Whats wrong with my queue?

**jjohan** · 09-25-2014

Hi all,

I'm trying to write a program that handles a queue. It works when I add an element to the queue (push), but not when I use the command pop(). The program doesn't seem to go into the function. When I write pop, the program just jumps to the next row in the command prompt and seems to be waiting for some new input. If I write pop a second time it looks like its working. Anyone can help me out? Thanks a lot!
Below is the code.

Code:

// A queue system
// Enter push a, for example to put the new char a in queue
// Enter pop to retrieve first element in queue

#include <stdio.h>
#define MAX 200

void push(char c);       // Puts a new element last in queue
char pop(void);            // Gets the first element in queue
int compare_str(char a[], char b[]);

static char s[MAX];
int first = -1;
int last = -1;

int main(void)
{
    char c, cpop, cont = 'y';
    char command[4];
    int result, i;
    
    printf("Enter push character to put a new element in in queue. Enter pop to retrieve firs element in queue:\n");
    
    while(cont == 'y')
    {
        scanf(" %s %c", command, &c);    // space between %s and %c to get rid of spaces
    
    
    
        // Get command
        result = compare_str(command, "push");
        printf("result: %d\n", result);
        if(result == 0)
            push(c);
    
        result = compare_str(command, "pop");
        printf("result: %d\n", result);
        if(result == 0)
        {    
            cpop = pop();
            printf("%c was withdrawn\n", cpop);
        }
        printf("Queue is now: \n");
        
        i = 0;
        while(s[i] != '\0')
        {
            printf("%c ", s[i]);
            i++;
        }
        
        printf("\nContinue? (y/n) ");
        scanf(" %c", &cont);
        printf("\ncontinue: %c\n", cont);
    }
    
        
    return 0;
    
}

int compare_str(char a[], char b[])
{
    int c = 0;
    
    while(a[c] == b[c])
    {
        if(a[c] == '\0' || b[c] == '\0')
            break;
        c++;
    }
    if(a[c] == '\0' && b[c] == '\0')
        return 0;
    else
        return -1;
    
}

void push(char c)
{
    last++;
    s[last] = c;
    
    printf("Inside push()\n");
}

char pop(void)
{
    first++;
    return s[first];
    
    printf("Inside pop()\n");
}

**Salem** · 09-25-2014

> return s[first];
> printf("Inside pop()\n");
It doesn't print anything because the print is AFTER the return statement.

> char command[4];
"push\0" takes 5 chars - which is a buffer overrun.

**jjohan** · 09-25-2014

Hm I changed it now, but still have the same problem.

**jjohan** · 09-25-2014

Here's the new code. Still the same problem with pop. I have to write pop then return, then pop again and return for it to work.

Code:

// A queue system
// Enter push a, for example to put the new char a in queue
// Enter pop to retrieve first element in queue

#include <stdio.h>
#define MAX 200

void push(char c);       // Puts a new element last in queue
char pop(void);            // Gets the first element in queue
int compare_str(char a[], char b[]);

static char s[MAX];
int first = 0;
int last = 0;

int main(void)
{
    char c, cpop, cont = 'y';
    char command[10];
    int result, i;
    
    printf("Enter push character to put a new element in in queue. Enter pop to retrieve firs element in queue:\n");
    
    while(cont == 'y')
    {
        scanf(" %s %c", command, &c);    // space between %s and %c to get rid of spaces
    
    
    
        // Get command
        result = compare_str(command, "push");
        if(result == 0)
            push(c);
        
        result = compare_str(command, "pop");
        if(result == 0)
        {
            cpop = pop();
            printf("%c was withdrawn\n", cpop);
        }
        printf("Queue is now: \n");
        
        i = first;
        while(s[i] != '\0')
        {
            printf("%c ", s[i]);
            i++;
        }
        
        printf("\nContinue? (y/n) ");
        scanf(" %c", &cont);
        printf("\ncontinue: %c\n", cont);
    }
    
        
    return 0;
    
}

int compare_str(char a[], char b[])
{
    int c = 0;
    
    while(a[c] == b[c])
    {
        if(a[c] == '\0' || b[c] == '\0')
            break;
        c++;
    }
    if(a[c] == '\0' && b[c] == '\0')
        return 0;
    else
        return -1;
    
}

void push(char c)
{
    
    s[last] = c;
    last++;
    
    printf("Inside push()\n");
}

char pop(void)
{
    printf("Inside pop()\n");
    return s[first++];
    
    
    
}

**jjohan** · 09-25-2014

Its this line that's causing my problems I figured out:

Code:

scanf(" %s %c", command, &c);    // space between %s and %c to get rid of spaces

I want the program to be able to read in a command + a char OR just a command.

**jjohan** · 09-25-2014

Solved it!

Code:

// A queue system
// Enter push a, for example to put the new char a in queue
// Enter pop to retrieve first element in queue

#include <stdio.h>
#define MAX 200

void push(char c);       // Puts a new element last in queue
char pop(void);            // Gets the first element in queue
int compare_str(char a[], char b[]);

static char s[MAX];
int first = 0;
int last = 0;

int main(void)
{
    char c, cpop, cont = 'y';
    char command[10];
    int result, i;
    
    printf("Enter push character to put a new element in in queue. Enter pop to retrieve firs element in queue:\n");
    
    while(cont == 'y')
    {
        scanf("%s", command);    // space between %s and %c to get rid of spaces
        if((compare_str(command, "push")) == 0)
        {
            scanf(" %c", &c);
            printf("Character: %c\n", c);
        }
        else;
    
    
        //Get command
        result = compare_str(command, "push");
        if(result == 0)
            push(c);
        
        result = compare_str(command, "pop");
        if(result == 0)
        {
            cpop = pop();
            printf("%c was withdrawn\n", cpop);
        }
        printf("Queue is now: \n");
        
        i = first;
        while(s[i] != '\0')
        {
            printf("%c ", s[i]);
            i++;
        }
        
        printf("\nContinue? (y/n) ");
        scanf(" %c", &cont);
        printf("\ncontinue: %c\n", cont);
    }
    
        
    return 0;
    
}

int compare_str(char a[], char b[])
{
    int c = 0;
    
    while(a[c] == b[c])
    {
        if(a[c] == '\0' || b[c] == '\0')
            break;
        c++;
    }
    if(a[c] == '\0' && b[c] == '\0')
        return 0;
    else
        return -1;
    
}

void push(char c)
{
    s[last] = c;
    last++;
}

char pop(void)
{
    return s[first++];    
}

**Matticus** · 09-25-2014

Line 32 - You don't need an empty "else".

**WoodSTokk** · 09-25-2014

Code:

        i = first;
        while(s[i] != '\0')
        {
            printf("%c ", s[i]);
            i++;
        }

Your while-loop run until a terminator is reached, but you have never set a terminator.
Your loop can be like this:

Code:

while(i <= last) …

**jjohan** · 09-25-2014

Thank you!

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