1.3. Conditionals and Loops

Table 1 shows that the syntax and semantics of if-else statements in C and Python are very similar. The main syntactic difference is that Python uses indentation to indicate "body" statements, whereas C uses curly braces (but you should still use good indentation in your C code).

Table 1. Syntax Comparison of if-else Statements in Python and C
Python version C version 
# Python if-else example


def main():


    num1 = input("Enter the 1st number:")
    num1 = int(num1)
    num2 = input("Enter the 2nd number:")
    num2 = int(num2)

    if num1 > num2:
        print("%d is biggest" % num1)
        num2 = num1
    else:
        print("%d is biggest" % num2)
        num1 = num2


# call the main function:
main()
 
/* C if-else example */
#include <stdio.h>

int main(void) {
    int num1, num2;

    printf("Enter the 1st number: ");
    scanf("%d", &num1);
    printf("Enter the 2nd number: ");
    scanf("%d", &num2);

    if (num1 > num2) {
        printf("%d is biggest\n", num1);
        num2 = num1;
    } else {
        printf("%d is biggest\n", num2);
        num1 = num2;
    }

    return 0;
}

The Python and C syntax for if-else statements is almost identical with only minor differences. In both, the else part is optional. Python and C also support multiway branching by chaining if and else if statements. The following describes the full if-else C syntax:

    // a one-way branch:
    if ( <boolean expression> ) {
        <true body>
    }

    // a two-way branch:
    if ( <boolean expression> ) {
        <true body>
    }
    else {
        <false body>
    }

    // a multibranch (chaining if-else if-...-else)
    // (has one or more 'else if' following the first if):
    if ( <boolean expression 1> ) {
        <true body>
    }
    else if ( <boolean expression  2> ) {
        // first expression is false, second is true
        <true 2 body>
    }
    else if ( <boolean expression  3> ) {
        // first and second expressions are false, third is true
        <true 3 body>
    }
    // ... more else if's ...
    else if ( <boolean expression  N> ) {
        // first N-1 expressions are false, Nth is true
        <true N body>
    }
    else { // the final else part is optional
        // if all previous expressions are false
        <false body>
    }

1.3.1. Boolean Values in C

C doesn’t provide a Boolean type with true or false values. Instead, integer values evaluate to true or false when used in conditional statements. When used in conditional expressions, any integer expression that is:

  • zero (0) evaluates to false

  • nonzero (any positive or negative value) evaluates to true

C has a set of relational and logical operators for Boolean expressions.

The relational operators take operand(s) of the same type and evaluate to zero (false) or nonzero (true). The set of relational operators are:

  • equality (==) and inequality (not equal, !=)

  • comparison operators: less than (<), less than or equal (<=), greater than (>), and greater than or equal (>=)

Here are some C code snippets showing examples of relational operators:

// assume x and y are ints, and have been assigned
// values before this point in the code

if (y < 0) {
    printf("y is negative\n");
} else if (y != 0) {
    printf("y is positive\n");
} else {
    printf("y is zero\n");
}

// set x and y to the larger of the two values
if (x >= y) {
    y = x;
} else {
    x = y;
}

C’s logical operators take integer "Boolean" operand(s) and evaluate to either zero (false) or nonzero (true). The set of logical operators are:

  • logical negation (!)

  • logical and (&&): stops evaluating at the first false expression (short-circuiting)

  • logical or (||): stops evaluating at the first true expression (short-circuiting)

C’s short-circuit logical operator evaluation stops evaluating a logical expression as soon as the result is known. For example, if the first operand to a logical and (&&) expression evaluates to false, the result of the && expression must be false. As a result, the second operand’s value need not be evaluated, and it is not evaluated.

The following is an example of conditional statements in C that use logical operators (it’s always best to use parentheses around complex Boolean expressions to make them easier to read):

if ( (x > 10) && (y >= x) ) {
    printf("y and x are both larger than 10\n");
    x = 13;
} else if ( ((-x) == 10) || (y > x) ) {
    printf("y might be bigger than x\n");
    x = y * x;
} else {
    printf("I have no idea what the relationship between x and y is\n");
}

1.3.2. Loops in C

Like Python, C supports for and while loops. Additionally, C provides do-while loops.

while Loops

The while loop syntax in C and Python is almost identical, and the behavior is the same. Table 2 shows example programs of while loops in C and Python.

