Basics of C

Created

Printing

printf can output a literal string and also allows for formatting printing of values, using placeholders in the fomrat string:

printf("There are %d students in class.", 36);

Placeholders begin with '%' and then may contain additional format information. The actual values corresponding to place holders are listed afte rthe format string (36 in this example).

  • Generate a literal % character by specifying two % characters in the format string.

Data Type Placeholders

Some common data type placeholders to use after the %:

  • d - decimal (integer type)
  • ld - long int
  • u - unsigned (integer type that dis allows negatives)
  • lu - for long unsigned
  • f - floating point
  • e - scientific notation
  • lf - for double
  • c - character
  • s - string

Variables

int num_students;

declares a variable with type of int and name of num_studnets. A variable has a value that may change through the program's lifetime. It's value can be printed with printf.

A two-word variable in C is often written using underscores rather than camel case (e.g., numStudents).

Types

Try to use the smallest data type. See all the data types here.

Integer types

  • int: signed integer, stored in 32 bits.
  • unsigned: unsigned integer. Always considered positive.
  • long: signed integer with significantly greater capacity than a plain int

Floating-point (decimal) types

  • float: single-precision floating point number. 4 bytes.
  • double: double-precision floating point number. 8 bytes. Can store more decimal digits.

Character types

  • char: holds a 1-byte character, A, B, $, ... They are stored in their unicode value.
  • Chars must be in single quotes. Double quotes are only for strings.
  • chars are basically integers, so they work for logic statements. This is valid: char digit = '4' - 1. ASCII standard governs the mapping between characters and integers. Use (int)char_variable to explicitly convert.
  • The C character library #include <ctype.h> contains a bunch of useful functions we can apply to character values. Boolean functions include isalpha, isdigit, islower, isspace, etc that return non-zero for true and zero for false. Conversion functions include tolower and toupper.

Boolean

  • #inlcude <stdbool.h to use this.
  • bool: value can be true or false
  • Integer types can function as bools, where 0 means false and non-0 means true.

Assignment

num_students = 32

= Is the assignment operator, which modifies a variable's value.

It's good practice to declare and assign at the same time:

int num_students = 32

This prevents "undefined" values (variable that has been declared but not yet assigned). "Undefined" values can cause mysterious fails.

Operators

3 + 4

3 and 4 are "operands" and constants (not variables) and + is the "operator".

Arithmetic operators include + (addition), - (subtraction), * (multiplication), / (division), and % (remainder).

  • Beware of integer division: 7 / 2 yields 3, not 3.5

Operator Precedence

Precedence can be found here. Use parentheses when in doubt.

Const

Put const before the type to say the variable to which it is applied cannot be modified.

To make a variable non-modifiable:

const int base = 32;
  • If applied to a local variable, it must be initialized when declared.
  • If applied to a parameter variable, it cannot be changed within the function. You can pass a non-const variable to a const parameter, but you cannot pass a const variable to a non-const parameter.

Formatted input with scanf

// scanf_d.c:
int i;
printf("Please enter an integer: ");
scanf("%d", &i);
printf("The value you entered is %d", i);

The scanf function works similarly to printf for reading formatted input. Use a format string (%d) followed by the memory location(s) (i) we are reading into. Use the & sign before the variable name.

  • Match the format string to the type of value you want to collect. The memory location should be able to accomodate this type.

The return value of scanf is the number of input items assigned.

  • Zero typically indicates that the input was available but invalid for the specific type.
  • EOF (-1) indicates that no input was available.

Linked from Intermediate Programming, Pointers