Difference between Expression and Statement in C

In C programming, expressions and statements are two fundamental concepts, but they serve different purposes.

What is an Expression in C?

An expression in C is any valid combination of variables, constants, operators, and function calls that produces a value.

Think of it like a math equation. You’re calculating something and expecting a result.

Example:

a + b

This is an expression. It adds the value of a and b and gives you a result. But on its own, it doesn’t do anything unless you use it in a statement.

x = 5    // Here, 'x = 5' is also an expression. It assigns 5 to x and returns 5.
x * y + 10  // Multiplies x and y, adds 10.

Expressions are used inside statements.

What is a Statement in C?

A statement in C is a complete instruction that tells the computer to perform an action. It ends with a semicolon (;).

In simple words, a statement is like a command or task.

Example:

x = a + b;

This is a statement. It tells the computer to add a and b, and store the result in x.

Types of Statements:

• Expression statement – like x = 5;
• Declaration statement – like int a;
• Control statement – like if, while, for, etc.
• Compound statement – a group of statements inside { }

Key Differences Between Expression and Statement in C

Here’s a comparison table highlighting the key differences between Expression and Statement in C.

Simple Code Example in C

#include <stdio.h>
 
int main() {
    int a = 10, b = 20;
    int sum;
 
    sum = a + b;      // 'a + b' is an expression, 'sum = a + b;' is a statement
    printf("%d\n", sum); // This is another statement
 
    return 0;
}

This C program adds two numbers and prints the result. It starts by including the standard input-output library. Inside the main() function, two integers a and b are given values 10 and 20. Their sum is stored in the variable sum. The printf() function then displays the sum, which is 30. The program ends with return 0;, showing that it ran successfully.

Side Effects, Expression Statements, and Null Statements in C

1. Side Effects in Expressions:

Some expressions, like assignment (x = 5), can have side effects. An expression that changes the state of variables is a typical side effect. For example:

x = 5;  // Expression with a side effect (it changes the value of x)

2. Expression Statements:

Expressions can be used as statements on their own. For example, a function call is an expression but can be used as a statement:

printf("Hello, World!");  // Expression statement

3. Null Statement:

The null statement (a semicolon ; alone) is a valid statement in C, but it does nothing. It’s typically used when a statement is syntactically required, but no action is needed:

for (int i = 0; i < 10; i++)
    ;  // Null statement, does nothing but is syntactically correct

4. Common Mistakes:

A typical mistake is confusing an expression with a statement, especially when semicolons are missing or incorrectly placed.

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