Input, Output and Variables

1. Input, Output and Variables Ethan Cerami New York University Ethan Cerami, NYU

2. This Week • Tuesday: • Basic Output: printf() • Introduction to Variables • Integer variables • Basic Input: scanf() • Thursday: • Basic Arithmetic • Float variables • Operator Precedence • Promotion/Casting Ethan Cerami, NYU

3. 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” Ethan Cerami, NYU

4. Sample Program /* Printing multiple lines with a single printf */ #include <stdio.h> main() { printf("Welcome\nto\nC!\n"); /* Wait for user */ getchar(); } Note the \n escape sequence creates a new line character. Welcome to C! Ethan Cerami, NYU

5. Escape Characters • The \ character indicates an Escape sequence. • Examples \n New Line Character \t Tab Character \a The Alarm\Bell Character \\ Outputs a single \ Character \" Outputs a single " Character Ethan Cerami, NYU

6. 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\""); Ethan Cerami, NYU

7. 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. Ethan Cerami, NYU

8. 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. • 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

9. 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. Ethan Cerami, NYU

10. 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 Ethan Cerami, NYU

11. 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; Ethan Cerami, NYU

12. Example 1: Basic Arithmetic #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); } Variable Declaration Variable Names Data Type Assignment Statements Ethan Cerami, NYU

13. 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! Ethan Cerami, NYU

14. 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. Name of variable to print. In this case, we print the variable x. Ethan Cerami, NYU

15. 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 Ethan Cerami, NYU

16. Basic Input: scanf() • Input: any user supplied data. • Keyboard input, mouse input. • scanf (): read in keyboard input. Ethan Cerami, NYU

17. Example 2: Using scanf() /* Addition program */ #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 */ } Ethan Cerami, NYU

18. Quick Review { 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 */ /* Addition program */ #include <stdio.h> #include <conio.h> main() Ethan Cerami, NYU

19. 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 Ethan Cerami, NYU

20. #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 • Represents another library that deals specifically with keyboard input and output. • Contains the getch() method which we will soon explore. Ethan Cerami, NYU

21. 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. Ethan Cerami, NYU

22. Variable Declaration /* Addition program */ #include <stdio.h> #include <conio.h> main() { 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 */ int integer1, integer2, sum; /* declaration */ Ethan Cerami, NYU

23. 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!) Ethan Cerami, NYU

24. Basic Input: scanf () /* Addition program */ #include <stdio.h> #include <conio.h> main() { int integer1, integer2, sum; /* declaration */ printf("Enter second integer\n"); /* prompt */ scanf("%d", &integer2); /* read an integer */ sum = integer1 + integer2; /* assignment of sum */ printf("Enter first integer\n"); /* prompt */ scanf("%d", &integer1); /* read an integer */ Ethan Cerami, NYU

25. 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. Ethan Cerami, NYU

26. 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. Note the & character. Without it, your computer may crash! (We will return to the exact meaning of the & character after the midterm.) Indicates where to store the data. In this case, if the user types 5, the number 5 is stored in integer1. Ethan Cerami, NYU

27. Two more tid-bits /* Addition program */ #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 */ Ethan Cerami, NYU

28. getch() Statement • getchar() • Contained in stdio.h • Waits for the user to Press ENTER • getch() • Contained in conio.h • Waits for the user to Press any key • Feel free to use either one. Ethan Cerami, NYU

29. 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 Ethan Cerami, NYU

30. 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.) Ethan Cerami, NYU

31. Return 0 • return 0 indicates that your program ended successfully. Hello World Program “Hello Windows, everything ran just fine” Windows Operating System Ethan Cerami, NYU

32. Basic Mathematical Operators • + - Addition / Subtraction • * Multiplication • / Integer Division • % Modulus Division Ethan Cerami, NYU

33. 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; Ethan Cerami, NYU

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

35. Example: Integer Division /* 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(); } 5/10: 0 5%10: 5 Ethan Cerami, NYU

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

37. 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. Ethan Cerami, NYU

38. Operator Precedence • Rules for evaluating mathematical expressions. • Every programming language has similar rules. • From left to right: • Parentheses are always evaluated first. • Multiplication, division and modulus are evaluated next. • Addition and subtraction are evaluated last. Ethan Cerami, NYU

39. Operator Precedence • Hence, option #2 is always correct: x = 7 + 3 * 6; Evaluates to x = 7 + 18 = 25 Ethan Cerami, NYU

40. 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: • money • long distances • weight, etc. Ethan Cerami, NYU

41. 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 Ethan Cerami, NYU

42. 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; Ethan Cerami, NYU

43. Finding an Average • Problem #2: • 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

44. 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; Ethan Cerami, NYU

45. 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; Ethan Cerami, NYU