C Updating Arrays

How it works

An array subscript selects one element object, so assigning through it changes that element. An index is only a position in the current layout. If readings are reordered, an old position may identify a different record. Before an update derived from outside input, validate the index against the element count. This lesson uses size_t throughout the index contract to avoid accepting a negative int and then accidentally converting it into a very large unsigned position.

Run it locally

Use a hosted C17 compiler in a separate lab folder. The command assumes a driver named cc; substitute gcc, clang or zig cc as appropriate. C does not currently run in Bhylu’s browser playground. Apache, PHP and MySQL are not needed for these C programs.

cc -std=c17 -Wall -Wextra -Wpedantic -Werror array-updates.c -o lesson

On Windows, use -o lesson.exe and run .\lesson.exe in PowerShell. On Unix-like systems run ./lesson. Math-library linking differs by toolchain; GCC on many Unix systems needs -lm after the source.

Download the complete C source. File-writing labs must run in an empty private directory, away from existing reports.

Example

#include <stdio.h>

int main(void) {
    int depths[] = {12, 19, 16};
    size_t count = sizeof depths / sizeof depths[0];
    size_t position = 1;
    if (position < count) { depths[position] = 17; }
    for (size_t i = 0; i < count; ++i) {
        printf("%zu: %d\n", i, depths[i]);
    }
    return 0;
}

Expected result

0: 12
1: 17
2: 16

The check happens before evaluating the selected element. Only the middle reading changes. The count remains three because assignment replaces a value without changing array storage. For a larger application, give each reading a record identifier and resolve its current position before applying an edit. A valid bound does not prove that an edit targets the right real-world reading.