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Basic Input/Output and Variables Ethan Cerami New York University @1998

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This Week Quick Review Integer Variables Basic Input: scanf() Float Variables Mathematical Operators

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Review: First Program in C /* First Program in C */ #include main() { printf("Hello, World!"); /* Wait for user to Press Enter */ getchar(); } Comments Include Library Files Main Function Pauses Program printf () Statement

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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.

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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.

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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

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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;

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Integer Program (Program 2.0) #include 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 TypeVariable Names Assignment Statements

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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

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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

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V. Basic Input: scanf() Input: any user supplied data. –Keyboard input, mouse input. scanf (): read in keyboard input.

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scanf() Example (Program 1.2) #include 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

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Using scanf() scanf ("%d", &integer1); %d indicates integer values (just like printf) Format Specifier Variable Name & Character is required

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Other tid-bits getch(); Waits for user to press any key. getch() requires the #include library : 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.

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VI. Mathematical Operators Basic Mathematical Operators + -Addition / Subtraction * Multiplication /Integer Division % Modulus Division

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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;

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Integer Division - The Solution To understand the solution, you need to remember your 3 rd Grade Math (really.) 7/4 = 1 (Integer Division) 7%4 = 3 (Modulus Division) 47 4 3 1 The answer: 1 remainder 3

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Integer/Modulus Division (Program 2.1) /* Integer and Modulus Division */ #include 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: 0 5%10: 5 Output

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Modulus Division (cont) Second Example: No matter what, you answers must be integers. 5/10 = 0 5%10 = 5 510 0 0 5

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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.

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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.

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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

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

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Float Example (Program 2.2) /* Float Example Program */ #include 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

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