CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. How programs read input and produce output, and the categories of instructions that make up a program.

Standard I/O Streams

• Standard input (stdin): default source of input data, normally the keyboard
• Standard output (stdout): default destination for normal program output, normally the screen
• Standard error (stderr): separate stream for error messages, so they can be redirected independently of normal output

I/O in C

• printf() — formatted output, uses format specifiers (%d int, %f float, %c char, %s string)
• scanf() — formatted input, requires the address-of operator & for non-string variables (e.g. scanf("%d", &age);)
• getchar() / putchar() — unformatted single-character input/output
• gets() / puts() — unformatted string input/output (gets() is unsafe/deprecated — no bounds checking)

I/O in C++

• cin >> — extraction operator, reads formatted input from stdin
• cout << — insertion operator, writes formatted output to stdout
• cerr — writes to stderr (unbuffered, for errors)
• getline(cin, str) — reads a full line including spaces (unlike cin >>, which stops at whitespace)

Formatted vs Unformatted I/O

• Formatted I/O: data is converted to/from a specific display format using format specifiers or stream operators (printf/scanf, cin/cout)
• Unformatted I/O: raw character-by-character transfer with no type conversion/formatting (getchar/putchar, cin.get())

Statement Types