10.28 Logic Loops Conditions

intro-to-computer-science-gtiit-03-2024.pdf

Logic Operators

Logic operators allow us to combine conditions:

EXPR || EXPR: logical 'or' of two expressions
EXPR && EXPR: logical 'and' of two expressions

These conditions can be used in the context of conditional statements and loops.

Logic operators have less priority than the comparison operators, so you can write the following without parentheses:

a == 10 || b >= 5 || c < 50
a == 10 && b >= 5 && c < 50

Interval Conditions

To express that some expression EXPR's value belongs to an interval [a, b], you need to write two comparison expressions:

EXPR is greater than or equal to a: EXPR >= a
EXPR is less than or equal to b: EXPR <= b

Then you combine both into a single condition with the logical 'and' operator:

EXPR >= a && EXPR <= b

Expression (a <= EXPR <= b) is syntactically correct in C, but it does not have the meaning one usually associates with such interval conditions.

Example

main() {
    int age;
    scanf("%d", &age);

    // check if user age belongs to range [18,60]
    if (age >= 18 && age <= 60)
        printf("You can apply to this job!\n");
    else
        printf("Sorry, you are too young or too old.\n");
}

Check if some value belongs to a set of values

If the set of options is very small, you can enumerate its elements and check whether a given value is equal to the first one, or equal to the second one, etc.

Example:

main() {
    int age;
    scanf("%d", &age);

    // check if user age belongs to set {20,30,40}
    if (age == 20 || age == 30 || age == 40)
        printf("We have a special gift for you!\n");
}

The while statement

while (CONDITION)
    [STATEMENT]
    [NEXT STATEMENTS]
...

while (CONDITION) {
    [STATEMENTS]
}
[NEXT STATEMENTS]
...

while statement example

main() {
    int count;
    printf("Please enter a number to do a countdown.\n");
    scanf("%d", &count);

    while (count >= 0) {
        printf("%d.\n", count);
        count = count - 1;
    }
}

While Statement: Things to Care About

A while statement repeats a sequence of statements (the "body" of the while loop) while some condition is true.
It stops repeating when the condition is false.
Condition is checked when the while statement is first reached, and after the whole body is executed, before it is repeated again.
If you write a while statement and the condition is always true, your program will never stop; then you need to use CTRL+c to terminate it (or use the kill command).

Manual Execution with while

int i = 0;
int a = 0;
while (i < 4) {
    if (i % 2 == 0)
        a = a + 1;
    else
        a = a - 1;
    i = i + 1;
}

i	a	i < 4	i % 2 == 0
0	-	-	-
0	0	-	-
0	0	true	-
0	0	-	true
0	1	-	-
1	1	-	-
1	1	true	-
1	1	-	false
1	0	-	-
2	0	-	-
2	0	true	-
2	0	-	true
2	1	-	-
3	1	-	-
3	1	true	-
3	1	-	false
3	0	-	-
4	0	-	-
4	0	false	-

How Conditions are Evaluated

The C language uses integers for boolean testing. What does this mean?
0 represents "false", and 1 represents "true".
Example: (1 > 0) evaluates to 1
Example: (1 != 1) has a value of 0
Consider (a <= EXPR <= b)
Read as ((a <= EXPR) <= b), the inner expression is evaluated first as a boolean value (0 or 1).
Then, evaluation continues with either (0 <= b) or (1 <= b).
This means something, but probably not what you want!

How Conditions are Evaluated (continued)

In a condition, 0 is interpreted as false and anything non-zero is interpreted as true.
That is: if (0) is always false, if (1) is always true, if (-2) too, etc.
Same for while (1).
Use gcc -Wall to detect possible problems (tcc is not good enough).

Conditions have values, continued

Let us read the manual of GCC (man gcc, does not work in JSLinux):

GCC options on the command line

GCC's most important options for us are:

-o name: specifies a name for the executable file generated; by default, it’s a.out
-Wall: enable all warnings detection
-Werror: make all warnings into errors

You can combine all of them:

gcc main.c -o program -Wall -Werror

TCCs

The TCC compiler has a "compile and execute" mode activated with the -run flag.
Useful in the typical case in which we want to quickly check the behavior of our program.
Usually, we do not care about warnings in that case, because we just want to see the program's output, except if there is an error. Hence we can use the -w flag to suppress any warning:

tcc -w -run main.c

Another Useful Warning

Another common error is to confuse assignment (=) with equality testing (==). For instance, the following program compiles but does not do what one would intuitively expect:

main() {
    int a = 10;
    if (a = 20)
        printf("You should not see this.\n");
}

This is because assignments a = b are expressions that also have an associated value.

The if statement takes assignment a = 20 (value 20, non-zero) as a true condition!

The Logical NOT Operator

Aside from the binary logical operators || and && ("or" and "and"), we also have the "not" operator, which changes the truth value of an expression:

if (!(a > b)) ...

It is very useful to express while loop conditions as a negated "until" condition:

while ( !(a == 0) )  // iterate until a == 0

The do-while loop

The do-while statement is similar to the while statement, except that the loop body is executed before the condition is checked for the first time.

Syntax:

do {
    STATEMENT
} while (CONDITION);

Using do-while to validate user input

You can put a scanf() statement in the do-while loop body and repeat it until the input value fulfills some condition.
For instance, asking for a non-negative integer:

do {
    scanf("%d", &x);
} while (!(x >= 0));  // repeat until x is non-negative

Equivalence

do {
    do_work();
} while (condition);

is equivalent to:

do_work();
while (condition) {
    do_work();
}

Remarks

Do-while loops are sometimes useful if you want the code to output some sort of menu to a screen so that the menu is guaranteed to show once.
If there is no need to use a do-while loop, then a regular while loop is preferred.

While loop examples

The following program uses a while loop to repeat a message a number of times:

#include <stdio.h>

int main() {
    int i = 0;
    while (i < 10) {
        printf("tick...\n");
        i = i + 1;
    }
    printf("BOOOOOM!\n");
}

While loop examples

Let's compute an integer summation using a while loop:

#include <stdio.h>

int main() {
    int i = 0;
    int sum = 0;
    int n;
    scanf("%d", &n);
    while (i <= n) {
        sum = sum + i;
        i = i + 1;
    }
    printf("The sum from i = 0 to %d of i is %d\n", n, sum);
}

While loop examples

Let's compute the average of the first n even (natural) numbers:

#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    int i = 0;
    int sum = 0;
    while (i < n) {
        sum = sum + 2 * i;
        i = i + 1;
    }
    printf("%d\n", sum / n);
}

While loop examples

Let's write a more challenging program to find the least common multiple:

#include <stdio.h>

int main() {
    int n, m;
    scanf("%d", &n);
    scanf("%d", &m);
    int i = n;
    while ((i % n != 0) || (i % m != 0)) {
        i = i + 1;
    }
    printf("least common multiple of %d and %d is: %d\n", n, m, i);
}

Final remarks: Recommendations for Loops

Make the loop condition clear, with regards to the specification of the problem.
If the problem specifies an amount of repeats of the loop, make the loop condition refer to the repeats.
If the problem is given as a bound condition, put the bound condition in the loop condition.
In general, try to make your code the most transparent possible translation of the problem specification. Avoid simplifying conditions in an effort to make your program more efficient.
Program readability is an important issue too!

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