Foundations

Conditionals

An if statement lets C++ choose between branches.

if statement An `if` statement runs one block when its condition is true and can use `else` for the other case.

Conditionals

temperature
conditionals.cpp
Replay: real traced execution (multi-file project)
#include <iostream>
#include <string>

int main() {
    int temperature = 72;
    std::string status = "";

    if (temperature >= 80) {
        status = "warm";
    } else {
        status = "comfortable";
    }

    std::cout << "temperature=" << temperature << std::endl;
    std::cout << "status=" << status << std::endl;
    return 0;
}

#include <iostream>
#include <string>

int main() {
    int temperature = 55;
    std::string status = "";

    if (temperature >= 80) {
        status = "warm";
    } else {
        status = "comfortable";
    }

    std::cout << "temperature=" << temperature << std::endl;
    std::cout << "status=" << status << std::endl;
    return 0;
}

#include <iostream>
#include <string>

int main() {
    int temperature = 90;
    std::string status = "";

    if (temperature >= 80) {
        status = "warm";
    } else {
        status = "comfortable";
    }

    std::cout << "temperature=" << temperature << std::endl;
    std::cout << "status=" << status << std::endl;
    return 0;
}

  1. temperature ← 72, status ← (empty)

    4int main() {5    int temperature→ 72 = 72; //@temperature=55, 906    std::string status→ (empty) = "";

  2. status ← comfortable

    9    status = "warm";10} else {11    status→ comfortable = "comfortable";12}

  3. std::cout << "temperature=" << temperature << std::endl;

    14    std::cout << "temperature=" << temperature72 << std::endl;15    std::cout << "status=" << statuscomfortable << std::endl;16    return 0;17}
    outputtemperature=72
status=comfortable

  1. temperature ← 55, status ← (empty)

    4int main() {5    int temperature→ 55 = 55;6    std::string status→ (empty) = "";

  2. status ← comfortable

    9    status = "warm";10} else {11    status→ comfortable = "comfortable";12}

  3. std::cout << "temperature=" << temperature << std::endl;

    14    std::cout << "temperature=" << temperature55 << std::endl;15    std::cout << "status=" << statuscomfortable << std::endl;16    return 0;17}
    outputtemperature=55
status=comfortable

  1. temperature ← 90, status ← (empty)

    4int main() {5    int temperature→ 90 = 90;6    std::string status→ (empty) = "";

  2. status ← warm

    8if (temperature90 >= 80) {9    status→ warm = "warm";10} else {

  3. std::cout << "temperature=" << temperature << std::endl;

    14    std::cout << "temperature=" << temperature90 << std::endl;15    std::cout << "status=" << statuswarm << std::endl;16    return 0;17}
    outputtemperature=90
status=warm

Follow the Branch

  1. temperature starts at 72.
  2. status starts as an empty string.
  3. C++ checks whether temperature >= 80.
  4. 72 is below 80, so the else branch sets status to comfortable.
  5. The program prints temperature=72 and status=comfortable. | temperature | check | status | | --- | --- | --- | | 55 | 55 >= 80 is false | comfortable | | 72 | 72 >= 80 is false | comfortable | | 90 | 90 >= 80 is true | warm |

Exercise: conditionals.cpp

Reproduce status=comfortable for temperature 72, then try 55 and 90 and predict the branch result.