Throw exception when mappings are incorrect

src/CsvFile.php

@@ -141,9 +141,18 @@ public function set_column_names($mapping) {
 			$this->column_names = $mapping;
 		} else {
 			$mapping_indecies = array_flip($this->column_names);
+
+			// User code wants to map part of the columns
 			if (count($mapping_indecies) !== count($mapping)) {
 				$mapping_indecies = array_intersect_key($mapping_indecies, $mapping);
 			}
+
+			// some of the columns that the user code wants to map are not found
+			if (count($mapping_indecies) !== count($mapping)) {
+				$bad_cols = array_diff_key($mapping, $mapping_indecies);
+				throw new Exception("The following column(s) are not present in the source file: " . implode(', ', $bad_cols));
+			}
+
 			$this->column_names = array_combine(
 				$mapping_indecies,
 				$mapping

tests/CsvFileTest.php

@@ -242,7 +242,17 @@ function test_non_utf8_encoded_file() {
 		], $csv->to_array() );
 	}
 
-
+	function test_missing_column_causes_exception() {
+		$this->expectException(Exception::class);
+
+		$csv = new CsvFile(__DIR__ . '/sample-data/info.csv');
+		$csv->use_first_row_as_header();
+		$csv->set_column_names([
+			'Last Name Typo' => 'lname',
+			'Address' => 'address',
+		]);
+	}
+
 	function test_map_partial_columns() {
 		$csv = new CsvFile(__DIR__ . '/sample-data/info.csv');
 		$csv->use_first_row_as_header();