Table 2. while Loop Syntax Comparison in Python and C
Python version C version 
# Python while loop example


def main():


    num = input("Enter a value: ")
    num = int(num)
    # make sure num is not negative
    if num < 0:
        num = -num

    val = 1
    while val < num:
        print("%d" % (val))
        val = val * 2


# call the main function:
main()
 
/* C while loop example */
#include <stdio.h>

int main(void) {
    int num, val;

    printf("Enter a value: ");
    scanf("%d", &num);
    // make sure num is not negative
    if (num < 0) {
        num = -num;
    }
    val = 1;
    while (val < num) {
        printf("%d\n", val);
        val = val * 2;
    }

    return 0;
}

The while loop syntax in C is very similar in Python, and both are evaluated in the same way:

while ( <boolean expression> ) {
    <true body>
}

The while loop checks the Boolean expression first and executes the body if true. In the preceding example program, the value of the val variable will be repeatedly printed in the while loop until its value is greater than the value of the num variable. If the user enters 10, the C and Python programs will print:

1
2
4
8

C also has a do-while loop that is similar to its while loop, but it executes the loop body first and then checks a condition and repeats executing the loop body for as long as the condition is true. That is, a do-while loop will always execute the loop body at least one time:

do {
    <body>
} while ( <boolean expression> );

For additional while loop examples, try these two programs:

for Loops

The for loop is different in C than it is in Python. In Python, for loops are iterations over sequences, whereas in C, for loops are more general looping constructs. Table 3 shows example programs that use for loops to print all the values between 0 and a user-provided input number:

Table 3. for Loop Syntax Comparison in Python and C
Python version C version 
# Python for loop example


def main():


    num = input("Enter a value: ")
    num = int(num)
    # make sure num is not negative
    if num < 0:
        num = -num


    for i in range(num):
        print("%d" % i)


# call the main function:
main()
 
/* C for loop example */
#include <stdio.h>

int main(void) {
    int num, i;

    printf("Enter a value: ");
    scanf("%d", &num);
    // make sure num is not negative
    if (num < 0) {
        num = -num;
    }

    for (i = 0; i < num; i++) {
        printf("%d\n", i);
    }

    return 0;
}

In this example, you can see that the C for loop syntax is quite different from the Python for loop syntax. It’s also evaluated differently.

The C for loop syntax is:

for ( <initialization>; <boolean expression>; <step> ) {
    <body>
}

The for loop evaluation rules are:

  1. Evaluate initialization one time when first entering the loop.

  2. Evaluate the boolean expression. If it’s 0 (false), drop out of the for loop (that is, the program is done repeating the loop body statements).

  3. Evaluate the statements inside the loop body.

  4. Evaluate the step expression.

  5. Repeat from step (2).

Here’s a simple example for loop to print the values 0, 1, and 2:

int i;

for (i = 0; i < 3; i++) {
    printf("%d\n", i);
}

Executing the for loop evaluation rules on the preceding loop yields the following sequence of actions:

(1) eval init: i is set to 0  (i=0)
(2) eval bool expr: i < 3 is true
(3) execute loop body: print the value of i (0)
(4) eval step: i is set to 1  (i++)
(2) eval bool expr: i < 3 is true
(3) execute loop body: print the value of i (1)
(4) eval step: i is set to 2  (i++)
(2) eval bool expr: i < 3 is true
(3) execute loop body: print the value of i (2)
(4) eval step: i is set to 3  (i++)
(2) eval bool expr: i < 3 is false, drop out of the for loop

The following program shows a more complicated for loop example (it’s also available to download). Note that just because C supports for loops with a list of statements for its initialization and step parts, it’s best to keep it simple (this example illustrates a more complicated for loop syntax, but the for loop would be easier to read and understand if it were simplified by moving the j += 10 step statement to the end of the loop body and having just a single step statement, i += 1).

/* An example of a more complex for loop which uses multiple variables.
 * (it is unusual to have for loops with multiple statements in the
 * init and step parts, but C supports it and there are times when it
 * is useful...don't go nuts with this just because you can)
 */
#include <stdio.h>

int main(void) {
    int i, j;

    for (i=0, j=0; i < 10; i+=1, j+=10) {
        printf("i+j = %d\n", i+j);
    }

    return 0;
}

// the rules for evaluating a for loop are the same no matter how
// simple or complex each part is:
// (1) evaluate the initialization statements once on the first
//     evaluation of the for loop:  i=0 and j=0
// (2) evaluate the boolean condition: i < 10
//     if false (when i is 10), drop out of the for loop
// (3) execute the statements inside the for loop body: printf
// (4) evaluate the step statements:  i += 1, j += 10
// (5) repeat, starting at step (2)

In C, for loops and while loops are equivalent in power, meaning that any while loop can be expressed as a for loop, and vice versa. The same is not true in Python, where for loops are iterations over a sequence of values. As such, they cannot express some looping behavior that the more general Python while loop can express. Indefinite loops are one example that can only be written as a while loop in Python.

Consider the following while loop in C:

int guess = 0;

while (guess != num) {
    printf("%d is not the right number\n", guess);
    printf("Enter another guess: ");
    scanf("%d", &guess);
}

This loop can be translated to an equivalent for loop in C:

int guess;

for (guess = 0; guess != num; ) {
    printf("%d is not the right number\n", guess);
    printf("Enter another guess: ");
    scanf("%d", &guess);
}

In Python, however, this type of looping behavior can be expressed only by using a while loop.

Because for and while loops are equally expressive in C, only one looping construct is needed in the language. However, for loops are a more natural language construct for definite loops (like iterating over a range of values), whereas while loops are a more natural language construct for indefinite loops (like repeating until the user enters an even number). As a result, C provides both to programmers.