Trying to printf a char in a specific index in a char Array

Question

The program works i allocate and reallocate data properly.

I wanted to break if c== -1, so i tried to compare c to "-" and data[i-1] == '1' but i got an error. So i tried to just print it to see what comes up.

   for (i=0;;i++) {
        c=getchar(); /* put input character into c */
        if (c== '1'  ) {               // need to find a way to change it to -1
            printf("n-1 is %c",data[i]); // ask
            break;
            //&& (data[i-1] == '-')
        }

The output is

n-1 is ?

*The ? is upside down

Any ideas. Thank you.

Edit:

char *getInput()
{

    char *data,*temp;
    data= malloc(sizeof(char));
    char c; /* c is the current character */
    int i; /* i is the counter */
    printf ("\n Enter chars and to finish push new line:\n");
    for (i=0;;i++) {
        c=getchar(); /* put input character into c */
        if (c== '1'  ) {               // need to find a way to change it to -1
            printf("n-1 is %c",data[i]); // ask
            break;
            //&& (data[i-1] == '-')
        }
        data[i]=c; /* put the character into the data array */
        temp= realloc(data,(i+1)*sizeof(char)); /* give the pointer some memory */
        if ( temp != NULL ) {
            data=temp;
        } else {
            free(data);
            printf("Error allocating memory!\n");
            return 0 ;
        }
    }

You don't assign data[i] in the code shown. What were you expecting to have printed out? — simonc
– simonc, Commented Jan 4, 2014 at 13:04
How is memory for data allocated? dynamically or statically? Can you show where you take inputs to data ? — thilinarmtb
– thilinarmtb, Commented Jan 4, 2014 at 13:06
what are you trying to achieve? Try detecting the '-' char and then raise a flag to detect a trailing 1 and halt, but this is poor design from my experience. — Reut Sharabani
– Reut Sharabani, Commented Jan 4, 2014 at 13:19

Jongware · Accepted Answer · 2014-01-04 13:39:16Z

1

If I understand it correctly, you want to test the specific input "-1" in your input loop. Then you need to test two characters, not only 1. To prevent data underrun, first test if i > 0 -- if you press 1 right away you are still in position #0 and so you are testing some random memory contents before your actual string:

if (i > 0 && c== '1' && data[i-1] == '-')
{
     data[i-1] = 0; /* Terminate string at correct position */
     break;
}

Don't forget to adjust your prompt.

Meta-question: why use '-1' to terminate input, as Enter is a much more logical choice?

answered Jan 4, 2014 at 13:39

Jongware

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vinayawsm · Accepted Answer · 2014-01-04 13:47:19Z

0

You haven't initiated data[] previously. And during execution if the first input user gives is '1' then it need to output some garbage. And that's the garbage, an inverted '?' .

answered Jan 4, 2014 at 13:47

vinayawsm

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Anirban Pal · Accepted Answer · 2014-01-04 13:51:01Z

0

You have not assigned any char to data[i] when executing printf. Use following code

     if (c== '1' && i>0 ) { 
          printf("n-1 is %c",data[i-1]); 
          break;
     }

instead of

      if (c== '1'  ) {               // need to find a way to change it to -1
        printf("n-1 is %c",data[i]); // ask
        break;
        //&& (data[i-1] == '-')
      }

answered Jan 4, 2014 at 13:51

Anirban Pal

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