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+---
+Title: 'isless()'
+Description: 'Returns true if first argument less than second argument, false otherwise.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Booleans'
+  - 'Functions'
+  - 'Math'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The C++ **`isless()`** function returns `true` if the first argument is strictly less than the second, and `false` otherwise. It performs a quiet floating-point comparison, never raises exceptions, and returns `false` if either value is `NaN`. The function is defined in the `<cmath>` header.
+
+## Syntax
+
+```pseudo
+bool isless(x, y);
+```
+
+**Parameters:**
+
+- `x, y`: Floating-point values (`double`, `float`, or `long double`) or integers, which are promoted to floating-point types.
+
+**Return value:**
+
+The `isless()` function returns `true` if `x < y` and both values are valid numbers. Returns `false` if the comparison is `false` or if either argument is `NaN`.
+
+## Example
+
+The following example uses the `isless()` function to compare and determine if the first argument is lesser than the second argument:
+
+```cpp
+#include <iostream>
+#include <cmath>
+using namespace std;
+
+int main() {
+  bool result;
+  int x, y;
+
+  x = 5;
+  y = 7;
+  result = isless(x, y);
+  cout << x << " is less than " << y << ": " << result << endl;
+
+  x = 9;
+  y = 3;
+  result = isless(x, y);
+  cout << x << " is less than " << y << ": " << result << endl;
+
+  double z = nan("1");
+  result = isless(x, z);
+  cout << x << " is less than nan: " << result << endl;
+
+  return 0;
+}
+```
+
+This produces the following output:
+
+```shell
+5 is less than 7: 1
+9 is less than 3: 0
+9 is less than nan: 0
+```
+
+## Codebyte Example
+
+This example uses `isless()` inside conditional logic to safely compare values, including NaNs:
+
+```codebyte/cpp
+#include <iostream>
+#include <cmath>
+using namespace std;
+
+void compare(double x, double y) {
+  if (isless(x, y)) {
+    cout << x << " is LESS than " << y << "\n";
+  } else if (isless(y, x)) {
+    cout << x << " is GREATER than " << y << "\n";
+  } else {
+    cout << x << " and " << y << " are NOT comparable (possibly NaN)\n";
+  }
+}
+
+int main() {
+  compare(3.0, 7.0);
+  compare(9.0, 2.0);
+  compare(nan(""), 5.0);
+}
+```
+
+Use `isless()` in situations where direct comparison operators might behave unpredictably:
+
+- When comparisons may involve `NaN`, since `isless()` guarantees a defined boolean result.
+- When writing safe comparison logic that avoids undefined behavior.
+- When building math-heavy code such as sorting, filtering, or numerical checks that require robust comparisons.