Control Flow - Execution Control

5.0 What This Teaches

Control flow determines which statements execute and how many times. This tutorial covers:

if, else if, else
switch and fall-through
while and do-while
for loop
Range-for over collections
break and continue

5.1 if and else

int score = 75;
if (score >= 90)      std::cout << "A\n";
else if (score >= 80) std::cout << "B\n";
else if (score >= 70) std::cout << "C\n";
else                  std::cout << "F\n";

The condition must evaluate to bool. Any non-zero integer converts to true, zero to false.

5.2 switch

int day = 3;
switch (day) {
    case 1: std::cout << "Monday\n";    break;
    case 2: std::cout << "Tuesday\n";   break;
    case 3: std::cout << "Wednesday\n"; break;
    default: std::cout << "other\n";
}

Each case must end with break or it falls through to the next case. default handles values not matched by any case. switch works on integer types and enum values only.

5.3 while and do-while

// while: condition checked before each iteration
int i = 0;
while (i < 5) {
    std::cout << i << " ";
    ++i;
}

// do-while: body executes at least once
int n = 1;
do {
    std::cout << n << " ";
    n *= 2;
} while (n <= 16);

Use while when the loop may not execute at all. Use do-while when the body must run at least once (e.g. menu prompts).

5.4 for

for (int j = 0; j < 5; ++j)
    std::cout << j * j << " ";   // 0 1 4 9 16

The three parts are init, condition, and increment, all optional. Any or all can be omitted: for (;;) is an infinite loop.

5.5 Range-for

std::vector<std::string> words = {"alpha", "beta", "gamma"};
for (const std::string& w : words)
    std::cout << w << " ";

// auto shorthand - same effect
for (const auto& w : words)
    std::cout << w << " ";

Use const auto& to avoid copies. Use auto& when you need to modify elements. Use auto (by value) only for cheap-to-copy types like int.

5.6 break and continue

for (int k = 0; k < 10; ++k) {
    if (k == 7) break;       // exit the loop entirely
    if (k % 2 == 0) continue; // skip even numbers
    std::cout << k << " ";   // prints: 1 3 5
}

break exits the innermost loop or switch. continue skips the rest of the current iteration and moves to the next one.

5.7 Example - All Together

// Control_Flow - if, switch, while, do-while, for, range-for, break, continue.

#include <iostream>
#include <vector>
#include <string>

int main() {
    // if/else
    int score = 75;
    if (score >= 90)      std::cout << "A\n";
    else if (score >= 80) std::cout << "B\n";
    else if (score >= 70) std::cout << "C\n";
    else                  std::cout << "F\n";

    // switch
    int day = 3;
    switch (day) {
        case 3: std::cout << "Wednesday\n"; break;
        default: std::cout << "other\n";
    }

    // while
    int i = 0;
    while (i < 5) { std::cout << i << " "; ++i; }
    std::cout << "\n";

    // for
    for (int j = 0; j < 5; ++j) std::cout << j * j << " ";
    std::cout << "\n";

    // range-for
    std::vector<std::string> words = {"alpha", "beta", "gamma"};
    for (const auto& w : words) std::cout << w << " ";
    std::cout << "\n";

    // break/continue
    for (int k = 0; k < 10; ++k) {
        if (k == 7) break;
        if (k % 2 == 0) continue;
        std::cout << k << " ";
    }
    std::cout << "\n";

    return 0;
}

C
Wednesday
0 1 2 3 4
0 1 4 9 16
alpha beta gamma
1 3 5

5.8 Exercise

Exercise

Write a loop that sums the integers 1 through 100 using a for loop and prints the result.
Use a while loop to print the Fibonacci sequence while values stay below 1000.
Use a range-for over a std::vector<int> to find and print the maximum value.

5.9 Common Mistakes

Missing break in switch

switch (x) {
    case 1: std::cout << "one\n";   // falls through to case 2!
    case 2: std::cout << "two\n"; break;
}

Without break, execution continues into the next case. Add [[fallthrough]]; to document intentional fall-through.

Off-by-one in for loops

for (int i = 0; i <= v.size(); ++i)   // reads one past the end
    std::cout << v[i];
// correct:
for (int i = 0; i < v.size(); ++i)

Modifying a container while range-for iterates it

Adding or removing elements from a std::vector inside a range-for invalidates iterators. Use an index-based for loop when modification is needed.

5.10 Key Terms

Term	Meaning
if / else	Conditional branch; executes one of two blocks
switch	Multi-way branch on an integer or enum value
fall-through	Execution continues into the next case when break is absent
while	Loop that checks condition before each iteration
do-while	Loop that checks condition after each iteration; runs at least once
for	Loop with init, condition, and increment in one line
range-for	Iterates over every element of a container or array
break	Exits the innermost loop or switch immediately
continue	Skips the rest of the current iteration