C Programming

1. C Programming Variables

2. What are variables? • Our way of asking the operating system to allocate memory for our C program • Each piece of allocated memory receives a name • Our program uses this memory to save values that it wants to use later in the program

3. Memory 0 4 8 my_int 32 5 my_double 152 3.5 Variables in memory int my_int = 5; double my_double = 3.5;

4. Memory 0 4 8 my_int 32 5 my_double 152 3.5 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 my_int=5 &my_int=32 my_double=3.5 &my_double=152

5. Char is also a number! • 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).

6. 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

7. The ASCII Table

8. More about character encoding • You don't care what the particular numbers are • The table above shows only 128 characters (7 bits). Some are non-printable. • Extended ASCII code contains 256 characters.

9. More about character encoding • ASCII code 0 is important – we will see it again. • Note contiguous sets of numbers, upper case and lower case characters.

10. char as a character and a number #include <stdio.h> int 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); return 0; } c as a character is b c as an integer is 98 The character after b is c

11. Another example /* Get the position of a letter in the abc */ #include <stdio.h> int 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); return 0; }

12. Exercise Write a program that accepts as input – • A lowercase letter and outputs – • The same letter in uppercase (e.g., if the input is ‘g’, the output should be ‘G’)

13. Solution /* Convert a letter to uppercase */ #include <stdio.h> int main() { char letter; printf("Please enter a lowercase letter\n"); scanf("%c", &letter); printf("This letter in uppercase is %c\n", letter - 'a' + 'A'); return 0; }

14. Arithmetic operators • An operator is an action performed on something (e.g. constants, variables). • That “something” is called an operand. • Common operators: • Assignment = • Addition + • Subtraction - • Multiplication * • Division / • Modulo %

15. Operations with different types • When operands of two different types are involved in an operation, the operand of the ‘weaker’ type is promoted to the other type (char → int → float → double). • The result of the operation is of the higher type. • When the operands are of the same type, the result is of that type as well.

16. Operations with different types For example - • 3 + 4 = 7 • 3.0 + 4 = 7.0 • 3 / 4 = 0 !!! • 3.0 / 4 = 0.75

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

18. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 1350 9876 Arbitrary numbers (garbage)

19. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 1350 0

20. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 1350 0

21. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 1350 0

22. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 369 0

23. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 369 0

24. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 369 9

25. Let’s see how it works int main() { int sum, num; sum = 0; printf("Enter a 3-digit number\n"); scanf("%d", &num); /* extract the first digit */ sum = sum + num % 10; num = num / 10; num sum 36 9

26. Let’s see how it works /* extract the second digit */ sum = sum + num % 10; num = num / 10; /* extract the third digit */ sum = sum + num % 10; printf("The digits sum is %d\n", sum); return 0; } num sum 36 15

27. Let’s see how it works /* extract the second digit */ sum = sum + num % 10; num = num / 10; /* extract the third digit */ sum = sum + num % 10; printf("The digits sum is %d\n", sum); return 0; } num sum 3 15

28. Let’s see how it works /* extract the second digit */ sum = sum + num % 10; num = num / 10; /* extract the third digit */ sum = sum + num % 10; printf("The digits sum is %d\n", sum); return 0; } num sum 3 18

29. 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. • Casting (int)f will cast f from float to int. Meaning it will round it to the closest integer.

30. Casting variables #include <stdio.h> int 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);return 0; } 1 / 2 = 0 1 / 2 = 0.5

31. Example – find what’s wrong #include <stdio.h> int main() { int a = 10; int b = 20; printf("The average of %d and %d is %d\n", a, b, (a + b) * (1 / 2)); return 0; }

32. Will this work? #include <stdio.h> int main() { int a = 10; int b = 20; printf ("The average of %d and %d is %d\n", a, b, (a + b)*(1.0 / 2)); return 0; }

33. 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;

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

35. 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 non-sense

36. Overflow – An example #include <stdio.h> int 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); return 0; } 1000000000 -1294967296 3000000000 705032704

37. Introduction to CS Flow Control (Tests)

38. Sequential Program int main() { Statement1; Statement2; … StatementN; } S1 S2 S3 S4 S5

39. Conditional Statements • Selects statements to execute based on the value of an expression • The expression is sometimes called the controlling expression • Conditional statements: • if statement • switch statement

40. T Expr F Statement Rest of Program Conditional statements: if • used to execute conditionally a statement or block of statements. if(expression) statement; if (grade >= 60) printf("Congratulations! You passed"); printf("Your grade is %d", grade);

41. Absolute Value - Example int main() { double num; printf("Please enter a real number: "); scanf("%lf", &num); if (num < 0) num = -num; printf("The absolute value is %g\n", num); return 0; }

42. Block • A sequence of statements enclosed within curly braces {} if (grade > 60){ printf("Congratulations! You passed"); printf("Hip Hip Hooray!"); } printf("Your grade is %d", grade);

43. if-else statement F T Expr if (expression) statement1 else statement2 • if expression is true, statement1 is executed • if expression is false, statement2 is executed • both statements can be (and very often are) replaced by blocks of statements (“compound statements”) Statement2 Statement1 Rest of Program

44. An example (fragment) int first, second, min; ... if (first < second) { min = first; printf ("The first number is smaller than the second.\n"); } else { min = second; printf ("The second number is not larger than the first\n"); } printf("The minimum is %d\n", min);

45. Nested if • The statement in the if statement’s body can be an if statement if (expression){ if (expression)statement1elsestatement2 } If (a>b) { if (b>c) printf(“a is the largest \n”); else printf(“b is the smallest \n”); }

46. Where does the else belong to? if (x == y) if (y == 0) printf("x == y == 0"); else printf("x == y; x,y != 0");

47. if-else statement (cont.) if (x == y) if (y == 0) printf("x == y == 0"); else printf("x != y”); • Misleading indentation • else is associated with the closest if • use {} for clarity and to change the meaning

48. if-else statement (cont.) if (x == y) { if (y == 0) printf("x == y == 0"); } else printf("x != y");

49. else if • if statements distinguish between exactly 2 cases and execute different code in each case • The else-if construction allows for a multi-way decision

50. else if if (expression) statement else if (expression) statement elseif (expression) statement else statement