C++ If-Else, Switch and Loops

Programs become useful when they can decide and repeat. A checkout needs to apply a discount only for large orders, and a report needs to visit every student in a class. Conditions and loops are the tools for both, and the same constructs power everything from small utilities to game loops.

if, else if and else

An if runs its block when the condition is true. In C++ any value that converts to bool can be a condition: zero is false and any other number is true. That flexibility is also a trap, as you will see in the mistakes section. Combine tests with && (and), || (or) and ! (not). Both && and || stop evaluating as soon as the result is known, which is called short-circuiting, and is how you write checks like if (ptr != nullptr && ptr->ready()) safely.

#include <iostream>
#include <string>
using namespace std;

int main() {
    int marks = 72;
    if (marks >= 90) {
        cout << "Grade A" << endl;
    } else if (marks >= 75) {
        cout << "Grade B" << endl;
    } else if (marks >= 60) {
        cout << "Grade C" << endl;
    } else {
        cout << "Needs improvement" << endl;
    }

    bool hasId = true;
    int age = 17;
    if (age >= 18 && hasId) {
        cout << "Entry allowed" << endl;
    } else {
        cout << "Entry denied" << endl;
    }

    string status = marks >= 40 ? "Pass" : "Fail";
    cout << status << endl;
    return 0;
}

Grade C
Entry denied
Pass

The chain is checked top to bottom and only the first matching branch runs, so 72 stops at marks >= 60. The last two lines use the conditional operator, written test ? ifTrue : ifFalse, which picks one of two values inside a single expression. Always use braces, even for one-line bodies, so that adding a second line later does not silently fall outside the if.

switch and the fall-through trap

switch compares one integer-like value, such as an int, a char or an enum, against several constant labels. It cannot switch on a std::string or a double, so use if/else for those. End every case with break unless you mean to carry on: leave it out and the program falls through, running the next label's statements as well, with no warning from the compiler.

#include <iostream>
using namespace std;

int main() {
    int day = 6;
    switch (day) {
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
            cout << "Weekday" << endl;
            break;
        case 6:
        case 7:
            cout << "Weekend" << endl;
            break;
        default:
            cout << "Invalid day" << endl;
    }

    char choice = 'r';
    switch (choice) {
        case 'b':
            cout << "Adding to cart" << endl;
            break;
        case 'r':
            cout << "Starting refund" << endl;
        case 'c':
            cout << "Cancelling order (fall-through!)" << endl;
            break;
        default:
            cout << "Unknown command" << endl;
    }
    return 0;
}

Weekend
Starting refund
Cancelling order (fall-through!)

The first switch uses fall-through on purpose, so cases 1 to 5 share one body. The second one is a bug: the 'r' case has no break, so it also runs the 'c' case. If you ever intend fall-through, mark it with the [[fallthrough]]; attribute (C++17) so readers and compilers know it is deliberate.

for, while and do-while

Reach for for when the repeat count is known up front, and for while when you are waiting on a condition to change - compounding savings until a goal is met, say. do-while is the odd one out: it tests *after* running the body, which guarantees a first pass. The range-based loop at the end of the example visits each element directly, and is the preferred way to walk a container.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    for (int i = 1; i <= 3; i++) {
        cout << "Round " << i << endl;
    }

    int balance = 1000;
    int years = 0;
    while (balance < 1500) {
        balance += balance / 10;
        years++;
    }
    cout << "Years: " << years << ", balance: " << balance << endl;

    int attempts = 0;
    do {
        attempts++;
    } while (attempts < 3);
    cout << "Attempts: " << attempts << endl;

    vector<int> marks = {45, 82, 67, 91, 30};
    int sum = 0;
    for (int m : marks) {
        sum += m;
    }
    cout << "Average: " << static_cast<double>(sum) / marks.size() << endl;

    for (int m : marks) {
        if (m < 40) {
            continue;
        }
        if (m > 90) {
            cout << "Topper found: " << m << endl;
            break;
        }
        cout << "Passed with " << m << endl;
    }

    for (int& m : marks) {
        m += 5;
    }
    for (const auto& m : marks) {
        cout << m << " ";
    }
    cout << endl;
    return 0;
}

Round 1
Round 2
Round 3
Years: 5, balance: 1610
Attempts: 3
Average: 63
Passed with 45
Passed with 82
Passed with 67
Topper found: 91
50 87 72 96 35

Follow the savings loop: 1000 grows by a tenth each pass (1100, 1210, 1331, 1464, 1610), and after five passes the balance is no longer below 1500. continue skips 30 because it is below 40, and break stops the search at 91, so nothing after it is visited.

Range-based for: copy, reference or const reference?

The loop variable in for (int m : marks) is a copy of each element, so changing it does not change the vector. To modify elements in place, take a reference: for (int& m : marks), which is how the last section added 5 grace marks. To read large elements such as strings without copying them, use a const reference: for (const auto& item : items). This one habit prevents a lot of silent bugs and wasted copies.

Common mistakes

• Using `=` instead of `==`. Unlike Java, if (marks = 90) compiles. It assigns 90 and the condition is then true. With -Wall you get warning: suggest parentheses around assignment used as truth value.
• A stray semicolon. if (x > 5); or for (...); makes the following block run unconditionally or only once. There is no error.
• Missing `break` in a switch. Execution falls into the next case, as the refund example showed.
• Switching on a string. switch (command) with a std::string fails with error: switch quantity not an integer. Use if/else if.
• Out-of-range indexing. for (int i = 0; i <= marks.size(); i++) reads one element past the end, which is undefined behavior and may crash or print garbage. Use <.
• Infinite loops. A while loop that never changes its condition variable runs forever. Stop it with Ctrl+C.

How this shows up in interviews and real projects

Beginner interview questions such as FizzBuzz, reversing an array, finding a maximum or checking for primes are all loops with a condition inside, and interviewers watch for correct boundaries and clean early exits. In production C++ you will often use standard algorithms like std::find or std::sort instead of hand-written loops, but they do exactly what these loops do. See coding interview prep for self-taught developers for how to practise.