Transcript Input, Output and Variables

Input, Output
and Variables
Ethan Cerami
New York University
7/21/2015
Ethan Cerami, NYU
1
This Week


Tuesday:
 Basic Output: printf()
 Introduction to Variables
 Integer variables
 Basic Input: scanf()
Thursday:
 Basic Arithmetic
 Float variables
 Operator Precedence
 Promotion/Casting
7/21/2015
Ethan Cerami, NYU
2
Basic Output: printf ()



printf () stands for “print formatted
text.”
Prints any String of text directly to the
computer screen.
String: Any text appearing between
quotes, e.g. “hello”
7/21/2015
Ethan Cerami, NYU
3
Sample Program
/* Printing multiple lines
with a single printf */
#include <stdio.h>
main()
{
printf("Welcome\nto\nC!\n");
/* Wait for user */
getchar();
}
7/21/2015
Ethan Cerami, NYU
Note the \n escape
sequence creates a
new line character.
Welcome
to
C!
4
Escape Characters


The \ character indicates an Escape
sequence.
Examples
\n
\t
\a
\\
\"
7/21/2015
New Line Character
Tab Character
The Alarm\Bell Character
Outputs a single \ Character
Outputs a single " Character
Ethan Cerami, NYU
5
Outputting a Quote Character



Quotes designate the beginning and end of a
String. Hence, if you try to output a quote,
the compiler gets confused.
For example:
printf ("He said, "Hello"");
will result in a compilation error.
To do it correctly, you must escape the “
character. For example:
printf ("He said, \“Hello\"");
7/21/2015
Ethan Cerami, NYU
6
Variables


Variable: a small piece or “chunk” of
data.
Variables enable one to temporarily
store data within a program, and are
therefore very useful.
Note: variables are not persistent. When you exit
your program, the data is deleted. To create
persistent data, you must store it to a file system.
7/21/2015
Ethan Cerami, NYU
7
Data Types


Every variable must have two things: a
data type and a name.
Data Type: defines the kind of data
the variable can hold.


7/21/2015
For example, can this variable hold numbers? can
it hold text?
C supports several different data types. We are
only going to look at one today.
Ethan Cerami, NYU
8
C’s Three Main Data Types




integers: the simplest data type in C.
Used to hold whole numbers, e.g. 5, 25.
floats: Used to hold fractional or
decimal values, e.g. 3.14, 10.25.
chars: Used to hold individual
characters, e.g. “c”, “e”
We will explore each one in detail in the
next week.
7/21/2015
Ethan Cerami, NYU
9
Bucket Analogy


It is useful to think of a variable as a
bucket of data.
The bucket has a unique name, and can
only hold certain kinds of data.
balance is a variable
containing the value
200, and can contain
only whole numbers.
200
balance
7/21/2015
Ethan Cerami, NYU
10
Variable Declaration


Before you use a variable, you must declare it. (Not
all languages require this, but C certainly does.)
Examples:
/* Creates an integer variable */
int number;
/* Creates a float variable */
float price;
/* Creates a character variable */
char letter;
7/21/2015
Ethan Cerami, NYU
11
Example 1: Basic Arithmetic
#include <stdio.h>
Variable Declaration
main () {
int x, y, z;
x = 5;
y = 10;
z = x + y;
printf ("x:
printf ("y:
printf ("z:
Data Type
Variable Names
Assignment Statements
%d\n", x);
%d\n", y);
%d\n", z);
}
7/21/2015
Ethan Cerami, NYU
12
Assignment Statements

Assignments statements enable one to initialize variables or
perform basic arithmetic.
x = 5;
y = 10;
z = x + y;

Here, we simply initialize x and y and store their sum within the
variable z.
Note: If you forget to initialize your variables, the
variable may contain any value. This is referred to as a
garbage value. Hence, always initialize your variables!
7/21/2015
Ethan Cerami, NYU
13
Printing Variables

To print a variable, use the printf()
statement:
printf ("x:
%d\n", x);
Format Specifier:
Indicates the type of
data to print. In this
case, %d indicates
whole digits or integer
values.
7/21/2015
Name of variable to
print. In this case, we
print the variable x.
Ethan Cerami, NYU
14
Rules for Naming Variables




Variable Names:
 Can contain letters, digits, or underscores _
 Cannot begin with a digit.
Examples of Valid Variable names
 int variable1, variable_2;
Examples of Invalid Variable names:
 int 1variable, variable#1
Remember, C is case sensitive, e.g. variable1 
VARIABLE1
7/21/2015
Ethan Cerami, NYU
15
Basic Input: scanf()


