Programming

1. Programming Variables

2. Variables • Named area in the computer memory, intended to contain values of a certain kind (integers, real numbers, characters etc.) • They contain the data your program works with

3. Type Identifier Variable Declaration • Before using a variable, one must declare it. • Variables declaration may only appear at the beginning of a block. • The declaration first introduces the variable type, then its name. • When a variable is declared, its value is undefined. int integer; float small_real; double big_real; char c1, c2;

4. Naming Rules • Letters, digits, underscores • i • CSE_5a • a_very_long_name_that_isnt_very_useful • fahrenheit • First character cannot be a digit • 5a_CSE is not valid! • Case sensitive • CSE_5a is different from cse_5a

5. Initialization Variables in Memory int my_int = 5; double my_double = 3.5; memory my_int 5 my_double 3.5

6. Variables in Memory • Whenever we write the variable name (e.g. my_int), we ask to read the value of that variable • If we write &variable_name, we ask for the address of that variable memory my_int 5 my_double 3.5

7. Example: Variable Declarations int i, j; char c; float f1 = 7.0, f2 = 5.2; double d;

8. Primitive Data Types • char – character (1 byte) • ‘a’, ‘b’, ‘c’ ..., ‘A’, ‘B’, ‘C’, ... • ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’, ‘8’, ‘9’, ‘0’ • ‘_’ ‘,’ ‘#’ ‘$’ ‘^’ ‘*’ ‘@’ ‘!’ .... • int – integer number (4 bytes). • 0, -1, 1, -2, 2, .... • float – real number (4 bytes) • 0.5, 3.2, 4.0, -5.122323, ... • double – double precision real number (8 bytes) char float int double

9. Example /* Convert cm to inches */ #include <stdio.h> void main() { double cm = 5.0, inches; inches = cm / 2.54; printf("5.0 cm are equal to %g inches\n", inches); }

10. Example /* Convert cm to inches */ #include <stdio.h> void main() { double cm = 5.0, inches; inches = cm / 2.54; printf("5.0 cm aer equal to %g inches\n", inches); } Initialization Assignment Assign the identifier on the left hand side the value of the expression on the right hand side.

11. Example /* Convert cm to inches */ #include <stdio.h> void main() { double cm = 5.0, inches; inches = cm / 2.54; printf("5.0 cm are equal to %g inches\n", inches); } What is THAT?

12. Printing Variable Values • printf – prints formatted data to the standard output (usually the screen) printf("This is equal to %g inches\n", inches); • The sequence %g is a special sequence, it is not printed! • It indicates to printf to print the value of a real variable following the printed string.

13. printf Conversion Codes • %c – a character • %d – an integer, %u – an unsigned integer. • %f – a float • %e – a float in scientific representation • %g – whichever is better between %e and %f • %lf – a double • %% - the ‘%’ character

14. Exercise • Write a program the converts 1250.25 USD into NIS. • The exchange rate is 4 NIS for USD

15. Solution Problem? #include <stdio.h> void main() { double usd = 1250.25, xrate = 4.0, nis; nis = usd * xrate; printf("%g USD = %g NIS (exchange rate = %g)\n", usd, nis, xrate); }

16. Getting Input From the User scanf("%lf", &usd); • This statement waits for the user to type in a double value, and stores it in the variable named ‘usd’. • To get 2 doubles from the user, use –scanf("%lf%lf", &usd, &xrate);

17. scanf Conversion Codes • %c – a character • %d – an integer, %u – an unsigned integer. • %f – a float • %e – a float in different representation • %lf – a double

18. Example printf/scanf void main() { int num, num1; /* Initialize the variables and display their contents. */ num = 3; num1 = 5; printf("Before scanf: num is %d and num1 is %d\n", num, num1); /* Get two values from the user and store them in the variables */ printf("Enter two numbers\n"); scanf("%d%d", &num, &num1); /* Display the content of the variables */ printf("After scanf: num is %d and num1 is %d\n", num, num1); }

19. Exercise Change your previous solution: Get the amount of USD and the exchange rate as input from the user.

20. Solution #include <stdio.h> void main() { double usd, xrate, nis; printf("Please enter amount of dollars: "); scanf("%lf", &usd); printf("Please enter the exchange rate: "); scanf("%lf", &xrate); nis = usd * xrate; printf("%g USD = %g NIS (exchange rate = %g)\n", usd, nis, xrate); }

21. Char • A char variable is used to store a text character: • Letters. • Digits. • Keyboard signs. • Non-printable characters. • But also small numbers (0 to 255 or -128 to 127).

22. Text as Numbers • Every character is assigned a numeric code. • There are different sets of codes: • ASCII (American Standard Code for Information Interchange) – most common. • EBCDIC – ancient, hardly used today. • Maybe others. • We will use ASCII

23. The ASCII Table

24. Character Encoding • Most of the time, you don't care what the particular encoding is. • The table above shows only 128 characters (7 bits). Some are non-printable. • Extended ASCII code contains 256 characters.

