Basic C Language Programs Hello World to Sorting

Program Shape and Compilation

Small C programs are useful only when each one teaches a concrete language or algorithm rule. Build, run, test boundaries, and explain the state changes instead of collecting disconnected snippets.

Every hosted C program starts at main. Include only the headers required by the functions you call, declare functions before use, return a meaningful exit status, and compile with warnings enabled. A compiler warning often identifies undefined behavior before it becomes a runtime defect.

Use a current language mode such as -std=c17.
Enable -Wall -Wextra -Wpedantic during practice.
Treat warnings as code defects, not decoration.

Input, Validation, and Output

Formatted input must match the destination type and must be checked. scanf returns the number of successful conversions; using an unchanged variable after a failed conversion is a logic error. For line-oriented input, fgets plus deliberate parsing gives better control over length and trailing text.

Choose types from the data range, check arithmetic boundaries when input is untrusted, and keep output labels clear enough that a failed test can be diagnosed.

Check every input operation.
Bound strings and array indexes.
Separate parsing from the calculation.

Number and Decision Programs

Programs such as even/odd, maximum of three values, leap-year checks, prime tests, factorial, and digit sums teach expressions, conditions, and loop invariants. Define the accepted domain first: factorial is not defined here for negative input, and a prime test must reject values below two.

Prefer an algorithm that expresses the rule directly. A prime test needs candidate divisors only through the square root; compare divisor <= value / divisor to avoid overflow from divisor * divisor.

Arrays, Strings, and Sorting

Array programs must carry both the storage and its element count. Searching returns a position or a clear not-found result. Sorting examples should state whether they modify the input, whether equal values remain stable, and their time and space cost.

C strings are null-terminated character arrays. Reserve space for the terminator, do not read beyond the destination, and distinguish byte length from allocated capacity.

Test Matrix

For every program, test the smallest valid input, a typical input, repeated values, an invalid input, and the largest practical boundary. Write the expected result before running the program. Use sanitizers when available to expose out-of-bounds access, use-after-free, and undefined arithmetic.

Validated Maximum of Three

#include <stdio.h>

      int main(void) {
          int a, b, c;
          if (scanf("%d %d %d", &a, &b, &c) != 3) {
              fputs("Expected three integers.\n", stderr);
              return 1;
          }

          int largest = a;
          if (b > largest) largest = b;
          if (c > largest) largest = c;
          printf("Largest: %d\n", largest);
          return 0;
      }

Insertion Sort with an Explicit Length

#include <stddef.h>
      #include <stdio.h>

      void insertion_sort(int values[], size_t count) {
          for (size_t i = 1; i < count; ++i) {
              int key = values[i];
              size_t j = i;
              while (j > 0 && values[j - 1] > key) {
                  values[j] = values[j - 1];
                  --j;
              }
              values[j] = key;
          }
      }

      int main(void) {
          int values[] = {7, 2, 7, -1, 4};
          size_t count = sizeof values / sizeof values[0];
          insertion_sort(values, count);
          for (size_t i = 0; i < count; ++i) printf("%d%c", values[i], i + 1 == count ? '\n' : ' ');
      }

Before you move on