Declarations and Expressions

Declarations and Expressions C and Data Structures Baojian Hua bjhua@ustc.edu.cn

Overview • Variable declaration • the way to introduce variables into programs • Operators • do some useful operations on variables and constants • Expressions • form complex expressions with variables, operators, constants and other expressions

Variables • Variable formation rules: • consists of one or more letter or digit, (“_” counts as a letter) • starts with a letter • lower and upper cases are distinct • different from key words • It’s common practice not to start with “_” • variable names in C library • Choose informative variable names • Compare “sldj_wdwp_” with “name”

Types and Constants

Types and Constants(2)

Variable Declaration • C obeys the variable use-after-declaration rule • Variable declarations consists of these parts: • Type name • Variable list (may only one) • initialization • Ex: int x, y, z; double f; char c = ‘c’; • Type qualifiers: • signed, unsigned (default is signed), const

Sample Variable Declaration int low, high; char line[1000]; double pi = 3.14; const double e = 2.71; const char digits[] = “0123456789abcdef”;

Variable Declaration • A declaration makes clear a variable’s type, size and allocates memory for it int i, j; $#*@? ?%*

Variable Initialization • Programmers’ duty to initialize that memory space int i, j; i = 9; 9 ?%*

Array Declaration int i, j; i = 9; int a[5]; 9 ?%* a $? #@ !~ &^ @$

Array Initialization int i, j; i = 9; int a[5]; for (int j=0; j<5; j++) a[j] = j*j; 9 ?%* a 0 1 4 9 16

Arithmetic Operators

Relational and Logical Operators

Relational and Logical Operators • Boolean operators are short-circuit • Computations stop as soon as true or false is known x = 9; if ((3>2) || (x=8)); // what’s x’s value? 8 or 9?

Relational and Logical Operators // another example on && x = 9; if ((1<0) && (x=8)); // what’s x’s value? 8 or 9? // a more fancy example int i = 0; int a[10]; while (i<10 && a[i]!=0){ … }

“!” • Not “!” converts 0 to 1, and non-zero values to 0 if (!a){ … } // reads “if a does not hold”, then … // be equivalent with if (0 == a) { … } // we’d see more examples later

Increment and Decrement int n = 9; int x; x = ++n; // x == 10 // n == 10 int n = 9; int x; x = n++; // x == 9 // n == 10

Bitwise Operators

Bitwise Operators Example // count num of 1’s in an integer x int count (int x) { int num = 0; unsigned int y = (unsigned int)x; while (y) { if (y & 0x1) num++; y >>= 1; } return num; }

Assignment Operators and Expressions • Assignment is a special kind of expression, so the following code fragment is legal int x; printf (“%d\n”, x=999); • Other kind of assignment expressions are of the form: e1 op= e2 • Equivalent to: e1 = e1 op e2 • Example: x += 9; ====> x = x + 9; • See the text for a complete list of op

Assignment Operators and Expressions // more examples on assignment operators: int a, b, c; a = b = 9; a = (b = 9) + (c = 8); a = (b = 9) / (c = 3); // even as the arguments of function calls: int a, b; x = sum (a = 1, b = 2);

Conditional Expressions if (a > b) max = a; else max = b; // C provides a more succinct way to do this: max = (a>b) ? a : b; // the general form is: e1 ? e2 : e3; // if e1 evaluates to true, then evaluates e2, // else evaluates e3.

Type Conversion (Cast) • When the type of some expression is not compatible with the one expected, automatic type conversion occurs • Example from our previous code: // all of c, ‘0’ and ‘9’ are automatic converted // into integer values by the compiler: if ((c>=‘0’) && (c<=‘9’)) a[c-’0’]++;

Type Conversion (Cast) • Generally, there are two kinds of automatic cast: • cast a “big” into “smaller” ones • long ==> int • cast a “small” into “bigger” ones • char ==> int • The former is not always safe

Type Conversion (Cast) • The general scheme for “safe” cast is: double float long unsigned int char

Type Conversion (Cast) // Dirty simple? However, it’s more subtle than // it first looks. Consider: #include <stdio.h> int main () { int i = -1; unsigned int j = 0; if (i < j) printf (“Never reach here, :-( \n”); else printf (“Shoot myself in the foot\n”); return 0; }

Type Conversion (Cast) • Don’t expect the compiler will always behave as you desire • Two general principals: • Always use the C’s type system as strong as possible • It’s your basic protection • One reason for strong type system’s popularity • Make use of C’s type conversion explicitly • To make clear what we are doing

Explicit Type Cast • General form of explicit type conversion: (type) expression // which converts the expression’s type to type // More examples: int i = (int)3333.14; char c = (char)i; int a[10]; a[(int)2.71] = 99; • As we see, data precision may be changed

Safety Issue • Type conversion is an infamous source of C programs bugs • especially with pointers • (int *)999 will send your passwd to BillG… • Any serious and well-designed C program should use type conversion really rarely • General principle: • do NOT use it, • always use the explicit form

Declarations and Expressions