c - How to avoid counting the non-alphanumeric characters -

i making crypting program cs50x , have small problem. trying crypt text using vigenere cipher(cipher uses word key). problem have that, whenever have space or non-alphabetic character, keeps incrementing. per spec, shouldn't... therefore, characters getting using wrong key... idea how can solve problem?

here small part of code understand problem.

for(int x=0, = 0 , textlen = strlen(text); x < textlen; x++ , i++ )      if (isalpha(text[x]))     {          if (islower(keyword[i % num_letters]))         {             if (islower(text[x]) && ((text[x] + (keyword[i % num_letters]) - 'a') <= 'z'))             {                 text[x]= (text[x] + (keyword[i % num_letters]) - 'a' );              }              else if (islower(text[x]) && ((text[x] + (keyword[i % num_letters]) - 'a') > 'z' ))             {                 text[x]= ('a'+ ((((text[x]) - 'a') + ((keyword[i % num_letters]) - 'a' )) % 26));              }         }     printf("%c", text[x]);     }     else   // print non-alpha char      {         printf("%c", text[x]);      } 

i hope piece of code understand. fragment code should enough understand problem , perhaps suggest solution.

thanks!

this may weird way solve it, think simple solution be, in else statement, decrement value. problem regardless of whether entering if/else statement, both finish , return top, incrementing if nothing changed. more readable solution take control of variable away loop , increment right before/after print char