actually set end. You pass it to a function, put this does not change its value unless it is a pointer. I would suggest changing the function from a void to a boolean here, then when you return, just check that value. — Hashman, Nov 21 '14 at 12:36

score 3 · Answer 1 · answered Nov 21 '14 at 12:37

3

In C, the variables are passed by value. So whatever value end will have in your parseCommand method, the outer function will not see it. An easy way to fix it is to make your function to return end:

bool parseCommand(char cmd[15], bool end) {
///
  return end;
}


// in main:

end = parseCommand(cmd, end);

answered Nov 21 '14 at 12:37

Ashalynd

12,363
2
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Oh yeah, it does. Thanks, but I changed it so that I pass a pointer to the bool instead. – OmniOwl Nov 21 '14 at 12:43

score 2 · Answer 2 · answered Nov 21 '14 at 12:36

2

You're trying to modify the value of a variable declared outside the function. You need a pointer to it.

answered Nov 21 '14 at 12:36

oopcode

1,912
16
26

Sourav Ghosh · Answer 3 · 2014-11-21T12:45:50.077

2

To compile, first add #include <stdbool.h>.

In your code, you're passing end using pass-by-value. This will create a local copy of end inside parseCommand() scope. Whatever it done with end value inside parseCommand() code will not be reflected to main().

change parseCommand(cmd, end); to parseCommand(cmd, &end);
change void parseCommand(char cmd[15], bool end) to void parseCommand(char cmd[15], bool *end)
change end = true; to *end = true;

Also, if(cmd != NULL) should be changed to if ((cmd != NULL) && (end != NULL))

edited Nov 21 '14 at 12:45

answered Nov 21 '14 at 12:38

Sourav Ghosh

133,132
16
183
261

@ANBU.SANKAR sorry i did not get you. – Sourav Ghosh Nov 21 '14 at 12:40
Why do I need to import stdbool.h first when the code compiles fine? – OmniOwl Nov 21 '14 at 12:48
@vipar `import` and `c` ? – Sourav Ghosh Nov 21 '14 at 12:53
You know what I mean. Why include stdbool.h when the code compiles? – OmniOwl Nov 21 '14 at 13:01
@Vipar maybe [this](http://stackoverflow.com/a/1608350/2173917) will end the confusion. – Sourav Ghosh Nov 21 '14 at 13:08
it doesn't really explain why I need to include it when the program runs fine without it. – OmniOwl Nov 21 '14 at 13:14
@Vipar well, it **does** explain. For me, it won't compile. with `gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)`, it throws error for undeclared `bool`, and as per the `c` tag, i'm suppossed to compile this gith `gcc`, not `g++`, right? [assuming you're on linux] – Sourav Ghosh Nov 21 '14 at 13:22

score 2 · Accepted Answer · answered Nov 21 '14 at 12:40

If you want 'parseCommand' to change the variable from the main function, you should pass a pointer of it.

void parseCommand(char cmd[15], bool * end) {
if(cmd != NULL) {
    if(strstr(cmd, "listaccounts") != NULL) {

    } else if(strstr(cmd, "addaccount") != NULL) {

    } else if(strstr(cmd, "removeaccount") != NULL) {

    } else if(strstr(cmd, "withdraw") != NULL) {

    } else if(strstr(cmd, "deposit") != NULL) {

    } else if (strstr(cmd, "exit") != NULL) {
        *end = true;
    } else {
        printf("Unknown Command: %s\n", cmd);
    }
} else {
    printf("cmd is null");
}
}

int main() {
    printf("Welcome to the Account Management System\n");
    printf("Please use one of the following commands:\n");
    bool end = false;
    while(true) {
        printf("\tlistaccounts\n");
        printf("\taddaccount\n");
        printf("\tremoveaccount\n");
        printf("\twithdraw\n");
        printf("\tdeposit\n");
        printf("\texit\n");
        char cmd[15];
        fgets(cmd,15,stdin);
        parseCommand(cmd, &end);
        if(end == true) {
            printf("Shutting down...");
            break;
        }
    }

    return 0;
}

That's the first time I've had to use a pointer. But given all the other answers to this question, I see now why I have to. Thank you for pointing it out! — OmniOwl, Nov 21 '14 at 12:43

score 1 · Answer 5 · answered Nov 21 '14 at 13:42

1

When you pass say void parseCommand(char cmd[15], bool end), you are creating duplicate variables local to the scope of the function (you may want to look up scope). Thus bool end becomes a duplicate local to the function, and when the function finishes, it cannot be accessed again. You could use a pointer and make bool end into bool *end, or you could have the function return a bool, by saying bool parseCommand(char cmd[15], bool end)

answered Nov 21 '14 at 13:42

Gophyr

406
2
11

I know what scope is, but I forgot that C passes by value :) – OmniOwl Nov 21 '14 at 13:44
I was going to up-vote you for a right answer, but don't ask for up-votes dude :P – OmniOwl Nov 21 '14 at 13:51

Program doesn't end after parsing "exit"

5 Answers5