Thread: if and while conditions

I'm trying to parse a string (polynomial function) for my program. Are these conditions allowed for if and while?

if (a >= '0' && a <= '9') ---> checks if char is a number
if ( a>= 'A' && a <= 'Z') ---> checks if char is a letter
if (a >= 'a' && a <= 'z')


while(r[i] != '\0') ---> repetition for all the characters.

Yes, they're allowed. The quick way to find out is to test them...

The only thing is you may want to add a couple of extra brackets just to gurantee the behaviour you want...

Code:

if (( a >= '0' ) && ( a <= '9' )) 
  { //whatever }

This guarantees the tests on a are completed before the && condition is tried instead of left to right evaluation which might give a different answer...

Also you could use the isdigit() and isalpha() functions from the std library for this.

oh thanks! I tried it and it worked. A little problem with the condition for numbers though.
When using this sample code, the numbers in the string passes the 2nd if, and also the else.

while(eq[i] != '\0') {
if(isNum(eq[i])) {printf("%c - number\n", eq[i]);}
if(isAlpha(eq[i])) {printf("%c - letter\n", eq[i]);}
else{ printf("%c - etc\n");}
i++;
}

Originally Posted by CommonTater

Also you could use the isdigit() and isalpha() functions from the std library for this.

I tried the isdigit and isalpha with the stdlib header file but the compiler says it's undeclared.

Originally Posted by firehydrant

oh thanks! I tried it and it worked. A little problem with the condition for numbers though.
When using this sample code, the numbers in the string passes the 2nd if, and also the else.

while(eq[i] != '\0') {
if(isNum(eq[i])) {printf("%c - number\n", eq[i]);}
if(isAlpha(eq[i])) {printf("%c - letter\n", eq[i]);}
else{ printf("%c - etc\n");}
i++;
}

Ok frst thing ... code tags. Please learn how to use them... << !! Posting Code? Read this First !! >> ... makes life a whole lot easier.

Second the setup of your code does not clearly show it's function... it may look ok now but will you still be able to follow it when you come back to it in a year?

Code:

while( eq[i] != '\0 ') {
   if( isNum( eq[i] ) ) 
       printf( "%c  - number\n", eq[i] );
   else if( isAlpha( eq[i] ) ) 
      printf( "%c  - letter\n", eq[i] );
   else
     printf( "%c  - etc\n"[b], eq[i] );
   i++;
}

The others will probably tell you I'm the last one to lecture on code formatting since in my own work I use a very unusual style but I'm sure you can see how much more readable your code is when it's set up nicely.

Originally Posted by firehydrant

I tried the isdigit and isalpha with the stdlib header file but the compiler says it's undeclared.

Sorry I meant the standard [set of] libraries... My bad.

They're in ctype.h

@quzah

isNum and isAlpha are functions I wrote in the program.
I'm new in this forum so sorry for that.

Originally Posted by firehydrant

@quzah

isNum and isAlpha are functions I wrote in the program.
I'm new in this forum so sorry for that.

They're also part of standard C99 ... while re-writing them might be an interesting exercise, there's not much point when they're already provided for you.

http://en.wikipedia.org/wiki/Ctype.h

this is already a bit off topic, but it's still part of the program i'm working on. I saw this sample code about parsing.

Code:

/* strtok example */
#include <stdio.h>
#include <string.h>

int main ()
{
  char str[] ="- This, a sample string.";
  char * pch;
  printf ("Splitting string \"%s\" into tokens:\n",str);
  pch = strtok (str," ,.-");
  while (pch != NULL)
  {
    printf ("%s\n",pch);
    pch = strtok (NULL, " ,.-");
  }
  return 0;
}

Sample code is not mine.
I got it here: http://www.cplusplus.com/reference/clibrary/cstring/strtok/

How do you store the tokens, say in an array, after parsing? Or is it already stored somewhere?

You have gotten good advice. I want to add one more comment.

Originally Posted by firehydrant

if ( a>= 'A' && a <= 'Z') ---> checks if char is a letter
if (a >= 'a' && a <= 'z')

The uppercase letters are not guaranteed to be contiguous. The reason the language standard did not require them to be is because there are C implementations designed for systems that use EBCDIC as either their source code and/or execution character set. In EBCDIC, the letters are not contiguous. They are in ASCII and most (all?) of the character sets based on them.

This is one of the advantages of using the isalpha(), isupper(), and islower() functions or macros from the standard library (mentioned ealier in the thread). They will deal with these changes in character sets in a portable way. This is only important if you need or want to be portable.

Originally Posted by pheininger

You have gotten good advice. I want to add one more comment.



The uppercase letters are not guaranteed to be contiguous. The reason the language standard did not require them to be is because there are C implementations designed for systems that use EBCDIC as either their source code and/or execution character set. In EBCDIC, the letters are not contiguous. They are in ASCII and most (all?) of the character sets based on them.

This is one of the advantages of using the isalpha(), isupper(), and islower() functions or macros from the standard library (mentioned ealier in the thread). They will deal with these changes in character sets in a portable way. This is only important if you need or want to be portable.

Thanks! I'll try the isupper and islower later.