CLang Lect04

Data input/output
Department of Information System

SoICT, HUT
1
Data input/ouput
• To read and write data in C, we use two standard
functions that include in the file <stdio.h>
• printf() – prints something to the screen. This

function accepts parameters as variables to display
their values
• scanf() – receives values from the standard input

and assign them to variables
2
Example
/* Calculate the area of circle */
#include <stdio.h>
int main()
{
float r, s;
printf(“Enter the radius of circle: ");

scanf("%f",&r);
s = 3.14*r*r;
printf(“The area of circle is: s=%f", s);
return 0;
}
3
Formatting with printf()
• Syntax
printf("string...",variables or numbers);
• The simplest use of printf is to just print out a
string:
printf ("Hello world!");
• Print out a single integer number:
int number = 42;
printf ("Some number = %d",number);
4
Conversion character
• Conversion characters (starts with %) do not display in the
screen but they are replaced by values
• Basic conversion character
• %d: signed decimal integer
• %u: unsigned decimal integer
• %x: hexadecimal integer
• %o: octal integer
• %s: string
• %c: single character
• %f: fixed decimal floating point
• %e: scientific notation floating point
• To print a character %, use %% in the format string
5
Print a value in different formats
• A same value can be printed in different format.
• Example
char ch = ‘A’;
printf ("%d\n", ch); → print out 65
printf ("%c\n", ch); → print out ‘A’
• %d is called a conversion character for integers
because it tells the compiler to treat the variable to
be filled into it as an integer
6
Print a value in different formats
#include <stdio.h>
int main()
{
char c = ‘A';
printf(“Print c in the char format: %c\n", c);

printf(“Print c in the interger format: %d\n", c);
printf(“Print c in the hexa format: %x", c);
return 0;
}
Output: Print c in the char format: A

Print c in the interger format: 65
Print c in the hexa format: 41
7
Formatting with printf
• Use special control characters such as \n, \t
• We can specify the field width by following:

% [-] [fwidth] [.p] X where:
• [fwidth] the field width

• [-] left justified.
• [.p] the number of decimal places or how many characters are to
be printed.
8
Example
Value Spec. Output
42 %6d | 42|
42 %-6d |42 |
'z' %3c | z|
'z' %-3c |z |
2.71828 %10f | 2.71828|
2.71828 %10.2f | 2.71|
2.71828 %-10.2f |2.71 |
2.718 %.4f |2.7180|
2.71828 %10e |2.71828e+00|
"printf" %s |printf|
"printf" %10s | printf|
9
Exercises
1. Write a program to display a menu of a restaurant,
including 3 columns: meal’s code, meal’s name,
price
MENU
Code Name Price
1 Aaa 45000.00
2 Bbb 12500.00
2. Initiate value for a character in a program. Display
it and its ASCII code in the form '0': 48 (in the
screen)
10
scanf()
• Syntax
scanf ("string...",pointers);
• Note: Not variables which are listed after the control string but
point to variables.
Example:
int i;
char ch;
float x;
scanf ("%d%c%f", &i, &ch, &x);
// enter an integer, a character, and a real number
• Notice the & characters which make the argument pointers
11
Formatting with scanf
• The conversion characters for scanf are not identical to those
for printf, but much more precise
• %d : decimal integer (int)
• %ld : long decimal integer (long)
• %x : hexadecimal integer
• %o : octal integer
• %h : short integer (short)
• %f : float type
• %lf : long float or double
• %c : single character
• %s : character string
12
Common errors
• Find errors in the following codes:
float a, b, c;
scanf("%f", a);
scanf("%d", &b);
scanf("%f", &c);
13
Example
Input octal integer, output integer as decimal
#include <stdio.h>
int main(){
int i ;
scanf("%o", &i);
printf("%d", i);
return 0;
}
Input: 70
Output: 56
14
Example
Input a letter, output its order in alphabetical table
#include <stdio.h>
int main(void)
{
char letter;
printf(“Enter a regular letter\n”);

scanf(“%c”,&letter);
printf(“The order of entered letter is: %d\n”,letter-

'a'+1);
return 0;
}
15
Scan input data
• Values stored in variables are scanned relying on input string
from user. The scan process is carries out sequentially and can
stop when an error occurs.
• Example:
int i = 0;
char ch = '*';
float x = 0;
scanf ("%d%c%f ",&i,&ch,&x);
printf ("%d %c %f\n ",i,ch,x);
If input : 1x2.3
We have output: 1 x 2.300000
If input : 1 x 2.3
We have output: 1 0.000000
16
Skipping Characters in Input Stream
• Skipping blank spaces
scanf("%d %d %d", &day, &month, &year);
• Skipping dashes ( Enter data as dd-mm-yyyy)
scanf("%d-%d-%d", &day, &month, &year);
• Example:
If input is 1-1-2000, then day=1, month=1, year=2000
• As usual, if the skip string cannot be matched, scanf
will abort, leaving the remaining characters in the
input stream.
17
Return value of scanf()
• The general form of the scanf function is:
n = scanf ("string...", pointers);
• The value n returned is the number of items
matched or the end of file character EOF, or NULL
if the first item did not match
• Example:
n=scanf("%d-%d-%d", &day, &month, &year);
• If input is 1-1-2000, then day=1, month=1, year=2000,
n=3
• If input is 1/1/2000, then day=1 and the scanf is broken,
return n=1
18
Checking input value
int n;
printf(“n = ”);
if (scanf(“%d", &n) != 1)
printf(“Can not get value for n”);
19
Exercises
1. Write a program to get a character from the user
and then display its ASCII code in the form '0':
48
2. Input a number and a string from the keyboard.

Display them to the screen.
3. Input two-time values from the keyboard and

display the distance (in seconds) between them.
The input time format is hh:mm:ss
20
Thank you
for your
attentions!

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Data input/output

Department of Information System

• printf() – prints something to the screen. This

• scanf() – receives values from the standard input

printf(“Enter the radius of circle: ");

printf(“Print c in the char format: %c\n", c);

Output: Print c in the char format: A

• We can specify the field width by following:

• [fwidth] the field width

printf(“Enter a regular letter\n”);

printf(“The order of entered letter is: %d\n”,letter-

2. Input a number and a string from the keyboard.

3. Input two-time values from the keyboard and

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