Slides sobre Funções, Scanf, EOF

1. Lesson 2
Functions, scanf and EOF
Introduction to Computer and Program Design
James C.C. Cheng
Department of Computer Science
National Chiao Tung University

2. The return of scanf
 The number of fields successfully converted and assigned
 In the case of an input failure before any data could be successfully
read, EOF is returned.
2
int a =1, b =2, c =3;
int n = scanf("%d %d %d", &a, &b, &c);
printf("%dn", n );
printf("%d, %d, %d", a, b, c);

3. Multiple input
 EOF: end of file
 usually EOF is defined as (-1)
 It is a signal to tell the system that no more data can be read from a
input source.
 In Windows, press Ctrl+Z and Enter
 In UNIX, Linux and Mac OS X, press Ctrl+D
3
int x = 0;
while( scanf("%d", &x) != EOF ){
…
}

4. Abnormal Input
 Please input a non-numeric line, ex: %T@#$@KI
 A non-stop loop occurred! Why?
 How to fix the problem?
4
int x = 0;
while( scanf("%d", &x) != EOF ){
…
}

5. Standard I/O Buffer
 Buffer:
 A memory block used to temporarily hold data while it is being moved
from one place to another.
 There three I/O memory buffers in the computer
 FILE* stdin : For the standard input device (generally, a keyboard).
 FILE* stdout : For the standard output device (generally, the screen).
 FILE* stderr : For output error and warning messages to the standard
output device
5
FILE* is a pointer to point a stream I/O buffer
0x 26E81E00
FILE* stdin
address data
26E81E00 04 A5 00 10
26E81E04 0B 00 56 4F
26E81E08 06 FF 00 00
26E81E0C B4 00 C7 FF
… …

6. 6
The Reading Mechanism of scanf -- 1
 In the format string of scanf:
 Format specifiers (%):
 A sequence formed by an initial percentage sign (%) indicates a format specifier,
which is used to specify the type and format of the data to be retrieved from stdin
and stored in the locations pointed by the additional arguments.
 The basic usage of format specifier:
%[*][l]type
where type can be c, d, e, E, f, g, G, o, s, x, X
EX: %d
 Read but ignored
%*type
int x = 0;
while( scanf("%*d%d", &x) != EOF ){
printf("%dn", x); // ?
}

7. The basic data types
 There are 13 basic data types in C
 In C++, bool represents boolean value, true or false.
 The size of bool depends on compiler, 1 byte in most case. 7
Name Size (byte) Range
char 1 -128 to 127
unsigned char 1 0 to 255
short 2 -32768 to 32767
unsigned short 2 0 to 65535
int 4 -231 to 231 - 1
unsigned int 4 0 to 232 - 1
long 4 -231 to 231 - 1
unsigned long 4 0 to 232 - 1
__int64, long long 8 -263 to 263 – 1
unsigned __int64 8 0 to 264 - 1
float 4 ±(1.175494351e-38 to 3.402823466e38 )
double 8 ±(2.2250738585072014e-308 to
1.7976931348623158e308 )
long double 12 in DevC++, 8 in MSC The same as double

8. The type of format specifier
8
type Qualifying Input Type of argument
c
Single character: Reads the next character. If a width
different from 1 is specified, the function reads width
characters and stores them in the successive locations
of the array passed as argument. No null character is
appended at the end.
char *
d
Decimal integer: Number optionally preceded with a + or
- sign.
int *
e,E,f,g,G
Floating point: Decimal number containing a decimal
point, optionally preceded by a + or - sign and optionally
folowed by the e or E character and a decimal number.
Two examples of valid entries are -732.103 and 7.12e4
float *
o Octal integer. int *
s
String of characters. This will read subsequent
characters until a whitespace is found (whitespace
characters are considered to be blank, newline and tab).
char *
u Unsigned decimal integer. unsigned int *
x,X Hexadecimal integer. int *

9. 9
The Reading Mechanism of scanf -- 2
 It starts after one string line inputted.
 a string which the last key is n
 In the format string of scanf:
 White-space characters, WC:
 blank (' '); tab ('t'); or newline ('n').
 一連串的WC可視為一個 WC
 一個在format string 的WC代表它可吸收零個以上的WC輸入
 輸入數值型態的資料時，前面可以鍵入零個以上的ＷC.
 The non-WCs in format, except the ’%’
 scanf 會從stdin 依輸入順序讀取字元，然後依序與format string 的非WC字元比較.
 若比較相同，輸入字元discarded，繼續下一字元.
 若比較不相同，scanf 失敗並結束，stdin剩下的資料包含現在比較失敗的字元都會
留在stdin

10. 10
Standard Input in C
 How to clear the stdin?
 getchar()
 read a character from stdin
 Usage:
 #include <stdio.h>
 int getchar( void );
 Returns the input character
 Ex:
 int c = getchar();
 Using getchar() to clear stdin
int x=0;
scanf("%d", &x);
printf("%dn", x);
{ int c; while((c=getchar())!='n'&&c!=EOF); } // clear stdin
printf("press enter to exit");
getchar(); // Waiting an Enter key

