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CS156 Loops: Loops

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 : conditional execution
  • switch : conditional execution
  • while : repeated execution
  • do…while : repeated execution
  • for : repeated execution

Three types of loops

while (expression) {
    statement;
    …
    statement;
}

do {
    statement;
    …
    statement;
} while (expression);

for (expr1; expr2; expr3) {
    statement;
    …
    statement;
}

Control Flow: while

• The while loop allows a set of statements to be repeated as long as some condition is true.

while (condition)
        statement;
 
while (condition) {
        statement;
        …
        statement;
}

Control Flow: while

% ./a.out
1
2
3

#include <stdio.h>
int main() {
        int number = 1;
        while (number <= 3) {
                printf("%d\n", number);
                number = number + 1;
        }
        return 0;
}

Control Flow: while

What’s the difference between the two?

int number = 0;
while (number < 3) {
    number = number + 1;
    printf("%d\n", number);
}

1
2
3

int number = 1;
while (number <= 3) {
    printf("%d\n", number);
    number = number + 1;
}

1
2
3

Example 6 : Count to 10

//  Count to 10 — with off-by-1 error
#include <stdio.h>
int main() {
        int number;
        while (number < 10) {
                printf("%d\n", number);
                number = number + 1;
        }
}

• What’s the error?
• How do you fix it?
• How do you change it to count to 100?

Example 7 : Count to 10 with error

#include <stdio.h>
int main() {
    int number = 1;
    printf("1\n");
    while (number < 10) {
        printf("%d\n", number);
        number = number + 1;
    }
}

• What’s the error?
• How do you fix it?
• How do you change it to count to 100?

While loop example

Reverse a number

    #include <stdio.h>
    int main() {
        int value=0;
        while (value <= 0) {
            printf("Enter number: ");
            scanf("%d", &value);
            if (value <= 0)
                printf("Must be positive\n");
        }
        while (value != 0) {
            int r_digit = value % 10;
            printf("%d", r_digit);
            value /= 10;
        }
        printf("\n");
        return 0;
    }

Example 6 : Fibonacci Numbers

#include <stdio.h>
int main() {
    int old = 1, current = 1;
    printf("1 ");
    while (current < 100) {
        printf("%d ", current);
        int next = current + old;
        old = current;
        current = next;
    }
    return 0;
}

1 1 2 3 5 8 13 21 34 55 89 

Control Flow: Common Problems

#include <stdio.h>
int main() {
    int i=1, j;
    while (i<5) {
        printf("%d\n", i);
        j++;
    }
}

• When does it stop? When i<5.
• It’s a shame that j is incremented, not i.
• This is an infinite loop.
• Press control-C to stop it.

Control Flow: for Loops

• C provides another syntax for constructing loops that execute a fixed number of times.

for (initialize; condition; increment) {
        statement;
        …
        statement;
}

• The initialize section is used to set up a counter variable.
• The condition is an expression that says how long to run the loop.
• The increment section is used to bump the counter by some amount.

for loop example

#include <stdio.h>
int main() {
    int i;
    for (i=0; i<5; i=i+1)
        printf("i: %d\n", i);
    return 0;
}

i: 0
i: 1
i: 2
i: 3
i: 4

for loop quiz

What does this print?

#include <stdio.h>
int main() {
    for (int i=1; i<=3; i=i+1)
        printf("alpha\n");
        printf("beta\n");
    return 0;
}

for loop quiz

#include <stdio.h>
int main() {
    for (int i=1; i<=3; i=i+1)
        printf("alpha\n");
        printf("beta\n");
    return 0;
}

alpha
alpha
alpha
beta

• It doesn’t matter how you indented it—the scope of the for loop is only the first printf, just like an if statement.
• If you don’t like that, then always use braces: { }.

Equivalency of for and while

The for and while loops are equivalent in what they do:

        for (initialize; condition; increment) {
                statement;
                statement;
        }

is the same as:

        initialize;
        while (condition) {
                statement;
                statement;
                increment;
        }

Equivalency of for and while

• The counter can be modified within a for loop, but this should be avoided.
• For clarity, the for loop should only be used for loops that iterate a fixed number of times.
• Use the while loop to express loops that may execute an undetermined number of times.

What is wrong with this?

• This code should print the squares of the number from 1 to 25.
• Why doesn’t it work?

        #include <stdio.h>
        int main() {
                for (int i=1; i<=25; i=i+1);
                        printf("%d", i*i);
                return 0;
        }

Loops inside Loops

What does this print?

for (int x=1; x<=12; x++) {
    for (int y=1; y<=5; y++)
        printf("%d", x);
    printf("\n");
}

Loops inside Loops

for (int x=1; x<=12; x++) {
    for (int y=1; y<=5; y++)
        printf("%d", x);
    printf("\n");
}

11111
22222
33333
44444
55555
66666
77777
88888
99999
1010101010
1111111111
1212121212

Careful!

for (int i=0; i<5; i++) {
    printf("%d", i);
    i += 1;
}

• What’s wrong? What does it print?
• It adds 1 to i twice each time through the loop, so:

% ./a.out
0
2
4

Loop Flags

• You can use a variable to say when to quit the loop:

        int done = 0;
        while (!done) {
            // insert code here
            if (x && !y || z)    // example test
                done = 1;        // we’re done with the loop
            // more code here
        }

A Bit More Control Flow

• Sometimes we need to break out of a loop based on a secondary condition.
• break — a keyword that causes the program to continue execution after the enclosing loop
• continue — a keyword causing execution to move to the next iteration of the loop

A Bit More Control Flow

while (1) {
    printf("Enter a number: ");
    scanf("%d", &control);
    if (control == 0)
        break;
    else if (control == 1)
        continue;
    printf("Your number was: %d\n", control);
}

• If the user enters 0 the program will exit the loop.
• If the user enters 1 the program will continue with the next iteration of the loop and ask for another number.
• For any other number the program prints a message.

Another way to do it:

for (;;) {
    printf("Enter a number: ");
    scanf("%d", &control);
    if (control == 0)
        break;
    else if (control == 1)
        continue;
    printf("Your number was: %d\n", control);
}