browsing through history commands

Hi everyone,

I want to implent dos like arrow keys for browsing between the previously typed text. What is a good way of doing this, I was thinking of storing them in pointers,

Code:

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char *p[16];

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and when counter is > 16, free and re-allocate 0, and start there again. This should keep 17 commands in memory.

Are there any good algorithms for this? Or can someone show an example of how this could be done?

Best regards.

• OS: MS Windows XP
  Compiler: MS VC++ 6.0

for 17 commands you need array of 17 members, not 16

And 2 indexes - last stored command and fisrt stored command that will be advanced in the cyclic manner.

The rest that you described seems to be OK

Keep an index counter. When the user types something, move it over to p[index]. Then increment index. If it goes beyond the limit, set index to 0. That way you have a circular array, and you're always pointing to the latest entry typed in.

Thanks for the responses,

I am a bit confused, can someone help me out;

Code:

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int main(void) {
        char *p[16];
        int pindex = 0;

        while(1) {
                if(pindex > 15) {
                        pindex = 0;
                }
                p[pindex] = (char *)malloc(sizeof(char)*128);
                fgets(p[pindex], 127, stdin);
                //if(!strcmp(p[pindex], "break")) break;

                if(pindex == 0) {
                        printf("There is no last item\n");
                }
                else {
                        printf("Last thing you typed: %s\n", p[pindex-1]);
                }

                pindex++;
        }

        for(pindex = 0;pindex < 16;pindex++) free(p[pindex]);

        return 0;
}

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How can I put the index to 15 when its a 'second-time' 0. (first time i initialize it to 0, second time it jumps to 0 when 16 is reached)

fgets is inclusive of the null terminator, so its,

Code:

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fgets(p[pindex], 128, stdin);

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Also don't cast malloc see the FAQ.

Not sure what you mean, but

Code:

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if(pindex > 15) {
pindex = 0;
}

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Isn't inclusive of 15, so 16 is a valid index (in-fact its not, its out-of-bounds)

Code:

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if(pindex >= 15)
    pindex = 0;

---

You also don't check if malloc fails (it returns NULL if it fails).

You have a memory leak if the user types more than 16 things. First of all, set every element of p to zero at the start. Then before you malloc(), check to see if p[pindex] is NULL. If it is, only then do you have to malloc().

To adjust pindex back to 15 for the last element, I would recommend an extra variable to calculate the last index.

Here's an altered version of your code:

Code:

---

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
        char *p[16] = { NULL };
        int pindex = 0, plast;
       
        while(1)
        {
                if(!p[pindex])
                {
                        p[pindex] = (char *)malloc(sizeof(char)*128);
                        if(!p[pindex])
                        {
                                printf("Out of memory.\n");
                                break;
                        }
                }
                fgets(p[pindex], 128, stdin);
                /* if(!strcmp(p[pindex], "break")) break; */
                plast = (pindex == 0) ? (15) : (pindex - 1);
                if((pindex == 0) && (!p[15]))
                {
                        printf("There is no last item\n");
                }
                else
                {
                        printf("Last thing you typed: &#37;s\n", p[plast]);
                }
                pindex = (pindex + 1) % 16;
        }
        for(pindex = 0;pindex < 16;pindex++)
        {
                free(p[pindex]);
        }

        return 0;
}

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First of all, thanks again for the responses..

Quote:

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Originally Posted by zacs7

Also don't cast malloc see the FAQ.

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Then, I get an error saying could not convert from void *, I guess its my compiler..

Quote:

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Originally Posted by MacGyver

Here's an altered version of your code:

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Thanks, I think I can sort it out from here, however, I do not understand this part,

Code:

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pindex = (pindex + 1) % 16;

---

what is the purpose of this?

Quote:

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Originally Posted by apsync

Then, I get an error saying could not convert from void *, I guess its my compiler..

---

Remember to include stdlib.h. If that doesn't fix it, you might be compiling this as a C++ program. C++ doesn't allow implicit casts from void *. C does.

Quote:

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Originally Posted by apsync

Thanks, I think I can sort it out from here, however, I do not understand this part,

Code:

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pindex = (pindex + 1) % 16;

---

what is the purpose of this?

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pindex will be raised by one, and then integer divided by 16 and the remainder will be assigned to pindex.

If pindex is 0, pindex + 1 is 1, and then integer divided by 16 is 0, with a remainder of 1. Hence pindex is just raised to one.

The trick is for when pindex is 15. pindex + 1 would be 16. 16 / 16 is 1, with a remainder of 0.

That's how you can get pindex to loop back to 0.

Ah, thank you everything is clear now..

Quote:

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Originally Posted by MacGyver

Remember to include stdlib.h.

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My bad, forgot to include that header.

to avoid checking null pointer on every iteration of the loop - you can allocate memory for all 16 strings before loop, check that all pointers are not null and then - run the loop already without check of the null-pointer...

And free the memory after the work is done - as you do now...