Funções, Scanf, EOF

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, EOFis returned.
2
int a =1, b =2, c =3;
int n = scanf("%d %d %d", &a, &b, &c);
printf("%d", 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+Zand 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("%d", x); // ?
}

7. The basic data types There are 13 basic data types in C

In C++, boolrepresents 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 -2
31
to 2
31
-1
unsigned int 4 0 to 2
32
-1
long 4 -2
31
to 2
31
-1
unsigned long 4 0 to 2
32
-1
__int64, long long 8 -2
63
to 2
63
–1
unsigned __int64 8 0 to 2
64
-1
float 4±(1.175494351e
-38
to 3.402823466e
38
)
double 8±(2.2250738585072014e
-308
to
1.7976931348623158e
308
)
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 specifi ed, 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
In the format string of scanf:

White-space characters, WC:
blank (' '); tab (' '); or newline ('').
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("%d", x);
{ int c; while((c=getchar())!=''&&c!=EOF); } // clear stdin
printf("press enter to exit");
getchar();// Waiting an Enter key

11. EOF in Windows Ctrl + z (^Z) in Windows:

^Z要搭配才有意義

按下^Z之後可一連串按下任意鍵再按下皆等同按下^Z，如
^Z!@#!@$!$!@等同^Z甚至^Z後面接一堆空格再加也等同^Z

他要能被當成EOF，之前除了一連串的或^Z，不可按其它任何鍵。否
則按下後是代表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, ^Z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, %d", 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("%d", 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, %d", x, y);
}
int x=0, y=0; while( scanf("%d , %d", &x, &y) != EOF){
printf("%d, %d", 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, %d", 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:
intF1(void);
intF2();
void F3(void);
void F3(int); // In C++, OK, but error in C
intF4(int, int, char);
or
intF4(intx, inty, 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("%d", 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 pl aced 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 =%d", x);
return 0;
}
int MyF(int a);
int main( void ){
int x = MyF(1);
printf("1+1 =%d", x);
return 0;
}
int MyF(int a){ return a+1;}
int main( void ){
int x = MyF(1);
printf("1+1 =%d", 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 =%d", 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 =%d", x);
return 0;
}
int MyF(int a){ return a+1; }
int main( void ){
int x = MyF(1);
printf("1+1 =%d", x);
return 0;
}

18. Function definition Default argument

Only C++ provides default argument for functions

A default argument is a value given in the declarationthat 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> ncauses contrary.
Therefore, a<=
n
n