11. EOF in Windows
 Ctrl + z (^Z) in Windows:
 ^Z要搭配n才有意義
 按下^Z之後可一連串按下任意鍵再按下n皆等同按下^Zn，如
^Z!@#!@$!$!@n等同^Zn甚至^Z後面接一堆空格再加n也等同^Zn
 他要能被當成EOF，之前除了一連串的n或^Z，不可按其它任何鍵。否
則按下n後是代表Substitute這個字元，ASCII code是26
 當要成為EOF前，^Z不產生任何ASCII code
 下列程式碼, ^Zn需按下兩次才能結束程式，為什麼?
11
int x=0, y=0;
while( scanf(" %d%d", &x, &y) != EOF ){
printf("%d, %dn", x, y);
}
a white space

12. EOF in Windows
 DO NOT let the last character of the format string be a WC
12
int x=0;
while( scanf("%d ", &x) != EOF ){
printf("%dn", x);
}
a white space

13. Exercises
 請根據指示輸入資料，並解釋為何會出現畫面上的結果?
13
int x=0, y=0;
while( scanf("%d,%d", &x, &y) != EOF ){
printf("%d, %dn", x, y);
}
int x=0, y=0;
while( scanf("%d , %d", &x, &y) != EOF ){
printf("%d, %dn", x, y);
}
請輸入下列資料:
1,2
3 ,4
5, 6
7 , 8
9 10
int x=0, y=0;
while( scanf("%d ,%d", &x, &y) != EOF ){
printf("%d, %dn", x, y);
}

14. 14
Function declaration
 Syntax:
return type + function name + (parameters);
where the parameters are:
1. nothing or void
2. typename1, typename2, … typenameN
3. the names of parameters are not necessary
 Function name must be unique , except C++.
 Ex:
int F1(void);
int F2();
void F3(void);
void F3(int); // In C++, OK, but error in C
int F4(int, int, char);
or
int F4(int x, int y , char c);

15. 15
Function declaration
 In C++, the function declaration must be placed before calling
int main( void ){
int MyF(int); // declaration
int x = 0;
printf("%dn", MyF(x));
return 0;
}
int MyF(int n){ return n+1 ;} // Function definition

16. 16
Function definition
 Function definition
 Syntax:
return type + function name + (parameters){ … }
where the name of the parameters not be ignored.
 Function definition can not be placed in any block (except C++).
int MyF(int n){
return n+1 ;
}

17. 17
Function definition
 Declaration, definition and invocation.
 Which of following are correct?
 In C, all of them are correct
 implicit declaration and the function is assumed to return int
 In C++, the third is failed
int MyF (int a);
int MyF(int a){ return a+1; }
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}
int MyF (int a);
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}
int MyF (int a){ return a+1;}
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}
int MyF (int a);
int MyF (int a){ return a+1; }
int MyF (int a){ return a+1; }
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}
int MyF (int a);
int MyF (int a){ return a+1; }
int MyF (int a);
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}
int MyF (int a){ return a+1; }
int main( void ){
int x = MyF (1);
printf("1+1 = %dn", x);
return 0;
}

18. Function definition
 Default argument
 Only C++ provides default argument for functions
 A default argument is a value given in the declaration that the compiler
automatically inserts if you don’t provide a value in the function call.
 A default argument cannot be placed before non-default argument
 The declaration of a default argument either in global function
declaration or in function definition.
 Local function declaration can has its own default argument list.
18
void F1(int a, int b =10, int c = 2){ … }
void F2(int a, int b =10, int c){ … } // Compiler error
void F3(int a, int b =10, int c=5); // Global function declaration
void F3(int a, int b =10, int c=5){ … } // Compiler error
int main(){
F1(1, 2, 3); // a = 1; b= 2; c =3
F1(1); // a = 1; b= 10; c =2
void F1(int a, int b =20, int c = 30); // Local function declaration
F1(1); // a = 1; b= 20; c =30
F1(1, ,3); // Compiler error
}

19. 練習題
1. 設計一個function，名稱為CheckN
int CheckN(int n);
若 n > 0 且任兩個連續的digit相加皆相同，則return任兩個連續的digit之和，反
之return 0。10<=n<100則return兩個digit之和，n<10則return0。注意，不可
用字串處理方式。
2. 設計一個function，名稱為TileN
int TileN(int RoomW, int RoomH, int TileW, int TileH);
RoomW及RoomH: 分別代表房間的寬及長(公分)
TileW及TileH : 分別代表地磚的寬及長(公分)
假設地磚不可分割，所有地磚排列方式需一致，則TileN會return 該房間可貼的
最多地磚數量。
3. 設計一個function，名稱為IsPrime
int IsPrime(int n);
根據右列的理論來判斷n是否為質數
若n為質數，return 1，反之return 0 19
If n = ab, a <= b.
Assuming that a > . .
Then, n = ab >= aa > n causes contrary.
Therefore, a <= n
n