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I'm new to C programming so if anything looks super wrong I'm sorry!

I need to make my array ( str[512] ) to be exactly 512 characters long after taking input from a text file with an unknown number of characters. After the text file input, there is supposed to be just lowercase 'x' until the 512 characters limit is reached. so if the text file is 'abcdf', my array needs to be 'abcdfxxxxxxxxxxxxxxxxxx(...)xxxxxxx' until there are 512 characters. When I run the program, it's a mess. First part of the code reads the text file, makes all the upper case letters lowercase, skips non alphabet characters and assigns the characters to the array. The second snippet is the problematic one that causes a messy output.

FILE * fp; char str[512]; int c, i = 0; fp = fopen("k1.txt", "r"); i = 0; while(!feof(fp)){ c = fgetc(fp); if (c == '\n') continue; c = tolower(c); if (c < 97 || c > 122) continue; str[i] = c; i++; 
for(i = 0; i < 512; ) { if (str[i] == '\0') str[i] = 'x'; i++; }
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    @Bananatoid, What text or who suggested using feof(fp)? Commented Jun 20, 2022 at 20:11
  • In your last section of code (the for loop), why are you resetting the value of i? Can you not start from the value at the end of the while loop, and then just set all chars remaining in the array to x (keeping in mind the NUL termination)? Commented Jun 20, 2022 at 20:16
  • if (str[i] == '\0') that is not going to work because you never actually fill in str with such NUL characters that you are checking for. Instead, i is already at the end of the data that was read from file (after you fix the feof problem so you should start the for loop from that index and not 0. Commented Jun 20, 2022 at 20:16
  • One easy way would be to first memset(str, 'x', sizeof str); and then to pass it as the buffer to fread at most the maximum size into like so: size_t len = fread(str, 1, sizeof str, stdin);. Now len is the number of bytes actually read, or 0 if the read failed or is short and you then check why. But be careful with the name str, because if it's not NUL-terminated it's not a string. Commented Jun 20, 2022 at 20:18
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    no one has linked it yet, see Why is while( !feof(file) ) always wrong? Commented Jun 20, 2022 at 20:26

2 Answers 2

Reset to default
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The condition of the while loop is incorrect

while(!feof(fp)){

At least you need to check that i is less than 512

Also this for loop

for(i = 0; i < 512; ) { if (str[i] == '\0') str[i] = 'x'; i++; }

does not make a sense because the array after the preceding while loop will not contain the zero character '\0'.

You should write something like the following

int c; size_t i = 0; while ( i < 512 && ( c = fgetc( fp ) ) != EOF ) { if ( isalpha( c ) ) { str[i++] = tolower( c ); } } while ( i < 512 ) str[i++] = 'x';
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6 Comments

Oh awesome, that makes sense, I was under the impression that all the empty spaces in the array were indeed the NUL 0\ values that's why wrote I my for loop like that.
@chux-ReinstateMonica c can be a negative number.
@chux-ReinstateMonica You are right. I have not read fgetc for much time.:)
I also need to wrap my output to be 80 letters long, what function should I use for that?
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After the text file input, there is supposed to be just lowercase 'x' until the 512 characters limit is reached.

Read the block of up to 512 characters in 1 step.

#define N 512 char str[N]; size_t count = fread(str, 1, N, fp); // Now copy in the x's memset(str + count, N - count, 'x');

If you want to print the array, use a precision as str[] is not a string.

printf("<%.*s>\n", N, str);
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