I'm just trying to scan strings into an array. What am I doing wrong?

Question

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

int main(void) {

   const int NUM_VALS = 20;
   int i;
   int actualInput;
   char userString[actualInput][NUM_VALS];
   int matchCount = 0;

   scanf("%d", &actualInput);

   for (i = 0; i < actualInput; ++i) {
      scanf("%s", userString[i]);
      printf("%s", userString[i]);
   }



   return 0;
}

Output:

b'hellohi\x80\x07@\xd2\x05@\x9a\x16[\xea\xccp\xa6\x15\xf6\x18+\xbf\x87\x8a#\x14)\x05@\xfe\x7f'b'\x92\x1fk\xb3\xfe\x7f\xfe\x7f\x118\x08\xe8\x03\x0eY\x03k\xb3\xfe\x7f\xfe\x7f\xb2Y{\xe8C}8\r\x8b-u{\x8cx86_64'F-8sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin/usr/sbin:/usr/bin:/sbin:/binsbin:/binTF-88tf8RELOAD=/usr/lib/x86_64-linux-gnu/coreutils/libstdbuf.so64-linux-gnu/coreutils/libstdbuf.sols/libstdbuf.soout

I've tried some variations replacing userString[i] with userString in the scanf function. The result is outputting 50,000 inputs of my last string. I don't understand what's happening.

Answer 1

The problem is this sequence of code:

int actualInput;
char userString[actualInput][NUM_VALS];
int matchCount = 0; 

scanf("%d", &actualInput);

The first line declares a variable called actualInput but doesn't assign a value to that variable.

The second line declares a variable length array (VLA) using the value in actualInput. Using the value of an uninitialized variable results in undefined behavior, which basically means that after that point in the code, anything can happen. What's likely happening (based on your description of the problem) is that actualInput is either zero, or a small number, so you get an array that's too small to hold your input.

The last line (with the scanf) finally assigns a value to actualInput. You may be thinking that the array will resize itself when actualInput is changed. That definitely does not happen. In C, after a VLA is created, its size cannot be changed.

The solution is simple, rearrange the code so that things are done in the proper order:

int actualInput;
scanf("%d", &actualInput);
char userString[actualInput][NUM_VALS];

int matchCount = 0;

As a side note, you should really do some error checking to make sure that the user inputs a reasonable number, before using that number to create an array. For example

int actualInput;
if (scanf("%d", &actualInput) != 1 || actualInput < 1 || actualInput > 1000)
{
    printf("That is not a valid array size\n");
    return 1;
}
char userString[actualInput][NUM_VALS];

Answer 2

you cant declare it as a 2D array then treat it as a normal array . each case should include only one letter but it can't be done automatically , I suggest you add this :

for (i = 0; i < actualInput; ++i) 
{
    gets(stri);
    for (k=0;k<strlen(stri);k++)
    userString[i][j]=stri[j];
}