Input: any user supplied data.
 Keyboard input, mouse input.
scanf (): read in keyboard input.
7/21/2015
Ethan Cerami, NYU
16
Example 2: Using scanf()
/* Addition program */
#include <stdio.h>
#include <conio.h>
main()
{
int integer1, integer2, sum;
printf("Enter first integer\n");
scanf("%d", &integer1);
printf("Enter second integer\n");
scanf("%d", &integer2);
sum = integer1 + integer2;
printf("Sum is %d\n", sum);
/* declaration */
/*
/*
/*
/*
/*
/*
prompt */
read an integer */
prompt */
read an integer */
assignment of sum */
print sum */
/* Wait for user to Any Key */
getch();
return 0; /* indicate that program ended successfully */
}
7/21/2015
Ethan Cerami, NYU
17
Quick Review
/* Addition program */
#include <stdio.h>
#include <conio.h>
main()
{
int integer1, integer2, sum;
/* declaration */
printf("Enter first integer\n");
scanf("%d", &integer1);
printf("Enter second integer\n");
scanf("%d", &integer2);
sum = integer1 + integer2;
printf("Sum is %d\n", sum);
/*
/*
/*
/*
/*
/*
7/21/2015
Ethan Cerami, NYU
prompt */
read an integer */
prompt */
read an integer */
assignment of sum */
print sum */
18
Quick Review: Comments
/* Addition program */
Comments. Never underestimate the power of
commenting!
Your programs should always be well
commented, and should include the following:

program description
author of program
date program was created
7/21/2015
Ethan Cerami, NYU


19
#include <conio.h>


As mentioned last time, the #include
statement includes another library so
that you can reuse its functionality.
conio.h: Console Input/Output


7/21/2015
Represents another library that deals
specifically with keyboard input and output.
Contains the getch() method which we will
soon explore.
Ethan Cerami, NYU
20
Quick Review: main ()



You always need a main () function.
Program execution will always begin at
main().
Note, however, that the main() function
can exist anywhere within your
program, even at the very bottom.
7/21/2015
Ethan Cerami, NYU
21
Variable Declaration
/* Addition program */
#include <stdio.h>
#include <conio.h>
main()
{
int integer1, integer2, sum;
printf("Enter first integer\n");
scanf("%d", &integer1);
printf("Enter second integer\n");
scanf("%d", &integer2);
sum = integer1 + integer2;
printf("Sum is %d\n", sum);
7/21/2015
/*
/*
/*
/*
/*
/*
/* declaration */
prompt */
read an integer */
prompt */
read an integer */
assignment of sum */
print sum */
Ethan Cerami, NYU
22
Variable Declaration



Variable declaration:
int integer1, integer2, sum;
This statements creates three variables, integer1,
integer2, and sum. All are set to the integer data
type.
integer1, integer2, and sum can therefore only
hold whole numbers.
Note: Variable declaration must occur at the very top of
your function. (you cannot stick it somewhere in the
middle of your function!)
7/21/2015
Ethan Cerami, NYU
23
Basic Input: scanf ()
/* Addition program */
#include <stdio.h>
#include <conio.h>
main()
{
int integer1, integer2, sum;
printf("Enter first integer\n");
scanf("%d", &integer1);
/* declaration */
/* prompt */
/* read an integer */
printf("Enter second integer\n"); /* prompt */
scanf("%d", &integer2);
/* read an integer */
sum = integer1 + integer2;
/* assignment of sum */
7/21/2015
Ethan Cerami, NYU
24
Basic Input: scanf()




If you want to read in data from the user, we
use the scanf() function.
scanf() reads in data from the keyboard, and
stores it in a specific variable.
Once stored in a variable, you can manipulate
the data any way you want.
Very useful for creating interactive
applications which require input from the
user.
7/21/2015
Ethan Cerami, NYU
25
scanf() Syntax

scanf has a very specific syntax, consisting of three
parts:
scanf("%d", &integer1);
Format specifier:
indicates the kind of
data to expect. In this
case, %d stands for
digits, e.g. whole
numbers or integers.
7/21/2015
Note the & character.
Without it, your
computer may crash!
(We will return to the
exact meaning of the &
character after the
midterm.)
Ethan Cerami, NYU
Indicates where to store
the data. In this case, if
the user types 5, the
number 5 is stored in
integer1.
26
Two more tid-bits
/* Addition program */
#include <stdio.h>
#include <conio.h>
main()
{
int integer1, integer2, sum;
printf("Enter first integer\n");
scanf("%d", &integer1);
printf("Enter second integer\n");
scanf("%d", &integer2);
sum = integer1 + integer2;
printf("Sum is %d\n", sum);
}
/* declaration */
/*
/*
/*
/*
/*
/*
prompt */
read an integer */
prompt */
read an integer */
assignment of sum */
print sum */
/* Wait for user to Any Key */
getch();
return 0; /* indicate that program ended successfully */
7/21/2015
Ethan Cerami, NYU
27
getch() Statement

getchar()



getch()



Contained in stdio.h
Waits for the user to Press ENTER
Contained in conio.h
Waits for the user to Press any key
Feel free to use either one.
7/21/2015
Ethan Cerami, NYU
28
Inter-Program Communication


When you run your program in Windows,
your program runs “on top” of Windows.
For example:
Hello World Program
Windows Operating
System
7/21/2015
Ethan Cerami, NYU
29
Communicating with Windows




