How to store user input in an array without a for loop in c

Question

I connected a display with a keyboard to a Raspberry Pi and when I press a key, it displays the key on the screen. I am using this library and parts of this program.

So I understood that this code passes the key to my keyboardKey function.

 if (reply->object == GENIE_OBJ_KEYBOARD)
      {
        if (reply->index == 0)  // Only one keyboard
          keyboardKey (reply->data) ;

The keyboardKey function then displays the key on the screen and saves the value into the buf array.

void keyboardKey (int key)
{
   char buf[4] ;
   int i ;
   printf("you typed %c\n",key) ;   //shows the typed key in the terminal
   sprintf(buf, "%c",key);         //transforms the key into a string which is required for using genieWrite    
   genieWriteStr (1, buf) ;       //writes the string to the screen. 1 is the index of the text box
   buf[i] = key ;                //stores the key in the array
   printf("%c\n",buf[0]);        //prints the array [0] in the terminal
}

The problem is it only shows one input. Everytime when a key is pressed it gets overwritten and the textbox only shows the last input. But what I want is for example to display a name.

So therefore I have to store each key that is passed to the function into the array without overwriting any of the previous ones.

I know how to store and print user input from the terminal with scanf or getchar and the use for loops to store and print the output, however here I am stuck. I was thinking of using i++ at the end but then what value should i get in the first place?

Anyone an idea or a term I could google for, I'm pretty sure this is a common problem?

Is the buf string used in keyboardKey() function or outside that function? If it is used inside, you can define a static string, and a static int index which initially set to 0. Each time you read a key, you copy that to ith place in the string and move i by 1. If you are useing buf outside the keyboardKey() function, you may need to define the string outside the function, and pass the string as a parament to the function. — Chuanzhen Wu
– Chuanzhen Wu, Commented Oct 31, 2015 at 16:49
You have char buf[4]; and are using it as a string. (e.g. printf("%c\n",buf[0]);) that means you have at most 3 characters you can store in buf (reserving the last, buf[3] for the null-terminating character 0). You can use static int i; causing i to retain it's last value on each function call, enabling you to use buf as a ring-buffer to that extent (e.g. at the end, i++; if (i == 3) i = 0;). Or, you can change the return char keyboardKey (int key) and store each key as it is returned. There are many ways to approach this. — David C. Rankin
– David C. Rankin, Commented Oct 31, 2015 at 16:59

Dimitrios Bouzas · Accepted Answer · 2015-10-31 18:00:33Z

1

Not quite sure what you're trying to accomplish. Anyway, if you want to store your buffer's state in each call of keyboardKey, you could declare it as static like below:

#define BUF_SIZE 4

void keyboardKey(int key) {
  static char buf[BUF_SIZE];
  static int  i = 0;

  if(i > ((int)sizeof(buf) - 2)) {
    i = 0;
    memset(buf, 0, sizeof(buf));
  }

  printf("you typed %c\n",key);
  buf[i] = key; 

  genieWriteStr (1, buf);

  buf[i++] = key;
  printf("%s\n",buf);
}

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edited Oct 31, 2015 at 18:00

answered Oct 31, 2015 at 17:54

Dimitrios Bouzas

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2 Comments

Tom Over a year ago

That works, thank you so much! but can you explain or maybe post a website to a tutorial what does this line mean (int)sizeof(buf) - 2)? Why -2?

Dimitrios Bouzas Over a year ago

@PaulBernhardWagner You have a buffer of N elements (i.e., N = sizeof(buf)), -1 element for the \0 terminating character in order for your buffer to be interpreted as a valid string, you have left with N - 1 usable elements. Now C counts from 0, thus the index i must take values from the sequence {0, 1, 2, ..., N - 2}. Consequently, when the index reaches a value that is greater than N - 2, the buffer along with the associated index must be reset. That's why the -2. Hope that cleared things out for you.

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