problem with interfacing keypad

Hey everoone! I'm using a lpc2138, And I wanna make a mini-project in this way that I have a keypad on port1(rows on P1.16 to P1.19)(columns on P1.20 to P1.23) and a lcd on port0, I want it that Once I click on a buttom of the keypad, the LCD shows the number of buttom (That Simple!).

The Lcd works, but I don't know what the problem is about the keypad! Actually I guess there is a problem in the subrootin that I have made for keypad.

It will be a pleasure if you tell me what the problem is!
thank you

Here is my code:

#include <lpc21xx.h>
#include "lcd4bit.h"
//////////////////////
int keypad(void)
{ int codes[4][4]={{1,2,3,4},{5,6,7,8},{9,10,11,12},{13,14,15,16}};
int i,j,a=20;
IODIR1=0XF<<16;
for(i=0;i<4;i++)
{ IOSET1=0XF<<16;
IOCLR1=1<<(16+i);
delay(2);
if(IOPIN1 & 0XF<<20 != 0XF<<20 )
{

for(j=0;j<4;j++)
{ if(IOPIN1 & (1<<(j+20))==0 ) a=codes[i][j];
} }
else
a=20;

}
return a;
} ///////////////////////////////
int main(void)
{ int i,k=20;
char s[20];
IODIR0=0XFFF;
lcd_init();
sprintf(s,"NUMBER");
lcd_print(s);
while(1)
{ while(k==20)
k=keypad();
if(k!=20)
{ sprintf(s,"I=%d",k);
lcd_gotoxy(1,0);
lcd_print(s);
} }
}