The C Language Basics - C Interview Questions and Answers

1. What is the difference between ++var and var++?

The ++ operator is called the increment operator. When the operator is placed before the variable (++var), the variable is incremented by 1 before it is used in the expression. When the operator is placed after the variable (var++), the expression is evaluated, and then the variable is incremented by 1.

The same holds true for the decrement operator (--). When the operator is placed before the variable, you are said to have a prefix operation. When the operator is placed after the variable, you are said to have a postfix operation.

For instance, consider the following example of postfix incrementation:

int x, y;
x = 1;
y = (x++ * 5);

In this example, postfix incrementation is used, and x is not incremented until after the evaluation of the expression is done. Therefore, y evaluates to 1 times 5, or 5. After the evaluation, x is incremented to 2.

Now look at an example using prefix incrementation:

int x, y;
x = 1;
y = (++x * 5);

This example is the same as the first one, except that this example uses prefix incrementation rather than postfix. Therefore, x is incremented before the expression is evaluated, making it 2. Hence, y evaluates to 2 times 5, or 10.

2. What does the modulus operator do?

The modulus operator (%) gives the remainder of two divided numbers. For instance, consider the following portion of code:

x = 15/7

If x were an integer, the resulting value of x would be 2. However, consider what would happen if you were to apply the modulus operator to the same equation:

x = 15%7

The result of this expression would be the remainder of 15 divided by 7, or 1. This is to say that 15 divided by 7 is 2 with a remainder of 1.

The modulus operator is commonly used to determine whether one number is evenly divisible into another. For instance, if you wanted to print every third letter of the alphabet, you would use the following code:

int x;
for (x=1; x<=26; x++)
if ((x%3) == 0)
printf("%c", x+64);

The preceding example would output the string "cfilorux", which represents every third letter in the alphabet.

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