Strange loop incrementing behaviour in C-CodePudding

I came across this strange loop behaviour in C. Here is the code:

char arr[SIZE];
fgets(arr, SIZE, stdin);
char brr[SIZE];
int i;
for(i=0; arr[i] != '\0'; i  ){
    if(arr[i] != ' ') brr[i] = (arr[i]   shift - 'a')&   'a';
    else brr[i] = ' ';
}
brr[i-1] = '\0';

The code is for Caesar cipher. It takes some text from the user and stores it in arr, then encodes it by shifting the characters, and this encoded version is put in brr. So, if user enters 'car' and shift is 2, brr is 'ect'.

The problem is in the last line. Normally, it should be brr[i] = '\0', but for some reason the loop will increase I once more after the condition is wrong. I have tried different combinations and it seems like the problem is somehow related to fgets.

Also, the problem is not specific to for loop, while behaves exactly the same.

Edit: Thanks for all the answers. I see now that I should have taken '\n' into account.

CodePudding user response：

somehow related to fgets.

Always test the return values of input functions.

// fgets(arr, SIZE, stdin);
if (fgets(arr, SIZE, stdin)) {
  // process arr[]
  ...

if user enters 'car' and shift is 2,

This is unlikely to only typed 'car' (3 characters). Certainly the user typed c a r Enter (4 characters) which filled arr[SIZE] with "car\n".

Rather than encode the 4 characters with if(arr[i] != ' '), test if arr[i] is a letter. Research isalpha().

Code could exit the for() loop early if '\n' is detected.

// for(i=0; arr[i] != '\0'; i  ){
for(i=0; arr[i] != '\0' && arr[i] != '\n'; i  ){
  ...
}
// brr[i-1] = '\0';
brr[i] = '\0';

Advanced

brr[i-1] = '\0'; is certainly a problem if i==0, possible if the first character read is a null character.