Qt Widgets: Setting Minimum Value for Integer Dialogs with QInputDialog::intMinimum

Purpose

• In Qt's QInputDialog class, specifically designed for creating integer input dialogs, intMinimum serves to establish the lowest allowable integer value a user can enter.

Functionality

• This helps guide the user towards valid input within a specific range and prevents them from entering nonsensical or out-of-range values.
• When you call the setIntMinimum(int min) function, you set the minimum boundary for the integer input field. Any value the user tries to enter that falls below this minimum will be rejected.

Example

#include <QApplication>
#include <QInputDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int minAge = 18; // Set the minimum age to 18

    int age = QInputDialog::getInt(nullptr, "Enter Age", "How old are you?", minAge, 120, 1); // Set minimum to 18, maximum to 120, step of 1

    if (age != -1) { // Check if dialog was accepted (not canceled)
        if (age < minAge) {
            // Handle case where user entered a value below the minimum
            qDebug() << "You must be at least 18 years old.";
        } else {
            qDebug() << "Your age is:" << age;
        }
    } else {
        qDebug() << "Input dialog canceled.";
    }

    return 0;
}

In this example:

1. We set the minimum age to 18 using minAge.
2. The QInputDialog::getInt function creates an integer input dialog with the label "Enter Age", a default value of minAge (18), a minimum of minAge, a maximum of 120, and a step size of 1.
3. If the user enters a value less than 18, the code handles the error by printing a message.

• intMinimum returns the current minimum value set for the dialog.
• You can use setIntRange(int min, int max) to set both the minimum and maximum values simultaneously.

---

Setting Minimum Based on User Input

#include <QApplication>
#include <QInputDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int level = QInputDialog::getInt(nullptr, "Enter Level", "What level do you want to start at?", 1, INT_MAX, 1);

    if (level != -1) {
        int minEnemies = level * 2; // Set minimum enemies based on level
        int numEnemies = QInputDialog::getInt(nullptr, "Enter Enemies", "How many enemies do you want to fight?", minEnemies, INT_MAX, 5);

        if (numEnemies != -1) {
            if (numEnemies < minEnemies) {
                qDebug() << "You must fight at least" << minEnemies << "enemies at this level.";
            } else {
                qDebug() << "You will fight" << numEnemies << "enemies.";
            }
        } else {
            qDebug() << "Number of enemies input canceled.";
        }
    } else {
        qDebug() << "Level input canceled.";
    }

    return 0;
}

• Then, we dynamically set the minimum number of enemies based on the level (e.g., level 2 requires at least 4 enemies).
• We first get the level from the user.

User Input Validation with Range

#include <QApplication>
#include <QInputDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int value = QInputDialog::getInt(nullptr, "Enter Value", "Enter a value between 10 and 50:", 20, 10, 50, 5); // Set min to 10, max to 50, step of 5

    if (value != -1) {
        qDebug() << "You entered:" << value;
    } else {
        qDebug() << "Input dialog canceled.";
    }

    return 0;
}

Here, we:

• The step size (5) allows users to only enter values in increments of 5.
• Set both the minimum (10) and maximum (50) values within the QInputDialog::getInt function.

Using intRange for Combined Minimum and Maximum

#include <QApplication>
#include <QInputDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int minVolume = 0;
    int maxVolume = 100;

    QInputDialog::setIntRange(minVolume, maxVolume); // Set range

    int volume = QInputDialog::getInt(nullptr, "Set Volume", "Enter volume level:", maxVolume / 2, minVolume, maxVolume, 5);

    if (volume != -1) {
        qDebug() << "Volume set to:" << volume;
    } else {
        qDebug() << "Volume setting canceled.";
    }

    return 0;
}

This example:

• The dialog then uses these predefined values for the range.
• Uses QInputDialog::setIntRange to define the minimum (0) and maximum (100) volume levels upfront.

---

Custom Validation with QValidator

• Set the validator for the QLineEdit used within the dialog using QLineEdit::setValidator().
• Create a custom validator subclass that inherits from QValidator and override the validate() method to implement your validation rules.
• If you require more complex validation logic beyond just a minimum value, you can leverage the QValidator class.

#include <QApplication>
#include <QInputDialog>
#include <QValidator>

class MinValueValidator : public QValidator {
public:
    MinValueValidator(int min) : QValidator(nullptr), minVal(min) {}

    QValidator::State validate(QString input, int&) const override {
        bool ok;
        int value = input.toInt(&ok);
        return (ok && value >= minVal) ? QValidator::Acceptable : QValidator::Invalid;
    }

private:
    int minVal;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int minAge = 18;
    MinValueValidator validator(minAge);

    QLineEdit* ageEdit = new QLineEdit;
    ageEdit->setValidator(&validator);

    QString ageStr = QInputDialog::getText(nullptr, "Enter Age", "How old are you?", QLineEdit::Normal, "", &ageEdit);

    if (!ageStr.isEmpty()) {
        int age = ageStr.toInt();
        if (age >= minAge) {
            qDebug() << "Your age is:" << age;
        } else {
            qDebug() << "You must be at least 18 years old.";
        }
    } else {
        qDebug() << "Input dialog canceled.";
    }

    return 0;
}

• We set this validator for the QLineEdit used within the dialog.
• We create a MinValueValidator subclass that validates if the input is an integer greater than or equal to minVal.

Lambda Function for Simple Validation

• The lambda function can check the input value and return a validation state (QValidator::State).
• For simpler validation scenarios, you can use a lambda function directly within the QInputDialog::getInt function.

#include <QApplication>
#include <QInputDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    int minAge = 18;

    int age = QInputDialog::getInt(nullptr, "Enter Age", "How old are you?", minAge, 120, 1,
                                     [minAge](int value) {
                                         return (value >= minAge) ? QValidator::Acceptable : QValidator::Invalid;
                                     });

    if (age != -1) {
        qDebug() << "Your age is:" << age;
    } else {
        qDebug() << "Input dialog canceled.";
    }

    return 0;
}

• The lambda function checks if the input value is greater than or equal to minAge and returns the corresponding validation state.

• For basic validation within the dialog itself, consider a lambda function.
• If you need more complex validation logic, opt for a custom QValidator subclass.
• Use QInputDialog::intMinimum for simple minimum value enforcement.