Basic Input/Outputand Variables

Ethan Cerami

New York University

@1998

This Week

• Quick Review

• Integer Variables

• Basic Input: scanf()

• Float Variables

• Mathematical Operators

Review: First Program in C

/* First Program in C */#include <stdio.h>

main(){ printf("Hello, World!");

/* Wait for user to Press Enter */ getchar();}

CommentsInclude Library Files

Main Function

Pauses Program

printf () Statement

printf () Escape Characters

• \ is an escape character reserved for special characters such as new line

and tab.

Examples: \n insert a newline \t tab \a bell sound \" double quote other escape characters listed on page 26 of text.

III: Variables

• Variable: a small piece or “chunk” of data– Could be a number (5, PI), or someone's name or

someone's address.

– Could be your bank account balance.

– Could be a list of the courses you have registered for this semester.

Data Types

• Every variable must have both a data type and a name.– Data type: defines the kind of data the variable can

hold

– Can this variable hold numbers? can it hold text?

• The “Bucket” Analogy

Integer Data Type

• Integer: data type that can hold only whole numbers, e.g. 5, 76, 1000.

• Cannot hold numbers with decimal points, such as 4.2 or 3.14. Cannot hold strings, such as “Hello” or “Ethan”

• Example:

int x = 5;

Integer Program (Program 2.0)#include <stdio.h>

main () {int x, y, z;

x = 5;y = 10;z = x + y;

printf ("x: %d\n", x);printf ("y: %d\n", y);printf ("z: %d\n", z);

/* Wait for user to Press Enter */ getchar();}

Variable Declaration

Data Type Variable Names

Assignment Statements

Printing Variables

To print integer variables: format specifier: indicates the data type. %d: print integer values you must include both a format specifier and a

variable name.

• Example:printf ("x: %d\n", x);

Format Specifier

Variable Name

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

C is case sensitive, e.g. variable1 VARIABLE1

V. Basic Input: scanf()

• Input: any user supplied data.– Keyboard input, mouse input.

• scanf (): read in keyboard input.

scanf() Example (Program 1.2)#include <stdio.h>#include <conio.h>

main(){ int integer1, integer2, sum; /* declaration */

printf("Enter first integer\n"); /* prompt */ scanf("%d", &integer1); /* read an integer */ printf("Enter second integer\n"); /* prompt */ scanf("%d", &integer2); /* read an integer */ sum = integer1 + integer2; /* assignment of sum */ printf("Sum is %d\n", sum); /* print sum */

/* Wait for user to Any Key */ getch();

return 0; /* indicate that program ended successfully */}

Variable Declaration

Using scanf()

scanf ("%d", &integer1);

%d indicates integer values (just like printf)

Format SpecifierVariable Name

& Character is required

Other tid-bits• getch();

Waits for user to press any key. getch() requires the #include <conio.h> library <conio.h>: Console Input/Output similar to getchar(), but getchar() requires that the user

press the ENTER key.

return 0; Returns the value 0 to the operating system. Tells the operating system that everything went well. Anything other than 0 indicates an error.

VI. Mathematical Operators

• Basic Mathematical Operators

+ - Addition / Subtraction

* Multiplication

/ Integer Division

% Modulus Division

Integer Division - The Problem

• Suppose you have the following code:

• Using a calculator, the answer is 1.75.

• But x can only hold integer values. 1.75 is clearly not an integer value.

int x;x = 7 / 4;

Integer Division - The Solution

• To understand the solution, you need to remember your 3rd Grade Math (really.)

• 7/4 = 1 (Integer Division)

• 7%4 = 3 (Modulus Division)

4 74

3

1

The answer: 1 remainder 3

Integer/Modulus Division (Program 2.1)

/* Integer and Modulus Division */#include <stdio.h>

main () {int x = 5, y =10;

printf ("5/10: %d\n", x/y); printf ("5%%10: %d\n", x%y); getchar();}

Note: %% will print a single %5/10: 05%10: 5 Output

Modulus Division (cont)

• Second Example:

• No matter what, you answers must be integers.

5/10 = 0

5%10 = 5

5100

0

5

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? Cleary, we cannot have this kind of ambiguity.

Operator Precedence

• Rules for evaluating mathematical expressions.

• From left to right:– Parentheses are always evaluated first.

– Multiplication, division and modulus are evaluated next.

– Addition and subtraction are evaluated last.

Operator Precedence

• Hence, option #2 is correct:

• To find a student’s grade average, what’s wrong with this?

• When in doubt, use parentheses.

x = 7 + 3 * 6;Evaluates to x = 7 + 18 = 25

avg = 90 + 95 + 95 + 100 / 4

Float Variables

• Float Data Type: Data type that can hold numbers with decimal values, e.g. 5.14, 3.14.

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;

sum = var1 + var2 + var3;

printf ("Sum: %.2f", sum);

getchar();}

%f: indicates floating values

%.2f displays a floating point value with 2 decimal points.

Output:

Sum: 276.50