you may do the following
//to skip the spaces
if(c==32)
{
j--;
continue;
}
char c = plainText[i];
int keyWordWrapper;
char keyC;
if(isalpha(c))
{
keyWordWrapper = j % strlen(keyWord);
keyC = keyWord[keyWordWrapper];
int key;
tolower(c);
if(islower(keyC))
{
key = keyC - 'a';
c = (c - 'a' + key) % 26 + 'a';
}
if(isupper(keyC))
{
key = keyC - 'A';
c = (c - 'A' + key) % 26 + 'A';
}
}