04.txt

一個計算機系教授的上課講義 主要是教導學生使用C語言編寫程序

CS 1355
Intro to Programming in C
Thursday 2006.9.21 (Week 2)
Lecture Notes (at http://r638-2.cs.nthu.edu.tw/C/notes/04.txt)

Introduction to C Programming (Chapter 2 cont'd)

Concepts
- Arithmetic expressions
- Operator associativity (left associative, right associative)
- Operator precedence
- Relational operators
- if statements
- statement blocks

Arithmetic expressions
- Expression: something that yields a value
  - basic:    literals (1, 2, 3, 100, 2345, ..), variables (a, b, x,...)
  - compound: operators (+ - * / ...), function calls ( f(x), ...)
- Actually, there are two kinds of numbers:
  - int:  integers (whole numbers)  e.g., 1  2  3   100  2034
  - float: floating-point (fraction) e.g., 3.1415   7.8899   0.0002324
- Arithmetic expression: evaluates to an arithmetic value (number)

Arithmetic operators:
- Binary operators
  add:        a + b
  subtract:   x - y
  multiply:   i * j
  divide:     m / n   (in case of int, the integer part of the quotient)
                      (if either m, n is a float, then result is a float)
  modulo:     m % n    => "remainder" (for integer division only)
- examples  (can try this out in Ch)
  > 7 / 4
  1
  > 7 % 4
  3
  > 7.0 / 4
  1.7500 
- Unary operators:
  negation:    -a    (if a is 3, then -a is -3)
  positive:    +a    (doesn't do anything)

Operator associativity
  x = 2 + 3 + 4;
  ==> x is 9.  Do (2+3)+4 = 5+4 = 9
  x = 8 - 2 - 1;
  ==> x is 5.  Do (8-2)-1 = 6-1 = 5.
   * This is called "left associative" (evaluate the left pair, then next, next,..)
   * If right associative, then it would be 
      (8 - (2 - 1)) = 8 - 1 = 7, which would be incorrect.
Operator Precedence
  x = 2 + 3 * 4;
  ==> x is 14, because it's implicitly
      2 + (3 * 4) = 2 + 12 = 14.
  It is NOT 
      (2 + 3) * 4 = 5 * 4 = 20.
  Operator precedence overrides associativity
  + -    are both left associative and equal precedence
  * / %  are all left associative, equal precedence,
         and higher precedence than + -
  Q: what is the evaluation order of
     x = 8 * 7 / 4 * 3 % 5 + 6 / 2 - 7;
  Answer:
     It is evaluated as
     (((((8 * 7) / 4) * 3) % 5) + (6 / 2)) - 7
     = ((((56 / 4 = 14) * 3 = 42) % 5 = 2) + (6 / 2 = 3) = 5) - 7
     = -2

Function evaluation:
- parameters are evaluated first
- the function is called
- then the return value of the function is used.
Example
#include <stdio.h>
int sum(int x, int y) {
        return x + y;
}
int main() {
        int a = 2 + sum(3*2-4, 4%3+5) * 4;
        printf("%d\n", a);
}
Run it
./a.out
34

Reason: 2 + sum(3*2-4, 4%3+5) * 4 
      = 2 + sum(2, 6) * 4 
      = 2 + 8 * 4
      = 34

Relational operators
- compare two numeric values
- Result: true or false value
- Six comparison operators:  ==  !=  >  <  >=  <=
  x == y       x is "equal to" y
  x != y       x is "not equal to" y
  x > y        x is "greater than" y
  x < y        x is "less than" y
  x >= y       x is "greater than or equal to" y
  x <= y       x is "less than or equal to" y
 where x and y are arithmetic expressions.

True/False value:
- what is the result of the relational operator?
- in C, anything zero is considered false
  - integer value 0
  - empty string ""
  - null character '\0'
- in C, anything else non-zero is considered true
  - by default, you get 1 for a true value
  - but you probably should not assume it's 1

Example: (try it in Ch, prompt is >)
  >    int x = 2;
  >    x > 3
  0                   /* This means false */
  >    x < 4
  1           << This means true
  >    x == 2
  1 
  etc

Operator precedence
- relational operators have lower precedence than arithmetic
  => evaluate arithmetic operators before comparing
- example (assume x = 2)
  x * 3 - 4 <= 3 + x * 2
  This is evaluated as
  (2 * 3 - 4) <= (3 + 2 * 2)
  (6 - 4) <= (3 + 4)
  2 <= 7
  True
- so, not necessary to put parentheses around the arithmetic expressions

If statements
- a way to decide whether to do something
- decision is based on a true/false value
Example
#include <stdio.h>
int main() {
        int age;
        printf("how old are you? ");
        scanf("%d", &age);
        if (age < 20) {
                printf("No, you are too young to drink alcohol\n");
        } else {
                printf("Yes, you are old enough to drink alcohol\n");
        }
}
Run it
% ./a.out
how old are you? 30
Yes, you are old enough to drink alcohol
% ./a.out
how old are you? 12
No, you are too young to drink alcohol
% ./a.out
how old are you? 20
Yes, you are old enough to drink alcohol
% ./a.out
how old are you? 19
No, you are too young to drink alcohol
%

How it works
- if, else  are keywords
- The structure looks like
  if (condition) {
    statement block  S1
  } else {
    statement block  S2
  }

- meaning: 
  if the condition expression has a true value, 
  then statement block S1 is executed; 
  otherwise (condition is false), statement block S2 is executed.
- Note:
  the ( ) around condition are required!!
  => it is a syntax error to say
	if age < 20 { ... }
     you must have ( ) around condition, as in
	if (age < 20) { ... }

- statement blocks S1 and S2 each can contain a sequence of statements
  actually, { }  enclose a statement block
  but { } itself is considered to be one (compound) statement.

Spacing and indentation:
- in C, spaces, tabs, and newlines are ignored
  (unless you are inside a string!)
- Example: the following programs are equivalent
/* program 1 */
#include <stdio.h>
int main() {
        int age;
        printf("how old are you? ");
        scanf("%d", &age);
        if (age < 20) {
                printf("No, you are too young to drink alcohol\n");
        } else {
                printf("Yes, you are old enough to drink alcohol\n");
        }
}

/* program 2 */
#include <stdio.h>
int
main()
{ int age; printf("how old are you? "); scanf("%d",&age);
  if (age<20) { printf("No, you are too young to drink alcohol\n"); }
  else        { printf("Yes, you are old enough to drink alcohol\n"); }}

/* program 3 */
#include <stdio.h>
int                        main() { int age;
printf("how old are you? ");
scanf("%d",
		&age);
if (age<20) 
{
printf("No, you are too young to drinnk alcohol\n");
}
else
{
printf("Yes, you are old enough to drink alcohol\n");
}
}

However, better to follow the first style, as recommended by the
inventor of C

1. put { at the end of a line to save some space
2. put } else  on the same line to save space
3. Statements in a statement block should be indented by one extra level.

** you can have a statement block within another statement block!