[C Series 5] Control Flow with Conditionals and Loops

Once you can calculate and compare values (Part 4), it is time to steer the program based on those results. In C, conditionals decide “what happens in this situation,” and loops decide “how many times should this happen.” This post focuses on if, else, for, and while so you can control execution paths.

New Terms In This Post

1. Condition expression: an expression like score > 80 that evaluates to true or false
2. Branch: the alternate path created by an if statement
3. Loop: syntax that repeats a block of code
4. Infinite loop: a loop without a terminating condition

Core Ideas

Study Notes

• Time required: 45–60 minutes
• Prerequisites: variables, comparison operators, and comfort with printf
• Goal: write branching and repetition patterns with conditionals and loops

Conditionals look at the current values and decide which block runs. Loops repeat work until a condition changes. Together they form the backbone of most programs—for example, “if the score is 80 or higher, print pass; otherwise encourage more study.”

We will cover five skills:

• Build branches with if, else if, and else
• Combine comparison and logical operators in condition expressions
• Repeat a known number of times with for
• Repeat while a condition is true with while
• Adjust loop flow with break and continue

Follow Along in Code

Split Paths with if

An if statement runs its block only when the condition is true.

#include <stdio.h>

int main(void) {
    int score = 88;

    if (score >= 80) {
        printf("Within the pass range.\n");
    }

    return 0;
}

score >= 80 is the condition. If it is true, the block runs; otherwise it is skipped. In a boolean context, any non-zero value is treated as true and 0 is treated as false. For now, keep using explicit comparisons like score > 80; later you will see code that uses the value directly.

Chain if, else if, else

Use chained conditionals when you have multiple cases.

#include <stdio.h>

int main(void) {
    int score = 72;

    if (score >= 90) {
        printf("Grade A.\n");
    } else if (score >= 80) {
        printf("Grade B.\n");
    } else if (score >= 70) {
        printf("Grade C.\n");
    } else {
        printf("More study required.\n");
    }

    return 0;
}

Conditions run from top to bottom. The first true block executes, and the rest are skipped, so order matters.

Comparison and Logical Operators

Build richer conditions using comparison and logical operators:

• ==, !=, >, <, >=, <=
• && (and), || (or), ! (not)

#include <stdio.h>

int main(void) {
    int age = 17;
    int has_id = 1;

    if (age >= 17 && has_id == 1) {
        printf("Entry granted.\n");
    } else {
        printf("Entry denied.\n");
    }

    return 0;
}

In many beginner examples, 1 represents true and 0 represents false, because conditions in C really are integers under the hood. Just remember that in a boolean context, any non-zero value counts as true.

Do Not Mix Up = and ==

This is the classic beginner mistake:

• = assigns a value.
• == compares two values.

int score = 80;   // assignment
if (score == 80) {
    // comparison
}

Using = in a condition assigns a value first, then evaluates that value as a condition—which is almost always a bug.

int score = 50;

if (score = 80) {
    printf("This might run unexpectedly.\n");
}

Here score becomes 80, which is non-zero, so the block runs. Watch compiler warnings to catch this early.

Repeat with for

Use a for loop when you know the iteration count. If you need to do the same thing 100 times, a loop is what keeps you from writing the same line 100 times.

#include <stdio.h>

int main(void) {
    int i;

    for (i = 1; i <= 5; i++) {
        printf("Print #%d\n", i);
    }

    return 0;
}

Read the structure as for (start; condition; update). The flow is set i = 1 -> check i <= 5 -> run body -> i++ -> check again. i++ is shorthand for i = i + 1.

Repeat with while

When the number of iterations is unknown and depends on the data, use while.

#include <stdio.h>

int main(void) {
    int countdown = 3;

    while (countdown > 0) {
        printf("%d\n", countdown);
        countdown--;
    }

    printf("Start!\n");
    return 0;
}

Always ensure something inside the loop makes the condition false eventually; otherwise you get an infinite loop.

break vs. continue

Inside loops you can fine-tune the flow:

• break: exit the loop immediately.
• continue: skip the rest of this iteration and move to the next.

#include <stdio.h>

int main(void) {
    int i;

    for (i = 1; i <= 5; i++) {
        if (i == 3) {
            continue;
        }

        if (i == 5) {
            break;
        }

        printf("i = %d\n", i);
    }

    return 0;
}

Here the loop skips printing when i == 3, and the entire loop stops when i == 5. If you later nest loops, remember that break exits only the innermost loop.

Practical Example: Count Warning Scores

#include <stdio.h>

int main(void) {
    int scores[5] = {95, 61, 48, 82, 55};
    int i;
    int warning_count = 0;

    for (i = 0; i < 5; i++) {
        if (scores[i] < 60) {
            warning_count++;
            printf("Warning score: %d\n", scores[i]);
        }
    }

    printf("Total warnings: %d\n", warning_count);
    return 0;
}

Even before diving deep into arrays, you can think of scores[i] as "the score at index i," where the first score is at index 0. The focus here is that we loop through multiple values and apply a condition to each one.

Why This Matters

Conditionals and loops give shape to your program. Without them, variables just hold values with no decision-making. Keep these five points:

• Conditionals run different code based on value states.
• Loops repeat work, either a set number of times or while a condition holds.
• Use for when the count is clear, while when the condition is the priority.
• break and continue fine-tune loop execution.
• Never confuse = (assignment) with == (comparison).

With this control-flow intuition, you are ready to package decisions and repetitions into reusable functions in the next installment.

Practice in CodeSandbox

The sandbox below uses CodeSandbox's Universal starter. For C, the key learning loop is still compile and run in the terminal, so recreate the lesson code as a source file and repeat that cycle directly.

Practice

• Follow: run the if, for, and while examples and note the execution order.
• Extend: modify the grading example to produce four ranges: >= 90, >= 80, >= 60, < 60.
• Debug: purposely write if (score = 80) and explain why it behaves differently.
• Completion check: describe if, for, while, break, and continue in one sentence each and write a simple branching + looping example yourself.

Wrap-Up

This post showed how to redirect execution with conditionals and loops. Focus on reading “when does the flow branch and when does it repeat?” rather than memorizing shapes. Next we will make these flows cleaner by grouping logic into functions.