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CS156 Control Structures

Control Flow

• Often, we wish to execute parts of a program repeatedly or only when some condition is true.
• There are special constructs in C for handling this type of behavior
  • if/else statement — conditional execution
  • switch statement — conditional execution
  • while loop — repeated execution
  • do…while loop — repeated execution
  • for loop — repeated execution

Control Flow: if-else

• Conditional execution is achieved with the if/else construction:

if (condition)
    statement;      // Do this if the condition is true

if (condition)
    statement;      // Do this if the condition is true
else
    statement;      // Do this if the condition is false

• The else statement; part is optional.
• A statement can be
  • a simple statement, like x=3 or printf,
  • another if/else,
  • or a sequence of statements enclosed in { }.

Control Flow: Conditions

• We have seen that an if statement uses a condition to determine whether certain statements are executed.
• C doesn’t really have a boolean (true/false) type, so we fake it with integers.
• A true condition is any expression with a non-zero value (e.g. 1, 15, -12, 92.34).
• A false condition is any expression with a zero value.
• C has many operators which are useful in forming conditional expressions.

Control Flow: Conditional Operators

Happens first

( )

* / %

+ -

> <= >=

== !=

!

&&

||

=

Happens last

Operation	Meaning
a == b	a is equal to b
a != b	a is not equal to b
a < b	a is less than b
a > b	a is greater than b
a <= b	a is less than or equal to b
a >= b	a is greater than or equal to b
!a	true if a is false, false if a is true
a && b	true if a and b are both true
a || b	true if a or b is true

Logical Operators

• zero: false
• Any non-zero value: true

x	!x
false	true
true	false

x	y	x && y	x || y
false	false	false	false
false	true	false	true
true	false	false	true
true	true	true	true

Example

What is the output of the following program?

int x=5, y=0;
printf("x=%d\n", x);
printf("y=%d\n", y);
printf("!x=%d\n", !x);
printf("!y=%d\n", !y);
printf("x||y=%d\n", (x||y));
printf("x&&y=%d\n", (x&&y));

x=5
y=0
!x=0
!y=1
x||y=1
x&&y=0

Example 4

printf("1 < 2 = %d\n", 1 < 2);
printf("2 >= 4 = %d\n", 2 >= 4);
printf("1<2 && 2<-3 = %d\n", 1<2 && 2<-3);
printf("1<2 || 2<-3 = %d\n", 1<2 || 2<-3);

1 < 2 = 1
2 >= 4 = 0
1<2 && 2<-3 = 0
1<2 || 2<-3 = 1

Example 5

int a=2, b=4;
if (a < b)
    printf("Good\n");
else
    printf("Bad\n");

Good

Control Flow: Common Problems

• Mismatched if-else — an else statement is matched to the closest if in the same block.

if (condition)                    if (condition) {
    if (condition2)                   if (condition2)
        statement;                      statement;
else                              }
    statement;                    else
                                      statement;

Control Flow: Common Problems

#include <stdio.h>
int main() {
   int x,y,z; char smallest;
   printf("Enter x: ");
   scanf("%d", &x);
   printf("Enter y: ");
   scanf("%d", &y);
   printf("Enter z: ");
   scanf("%d", &z);
   if (x < y)
        if (x < z)
            smallest = 'x';
   else if (y < z)
            smallest = 'y';
   else 
            smallest = 'z';
    printf("%c is smallest\n", smallest);
    return 0;
}

• Example: find the smallest number of three numbers: x, y and z.
• What’s the error? How do you fix it?

Comparing characters

You can compare characters to each other:

    char grade='C';
    if (grade > 'C')          // failing grade?
        printf("You'll be back!\n");

Order of characters

• You may assume:
  • 'A' < 'B' < 'C' < … < 'X' < 'Y' < 'Z'
  • 'a' < 'b' < 'c' < … < 'x' < 'y' < 'z'
  • '0' < '1' < '2' < … < '7' < '8' < '9'
• Do not assume that 'A' == 'a'. They’re different.
• It’s not sensible to ask if 'a' > 'D', or if '@' < '$'.
• Do not assume that 4 == '4'. 4 is an int. '4' is a char.

Example - characters

Need to handle reading in the enter key!

