6. Loops & Jump Statements

Loops in C

Loops are used to execute a block of code repeatedly as long as a specified condition is true. Loops are divided into two categories:

A. Entry-Controlled Loops

In these loops, the condition is checked before executing the loop body. If the condition is initially false, the loop body doesn't execute at all. (Examples: for loop, while loop).

1. while loop

A basic entry-controlled loop. Best used when the number of iterations is not known beforehand.

Syntax:

while (condition) {
    // Loop body
    // increment/decrement
}

Example:

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

2. for loop

A compact entry-controlled loop where initialization, condition, and increment/decrement happen in one line. Best used when the number of iterations is known exactly.

Syntax:

for (initialization; condition; increment/decrement) {
    // Loop body
}

Example:

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

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B. Exit-Controlled Loops

In these loops, the condition is checked after the execution of the loop body. This guarantees that the loop body will execute at least once, even if the condition is false. (Example: do-while loop).

3. do-while loop

Syntax:

do {
    // Loop body
    // increment/decrement
} while (condition); // Notice the semicolon here!

Example:

int i = 10;
do {
    printf("%d ", i); // Will output 10 once, then condition fails
    i++;
} while (i <= 5);

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Jump Statements

Jump statements alter the normal flow of the program by transferring control to another part of the code unconditionally.

1. break statement

Used to immediately exit the innermost enclosing loop or switch case.

Example:

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break; // Loop stops when i is 5
    }
    printf("%d ", i); // Prints 1 2 3 4
}

2. continue statement

Skips the current iteration of the loop and jumps directly to the next iteration's condition/increment check.

Example:

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue; // Skips printing 3
    }
    printf("%d ", i); // Prints 1 2 4 5
}

3. goto statement

Allows an unconditional jump to a labeled statement within the same function. It is generally discouraged in modern programming as it makes code hard to trace (Spaghetti code).

Syntax:

goto label_name;

label_name:
    // statements

Example:

int i = 1;
loop_start:  // A label
    if (i <= 5) {
        printf("%d ", i);
        i++;
        goto loop_start; // Jumps back up
    }