At the end of your program, you can tell Windows
whether your program was successful or not.
You communicate with windows via a return
statement.
Generally, return 0 indicates that everything ran just
fine. -1 indicates that an error occurred.
You will notice that all programs in the Text Book
include a return statement. But, the programs in the
packet may or may not (for this course, either option
is fine, but some compilers may require a return
statement.)
7/21/2015
Ethan Cerami, NYU
30
Return 0

return 0 indicates that your program
ended successfully.
Hello World Program
“Hello Windows, everything ran
just fine”
Windows Operating
System
7/21/2015
Ethan Cerami, NYU
31
Basic Mathematical Operators

+ -Addition / Subtraction

* Multiplication

/

% Modulus Division
7/21/2015
Integer Division
Ethan Cerami, NYU
32
Integer Division - The Problem

Suppose you have the following code:
int x;
x = 7 / 4;


Using a calculator, the answer is 1.75.
But x can only hold integer values.
1.75 is clearly not an integer value.
7/21/2015
Ethan Cerami, NYU
33
Integer Division - Solution

To understand the solution, you need to
remember your 3rd Grade Math (really.)
1
4
7
4
The answer: 1 remainder 3
3


7/4 = 1 (Integer Division)
7%4 = 3 (Modulus Division)
7/21/2015
Ethan Cerami, NYU
34
Example: Integer Division
/* Integer and Modulus Division */
#include <stdio.h>
5/10: 0
5%10: 5
main () {
int x = 5, y =10;
printf ("5/10: %d\n", x/y);
printf ("5%%10: %d\n", x%y);
getchar();
}
7/21/2015
Ethan Cerami, NYU
35
Modulus Division (cont.)

Second Example:
5/10 = 0
5%10 = 5

10
0
5
0
5
No matter what, you answers must be
integers.
7/21/2015
Ethan Cerami, NYU
36
Operator Precedence


Here’s another problem. What’s the answer to this?
x = 7 + 3 * 6;
Two Options (depending on the order of operations):



Perform addition first: 7 + 3 = 10  10 * 6 = 60
Perform multiplication first: 3*6 =18  7+18 = 25
Which option is correct? Clearly, we cannot have this
kind of ambiguity.
7/21/2015
Ethan Cerami, NYU
37
Operator Precedence



Rules for evaluating mathematical
expressions.
Every programming language has similar
rules.
From left to right:



7/21/2015
Parentheses are always evaluated first.
Multiplication, division and modulus are evaluated
next.
Addition and subtraction are evaluated last.
Ethan Cerami, NYU
38
Operator Precedence

Hence, option #2 is always correct:
x = 7 + 3 * 6;
Evaluates to x = 7 + 18 = 25
7/21/2015
Ethan Cerami, NYU
39
Float Data Types


Float Data Type: Data type that can
hold numbers with decimal values, e.g.
5.14, 3.14.
Floats can be used to represent many
values:



7/21/2015
money
long distances
weight, etc.
Ethan Cerami, NYU
40
Float Example (Program 2.2)
/* Float Example Program */
#include <stdio.h>
main ()
{
float var1, var2, var3, sum;
var1 = 87.25;
var2 = 92.50;
var3 = 96.75;
%f: indicates floating
values
%.2f displays a floating
point value with 2 decimal
points.
Output:
Sum: 276.50
sum = var1 + var2 + var3;
printf ("Sum:
%.2f", sum);
getchar();
}
7/21/2015
Ethan Cerami, NYU
41
Challenge: Find an Average

Suppose you want to determine a student’s
average.
int num = 4;
float avg = 90+92+95+100/num;

Problem #1: Operator Precedence
By rules of operator precedence, 100/4 is
evaluated first. Hence, avg is set to: 302.
 To solve this problem, use ():
float avg = (90+92+95+100)/num;

7/21/2015
Ethan Cerami, NYU
42
Finding an Average

Problem #2:




7/21/2015
90, 92, 95, 100 and 4 are all integers.
Hence, this is integer division.
Integer division can result in data
truncation (or loss of data.)
Hence, avg is set to: 94, but the students
real average is 94.25.
There are actually three ways to solve this
problem.
Ethan Cerami, NYU
43
Rules of Promotion


Promotion: when mixing ints and
floats, everything is promoted to floats.
In our average example, there are three
ways to force promotion, and get the
right answer of 94.25:
1. change num to a float:
float num = 4.0;
float avg = 90+92+95+100/num;
7/21/2015
Ethan Cerami, NYU
44
Rules of Promotion
2. Use a Cast Operator
int num = 4;
float avg = 90+92+95+100/(float)num;
In this case, num is explicitly cast to a float. And, because we
have one float, everything else is promoted. Note that you can
also use the (int) cast to cast a float to an integer.
3. Divide by 4.0
float avg = 90+92+95+100 / 4.0;
7/21/2015
Ethan Cerami, NYU
45