    #include <stdio.h>
    int main() {
        char letter, enter;
        printf("Enter a letter: ");
        scanf("%c%c", &letter, &enter);
        while (letter != '.') {
            if (letter >= 'a' && letter <= 'z')
                printf("%c is LOWER case\n", letter);
            else if (letter >= 'A' && letter <= 'Z')
                printf("%c is UPPER case\n", letter);
            else
                printf("%c is not in my alphabet.\n", letter);
            printf("Enter another letter (or . to quit): ");
            scanf("%c%c", &letter, &enter);
        }
        return 0;
    }

No shortcuts

Here are some things that you’d think might work, but they don’t:

Wrong	Right
if (x==1||2)	if (x==1 || x==2)
if (a<b<c)	if (a<b && b<c)

The old switch-er-roo!

The switch construct lets you pick one from a number of alternatives:

    switch (some value) {
        case this value: 
            statements;
            break;
        case that other value: 
            statements;
            break;
        default: 
            statements;
    }

The value must be an int or char, or an expression, e.g., j+2, or toupper(c).

The Switch Statement

A switch statement makes a multi-way decision:

int year;
scanf("%d", &year);
switch (year) {
    case 1957: printf("Finest year in all history\n");  break;
    case 1969: printf("One small step for [a] man\n");  break;
    case 2003: printf("Punks born in this year\n");     break;
    case 2004: printf("Slackers born in this year\n");  break;
    case 2005: printf("Clowns to the left of me\n");    break;
    case 2006: printf("Jokers to the right\n");         break;
    case 2025: printf("Welcome to the FUTURE!!!\n");    break;
    default:   printf("Boring year\n");                 break;
}

Switching on chars

char grade = 'A';
switch (grade) {
    case 'A': printf("Good Job!\n");  break;
    case 'B': printf("Also Good.\n"); break;
    default : printf("Study more.\n");
}

Good Job!

char grade = 'C';
switch (grade) {
    case 'A': printf("Good Job!\n");  break;
    case 'B': printf("Also Good.\n"); break;
    default : printf("Study more.\n");
}

Study more.

Break is important

char grade = 'A';
switch (grade) {
    case 'A': printf("Good Job!\n");
    case 'B': printf("Also Good.\n");
    default : printf("Study more.\n");
}

Good Job!
Also Good.
Study more.

The break statement is important. Without it, execution would continue through the statements of the switch until the end.

The Switch Statement : Example: vowels.c

#include <stdio.h>
int main() {
   char letter, enter;
   printf("Enter a character: ");
   scanf("%c%c", &letter, &enter);
   switch (letter) {
      case 'a': case 'A': case 'e': case 'E':
      case 'i': case 'I': case 'o': case 'O':
      case 'u': case 'U': 
         printf("A vowel: %c\n", letter);
         break;
      default:
         printf("Not a vowel: %c\n", letter);
   }
   return 0;
}

Sometimes we want to leave out the break statement.

Careful!

• Dangling else statements are easily created and difficult to debug. Use squigglys to clarify, even if not necessary:

if (a > b)
    if (b > c)
        min = c;
else
    min = a;

Careful!

• Dangling else statements are easily created and difficult to debug. Use squigglys to clarify, even if not necessary
• Fix:

if (a > b) {
    if (b > c)
        min = c;
}
else
    min = a;

Careful!

int a = 5;
int b = 9;
if (a = b)
    val = b;

• Watch out for side effects!
• What does this code do?

Careful!

int a = 5;
int b = 9;
if (a = b)      // Perhaps a == b would be better
    val = b;

Look at that if statement carefully: that’s =, not ==. That will:

• Assign b to a.
• a is now 9.
• Is 9 true or false?
• It’s non-zero, hence true.
• Therefore, val = b is executed.

Careful!

• Watch out for ; at end of if
• This compiles just fine—what does it do?

int x=3;
if (x >= 4);
    printf("Hello\n");

Hello

Careful!

int x=3;
if (x >= 4);
    printf("Hello\n");
// Is just like this:
int x=3;
if (x >= 4)
    /* Nothing */;
printf("Hello\n");

• That extra semicolon is a whole statement.
• The fact that you indented the printf is great, but it doesn’t impress the compiler at all. The compiler ignores indentation and looks at semicolons.
• You still have to indent your programs, however.

Careful!

Remember that scanf requires the ampersand, but printf forbids it.

To illustrate the difference:

int x=11, y=22, z=33;
printf("%d %d %d\n", x, y, z);
printf("%p %p %p\n", &x, &y, &z);

11 22 33
0x7ffce4b75f7c 0x7ffce4b75f78 0x7ffce4b75f74

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