25. Encoding Example #include <stdio.h> void main() { char c = 'b'; printf("c as a character is %c\n", c); printf("c as an integer is %d\n", c); printf("The character after %c is %c\n", c, c + 1); } c as a character is b c as an integer is 98 The character after b is c

26. Another example /* Get the position of a letter in the abc */ #include <stdio.h> void main() { char letter; printf("Please enter a lowercase letter\n"); scanf("%c", &letter); printf("The position of this letter in the abc is %d\n", letter - 'a' + 1); }

27. Exercise Write a program that accepts as input – • A lowercase letter and outputs – • The same letter in uppercase Do not use the ASCII table directly (e.g., if the input is ‘g’, the output should be ‘G’)

28. Solution /* Convert a letter to uppercase */ #include <stdio.h> void main() { char letter; printf("Please enter a lowercase letter\n"); scanf("%c", &letter); printf("This letter in uppercase is %c\n", letter - 'a' + 'A'); }

29. Expressions • “Things” that have value and type • 1, 2.5, ‘a’ • cm, letter • We can build complex expression from simple ones using operators.

30. Arithmetic Operators • + Addition • - Subtraction • * Multiplication • / Division • % Modulo • = Assignment

31. Complex Expressions • 1 + 2 • letter + 1 • cm / 2.54 • a = b • The value of assignment expression is the assigned (right hand side) value

32. Mixing Types 1 + 0.5 cm * 3 • When operands of two different types are involved in an operation, the operand of the ‘weaker’ type is promoted to the other typechar → int → float → double. • The result of the operation has the higher type. • When the operands are of the same type, the result is of that type as well.

33. Mixing Types - Example • 3 + 4 = 7 (int + int → int) • 3.0 + 4 = 7.0 (double + int → double) • 3 / 4 = 0 (int / int → int) • 3.0 / 4 = 0.75 (double / int → double)

34. Example - • A program that sums the digits of a number with three digits. • For example: • The input 369 yields the output 18

35. Sum Digits void main() { int sum = 0, num; printf("Enter 3-digits number\n"); scanf("%d", &num); sum = sum + num % 10; num = num / 10; sum = sum + num % 10; num = num / 10; sum = sum + num % 10; printf("The sum of the digits is %d\n", sum); }

36. Exercise • Copy the above program • Run in the debugger and see how it works

37. Casting • Sometimes it is desirable for a variable of one type to be considered as belonging to another in an operation • We say the variable is castto the new type. • The casting operator is of the form: (type) • For example, (float)i casts the variable i to a float.

38. Casting Variables #include <stdio.h> void main() { int a = 1, b = 2; printf("%d / %d = %d\n", a, b, a/b); printf("%d / %d = %g\n", a, b, (float)a / b); } 1 / 2 = 0 1 / 2 = 0.5

39. Find The Problem #include <stdio.h> void main() { int a = 10; int b = 20; printf("The average of %d and %d is %d\n", a, b, (a + b) * (1 / 2)); }

40. Will This Work? #include <stdio.h> void main() { int a = 10; int b = 20; printf("The average of %d and %d is %d\n", a, b, (a + b)*(1.0 / 2)); }

41. value sign (+/-) The unsigned qualifier • Normally, the last bit of a variable serves as a sign bit. • We can use all the bits to represent the value by declaring a variable as unsigned. • To declare a variable as unsigned we add the ‘unsigned’ keyword before its type. • unsigned int; • unsigned char;

42. Unsigned Range • Char (256 different values) • signed -127..+128 • unsigned 0..+255 • Int (4294967296 different values) • signed -2147483648.. +2147483647 • unsigned 0.. +4294967295

43. Unsigned - output • When using printf We use %d for signed variables and %u for unsigned ones • void main() • { • unsigned char u = 200; • char s; • printf("%d\n", u); • printf("%u\n", u); • s = u; • printf("%d\n", s); • printf("%u\n", s); • } 200 -56 4294967240

44. Overflow • Happens when a variable gets assigned a value that is outside of its range • This is equivalent to saying that the number of bits required to encode the value exceeds the number of bits in the variable • The value of the variable will usually be corrupted

45. Overflow Example #include <stdio.h> void main() { int iA = 1000, iB = 1000000, iC = 3000000, iD = 5000000; printf ("%d * %d = %d\n", iA, iB, iA*iB); printf ("%d * %d = %d\n", iA, iC, iA*iC); printf ("%d * %d = %u\n", iA, iC, iA*iC); printf ("%d * %d = %u\n", iA, iD, iA*iD); } 1000000000 -1294967296 3000000000 705032704