Control Flow I: if, switch, ternary

সিদ্ধান্ত গ্রহণ — if, else, switch

Read: ~25 min 12 practice problems

1. The if Statement

if.cpp

#include <iostream>
int main() {
    int n = 42;
    if (n > 0)       std::cout << "positive\n";
    else if (n < 0) std::cout << "negative\n";
    else             std::cout << "zero\n";
}

Classic bug if (x = 5) assigns 5 to x and tests truthiness — it's not a comparison! Use ==. Compile with -Wall to catch this.

2. C++17 if-init

Initialize a variable scoped to the if-block:

ifinit.cpp

#include <iostream>
#include <map>

int main() {
    std::map<std::string, int> m = {{"a", 1}, {"b", 2}};
    if (auto it = m.find("a"); it != m.end()) {
        std::cout << "Found: " << it->second << "\n";
    }
    // it is not visible here — clean!
}

3. switch — When You Have Many Cases

switch.cpp

#include <iostream>

int main() {
    int day = 3;
    switch (day) {
        case 1: std::cout << "Sun\n"; break;
        case 2: std::cout << "Mon\n"; break;
        case 3: std::cout << "Tue\n"; break;
        case 4: std::cout << "Wed\n"; break;
        default: std::cout << "unknown\n";
    }
}

Fall-through Missing break falls through to next case. Use [[fallthrough]]; (C++17) when intentional.

4. The Ternary Operator

ternary.cpp

#include <iostream>
int main() {
    int a = 7, b = 3;
    int max_val = (a > b) ? a : b;
    std::cout << "Max = " << max_val << "\n";
}

When to use ternary For simple value selection only. Multi-line logic = use full if. Nested ternaries hurt readability.

5. Boolean Idioms

Bad	Good
`if (flag == true)`	`if (flag)`
`if (flag == false)`	`if (!flag)`
`if (count != 0)`	`if (count)` (or be explicit)
`return x > 0 ? true : false;`	`return x > 0;`

6. Practice Problems

Write a program that classifies a number as positive, negative, or zero.

✨ Show Answer
See the example in section 1 above — same idea.

FizzBuzz: print 1..15. For multiples of 3 print "Fizz", multiples of 5 print "Buzz", multiples of both print "FizzBuzz".

✨ Show Answer

fizzbuzz.cpp

#include <iostream>
int main() {
    for (int i = 1; i <= 15; ++i) {
        if      (i % 15 == 0) std::cout << "FizzBuzz\n";
        else if (i %  3 == 0) std::cout << "Fizz\n";
        else if (i %  5 == 0) std::cout << "Buzz\n";
        else                 std::cout << i << "\n";
    }
}

Why must case values in switch be compile-time constants?

✨ Show Answer
The compiler often builds a jump table for fast dispatch. Constants are required so it can compute the table at compile time.
What's the bug in: if (a = 0) {...}?

✨ Show Answer
Assignment, not comparison. a becomes 0, the if is always false. Use ==.
Use the ternary to print "even" or "odd" for n=7.
✨ Show Answer
```
std::cout << (n % 2 == 0 ? "even" : "odd") << "\n";
```

Write a switch that prints the season for month numbers 1-12.

✨ Show Answer

switch (m) {
    case 12: case 1: case 2: std::cout << "winter\n"; break;
    case  3: case 4: case 5: std::cout << "spring\n"; break;
    case  6: case 7: case 8: std::cout << "summer\n"; break;
    case  9: case10: case11: std::cout << "autumn\n"; break;
    default: std::cout << "invalid\n";
}

Multiple case labels can fall into the same block intentionally.

Refactor: return x > 5 ? true : false;

✨ Show Answer
return x > 5; — the comparison already produces a bool.

Write code that checks if a year is a leap year.

✨ Show Answer

bool isLeap(int y) {
    return (y % 4 == 0 && y % 100 != 0) || (y % 400 == 0);
}

When is switch better than chained if-else?

✨ Show Answer
When testing one variable against many integer/enum constants. Compilers can generate jump tables, making it O(1) instead of O(n).
What does [[fallthrough]]; do?

✨ Show Answer
An attribute (C++17) telling the compiler "I intentionally want fall-through here, don't warn me". Place it as the last statement of a case before falling into the next.
Use if-init to safely call a function that returns -1 on error.
✨ Show Answer
```
if (int result = compute(); result != -1) {
    use(result);
}
```

Rewrite without ternary: int sign = (x > 0) ? 1 : (x < 0 ? -1 : 0);

✨ Show Answer

int sign;
if (x > 0)      sign =  1;
else if (x < 0) sign = -1;
else             sign =  0;

Summary

Branches: if/else, switch, ternary ?:. Use switch for many integer cases. if (init; cond) in C++17 scopes setup variables tightly. Always compile with -Wall to catch = vs == bugs.

Next Module → Control Flow II: Loops & Range-based for.

Previous Next: Loops