Difference between %c and %s in scanf()

[akk003]

Hello

I'am a returning coder to C programming language and would appreciate if you could help me understand why does my while loop behave normally in the following code:

int loop = 1;
char userIN;
while(loop)
{
printf("\nIf you want to retrieve tuples prompt y/n: \t");
//items_read = scanf("%s",&userIN);
scanf("%s",&userIN);
if(userIN=='n'||userIN=='N')
{
loop = 0;
printf("\n user prompt: %c \n",userIN);
break;
}
else if((userIN=='y')||(userIN=='Y'))
{
loop = 1;
printf("\n user prompt: %c \n",userIN);
}
else
{
printf("Invalid Response, exiting:\t");
printf("\n user prompt: %c \n",userIN);
break;
} 
}

and why does it break in this code(only after the first prompt):

int loop = 1;
char userIN;
while(loop)
{
printf("\nIf you want to retrieve tuples prompt y/n: \t");
//items_read = scanf("%s",&userIN);
scanf("%c",&userIN);
if(userIN=='n'||userIN=='N')
{
loop = 0;
printf("\n user prompt: %c \n",userIN);
break;
}
else if((userIN=='y')||(userIN=='Y'))
{
loop = 1;
printf("\n user prompt: %c \n",userIN);
}
else
{
printf("Invalid Response, exiting:\t");
printf("\n user prompt: %c \n",userIN);
break;
} 
}
 

The only difference between the two codes is in the scanf().

Thanks!

[gpraghuram]

If you try to read a string with a character then what happens is array overwrite.
Since you are passing the character variables address you wont even get a compiler error for that.

Raghuram

[romcab]

%c - accepts a character
%s - accepts a string

Hope this helps.

Hello

I'am a returning coder to C programming language and would appreciate if you could help me understand why does my while loop behave normally in the following code:

int loop = 1;
char userIN;
while(loop)
{
printf("\nIf you want to retrieve tuples prompt y/n: \t");
//items_read = scanf("%s",&userIN);
scanf("%s",&userIN);
if(userIN=='n'||userIN=='N')
{
loop = 0;
printf("\n user prompt: %c \n",userIN);
break;
}
else if((userIN=='y')||(userIN=='Y'))
{
loop = 1;
printf("\n user prompt: %c \n",userIN);
}
else
{
printf("Invalid Response, exiting:\t");
printf("\n user prompt: %c \n",userIN);
break;
} 
}

and why does it break in this code(only after the first prompt):

int loop = 1;
char userIN;
while(loop)
{
printf("\nIf you want to retrieve tuples prompt y/n: \t");
//items_read = scanf("%s",&userIN);
scanf("%c",&userIN);
if(userIN=='n'||userIN=='N')
{
loop = 0;
printf("\n user prompt: %c \n",userIN);
break;
}
else if((userIN=='y')||(userIN=='Y'))
{
loop = 1;
printf("\n user prompt: %c \n",userIN);
}
else
{
printf("Invalid Response, exiting:\t");
printf("\n user prompt: %c \n",userIN);
break;
} 
}
 

The only difference between the two codes is in the scanf().

Thanks!

[akk003]

If you try to read a string with a character then what happens is array overwrite.
Since you are passing the character variables address you wont even get a compiler error for that.

Raghuram

Thankyou for your prompt reply. But userIN happens to be a char variable and it isn't an array.

[Parul Bagadia]

Even i had got the same problem ; when i had used do-while loop;
actually what happens is; many times when we say %c; a value is taken from the buffer; any garbagde value which is already stored there.
Where as a new value is accepted in case of %s..

[akk003]

thankyou all for the quick responses !!!
I now understand the problem which Parul just explained.

[worldwidegeneration]

Hey, you could try using gets() for a string or getch() for a character, and getche() to echo the reply back to the user(so they see what they typed in). Also, for those functions you might need to

 #include <conio.h> 

i tend to only use scanf() when reading in number data.

goodluck