variables value unexpectedly changes? - c

Question

The program below uses a keypad and arduino to test whether an input is equal to the password. Whenever '#' is pressed the input is checked and then the variable storing the input resets to an empty string (char input[257] = ""). The program then loops back to the beginning of the void loop() code block. I am having a problem when my program loops back to the beginning of the "void loop()" code block. When i reset input to an empty string, input is reset to an empty string as it should. However, when the program loops the value of input is changed to what it was before. So if i originally entered "123abc" and hit '#' the program would tell me that the input was incorrect and then the program would reset the variable to an empty string, but when the program loops the variable storing the empty string is changed back to "123abc". What's happening? Why doesn't the variable remain an empty string?
#include <Keypad.h>
const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns
char keys[ROWS][COLS] = {
{'1','2','3','A'},
{'4','5','6','B'},
{'7','8','9','C'},
{'*','0','#','D'}
};
byte rowPins[ROWS] = {5, 4, 3, 2}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {9, 8, 7, 6}; //connect to the column pinouts of the keypad
char password[9] = "3994A", input[257]="";
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );
int x;
void setup(){
Serial.begin(9600);
}
void loop(){
x = 0;
Serial.println(input);
while (1)
{
char key = keypad.getKey();
if (key)
{
if (key == '#')
{
break;
}
input[x] = key;
x+=1;
}
}
if (strcmp(password,input) == 0)
{Serial.println("Access Granted");
}
else
{Serial.println("Access Denied");
}
char input[257] = "";
Serial.println(input);
}

Answer 1

Not the same variable. You've got one input in your top block and another in your loop function.

Answer 2

This line
char input[257] = "";
makes a new local variable called input, but you dont want that. You already made a global one here:
char password[9] = "3994A", input[257]="";
Change
char input[257] = "";
to
memset(input,0,257);
So you dont lose the new variable off the stack and instead use the global one you declared earlier.

Answer 3

The second char input[257] = ""; declares a new variable named input. It does not change the contents of the existing variable named input. Use memset() instead.

Answer 4

I agree with the error in declaring again the input variable
char input[257] = "";
but i would not solve it with memset() but doing
input[0]='\0';
because this is faster and doesn't require to include the memset() function on the code. we are talking about a microcontroller, cpu and memory is precious and it is a good habit to write fast and light code.

Answer 5

Here, how could I achieve this:
Go to the official Arduino Website Download Keypad.h http://playground.arduino.cc/Code/Keypad#Download (ctrl+f and type: download) Download the library to put on C:\Program Files (x86)\Arduino\libraries Close your current Arduino screen then re-open the screen. Rewrite your code and put the code first. I okey with this method.
#include <Keypad.h>