I/O

Standard Streams

printf, scanf only works with stdin/stdout. They use the standard stream.

They don't have to be closed or opened; C handles that You can refer to them by these names, defined in stdio.h:

• stdin
• stdout
• stderr

For example, fprintf can write to stdout like printf:

fprintf(stdout, "Hello World\n");

C File I/O

fprintf, fscanf works with any files. Good for big inputs and outputs.

fopen

fopen("output.txt", "w");

Use relative file path for the first parameter. The second parameter indicates the modes, which are:

• "w" opens file for writing
• "r": reading
• "r+": open for reading and writing.
• "w+": open file for reading & writing.

Note: "r" and "w" are common. "w" and "w+" causes the named file to be overwritten if it already exists.

fopen returns a FILE*, a pointer to the FILE struct. Returns NULL and crashes if fopen failed.

• NULL is a special pointer value, usually equal to 0, and is a common way to indicate failure for functions with pointer return type.

FILE* input = fopen("numbers.txt", "r");

if (input == NULL) {
	printf("Error: could not open input file\n");
	return 1; // indicate error 
}

fscanf

fscanf(input, "%d%d", &destination1, &destination2) 

• reads each line in the file named input.
• scanf returns the number of input collected. Do while (numCollected ==2) to make a loop that keeps collecting two inputs from each line. Then, check if (numCollected != EOF) because it would indicate that the program could not parse line instead of reaching the end of file.

Other Functions

• feof(fileptr) returns non-zero if we've already read past the end of the file.
  • Starts reading from the beginning of the file, and as you start reading, the cursor moves.
• ferror(fileptr) returns non-zero if file is in an error state,
  • e.g. if we've opened file for writing but then attempt a read.
• rewind(fileptr) returns fileptr to beginning of file.
• fclose(input) closes input file.

Assertions

assert(boolean expression);

Assertion statements help catch bugs as close to the source as possible.

• #include<assert.h> is required to use.
• boolean expression is an expression that should be true if everything is okay.
• If it's false, program immediately exits with an error message indication failed assertion.
• Create test cases using assert to make sure certain states exists.

They make your assumptions clear:

int sum = a*a +b*b;
assert (sum >= 0);

We should use assert when we are verifying something about the logic of your program (checking for something that implies your program is incorrect). Use if statements to check the external environment and for bad user input.

• For example, when using fopen to open a file, we use if(input = NULL) instead of an assert (input != NULL) because if there is a problem, it would be the fault of the file (missing or corrupt), and not the program.