Thread: would you explain to me? hexadecimal to decimal

The below coding can correctly print out the result. But i do not understand the last part.

i--;
for(;i>=0;i--)
{
h=h+a[i]*m;
m=m*16;
}

how the i-- and m functions?

Code:

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


void hd();

void main()
{
		
		printf("Enter Hexadecimal to Decimal:\n");
		
		{
		 int i,a[20];
		 unsigned long int h=0,m=1;
		 char s[20];
		 
		 printf("Convertion from Hexadecimal to Decimal\n");
		 printf("Enter a Hexadecimal number\n");
		 scanf("%s",s);
		 printf("Decimal Equivalent=");
		 for(i=0;s[i]!=NULL;i++)
		 {
		  switch(s[i])
		   {
		    case '0':
		     a[i]=0;
		     break;
		    case '1':
		     a[i]=1;
		     break;
		    case '2':
		     a[i]=2;
		     break;
		    case '3':
		     a[i]=3;
		     break;
		    case '4':
		     a[i]=4;
		     break;
		    case '5':
		     a[i]=5;
		     break;
		    case '6':
		     a[i]=6;
		     break;
		    case '7':
		     a[i]=7;
		     break;
		    case '8':
		     a[i]=8;
		     break;
		    case '9':
		     a[i]=9;
		     break;
		    case 'a':
		    case 'A':
		     a[i]=10;
		     break;
		    case 'b':
		    case 'B':
		     a[i]=11;
		     break;
		    case 'c':
		    case 'C':
		     a[i]=12;
		     break;
		    case 'd':
		    case 'D':
		     a[i]=13;
		     break;
		    case 'e':
		    case 'E':
		     a[i]=14;
		     break;
		    case 'f':
		    case 'F':
		     a[i]=15;
		     break;
		    
		   }
		 }
		 i--;
		 for(;i>=0;i--)
		 {
		  h=h+a[i]*m;
		  m=m*16;
		 }
		  printf("%ld ",h);
}
}

The last part of the code multiples the value of each digit by an increasing power of 16.

For example if you enter 0xA25. The final loops calculates.

10*(16*16) + 2*16 + 5 * 1 = 2560 + 32 + 5 = 2597