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Every C programmer has stared at a program that compiles perfectly and still gives the wrong answer.

The good news: beginners make the same few mistakes again and again. Once you know them, you will spot them in seconds — in your own code and in exam questions that are designed to trick you.

Here are the ten most common ones, each with the wrong code, the fix, and the reason.

1. Using = instead of ==

int marks = 35;

if (marks = 40)          // WRONG
    printf("Pass\n");

This always prints Pass. A single = is assignment: it puts 40 into marks, and the value of the whole expression is 40 — which is non-zero, so C treats it as true.

if (marks == 40)         // RIGHT: comparison

Tip: compile with warnings turned on (gcc -Wall prog.c). The compiler will warn you about an assignment inside a condition.

2. Forgetting the & in scanf

int age;
scanf("%d", age);        // WRONG
scanf("%d", &age);       // RIGHT

scanf has to store the number in your variable, so it needs the variable's address — that is what & gives. Without it, scanf uses the garbage value inside age as an address and writes somewhere random. The program may crash or silently corrupt other data.

Exception: strings. scanf("%19s", name) has no &, because an array name already is an address.

3. Integer division

int a = 7, b = 2;
float avg = a / b;       // avg = 3.000000, not 3.5!

When both operands are integers, C does integer division and throws the fraction away. Storing the result in a float afterwards does not bring it back.

float avg = (float) a / b;   // 3.500000
float avg2 = a / 2.0;        // also works: 2.0 is a double

The same trap catches 1 / 2, which is 0, so a formula like area = 1 / 2 * base * height always gives 0.

4. A semicolon after if, for or while

if (x > 10);             // WRONG: the ; is an empty statement
    printf("big\n");     // always runs

for (int i = 0; i < 5; i++);   // WRONG: loops 5 times doing nothing
    printf("Hi\n");            // prints once

The semicolon ends the if or the loop right there. Remove it — and always use braces { }, even for one line:

if (x > 10)
{
    printf("big\n");
}

5. Going one step past the end of an array

int a[5];

for (int i = 0; i <= 5; i++)   // WRONG: also touches a[5]
    a[i] = 0;

An array of size 5 has indexes 0 to 4. a[5] is not part of the array — and C does not check. Writing there quietly changes other variables, which leads to some of the strangest bugs you will ever see.

for (int i = 0; i < 5; i++)    // RIGHT

6. Using a variable before giving it a value

int sum;                       // WRONG: sum holds garbage

for (int i = 1; i <= 10; i++)
    sum += i;

Local variables in C are not set to 0 automatically. They contain whatever was in that memory before.

int sum = 0;                   // RIGHT

The same goes for counters, products (start at 1) and "largest so far" values (start with the first element, not 0 — otherwise an array of negative numbers gives the wrong maximum).

7. Forgetting break in a switch

switch (choice)
{
    case 1: printf("Tea\n");
    case 2: printf("Coffee\n");
    default: printf("Water\n");
}

With choice = 1 this prints Tea, Coffee and Water. Once a case matches, C keeps running into the following cases ("fall-through") until it meets a break.

case 1: printf("Tea\n");    break;
case 2: printf("Coffee\n"); break;
default: printf("Water\n");

8. scanf("%c") reading the leftover Enter key

int age;
char grade;

scanf("%d", &age);
scanf("%c", &grade);     // WRONG: reads the '\n' left by Enter

When you type 20 and press Enter, %d reads 20 but leaves the '\n' in the input buffer. The next %c happily reads that newline instead of waiting for you.

scanf(" %c", &grade);    // RIGHT: the space skips spaces and newlines

9. Comparing strings with ==

char answer[10];
scanf("%9s", answer);

if (answer == "yes")             // WRONG: compares addresses

This is never true, even when the user types yes. == compares where the two strings are stored, not the text.

#include <string.h>

if (strcmp(answer, "yes") == 0)  // RIGHT: 0 means equal

10. Ignoring compiler warnings

This one is not a single bug — it hides all the others.

Beginners often see a warning, notice the program still runs, and move on. But most of the mistakes in this list produce a warning first:

  • assignment used as a condition (if (x = 5))
  • format %d expects int * (missing & in scanf)
  • variable used uninitialised
  • implicit declaration of a function (missing #include or prototype)

Compile like this and read every warning:

gcc -Wall -Wextra prog.c -o prog

A program with zero warnings is not guaranteed to be correct — but a program with warnings is very often wrong.

A simple debugging habit

When a program prints the wrong answer, don't change random lines and hope. Instead:

  1. Print the values of your variables at important points: printf("i=%d sum=%d\n", i, sum);
  2. Trace by hand with a small input, writing each variable's value in a table.
  3. Find the first line where the real value differs from what you expected. The bug is there.

Learn to spot these mistakes visually

Many of these bugs are much easier to understand when you can watch the program run. In our free interactive C course every lesson has a "Common mistakes" section and a lab that runs the code line by line, for example:

Quick checklist

Before you submit a program, check:

  • == in every comparison
  • & before every non-array variable in scanf
  • at least one double/float operand in divisions that need decimals
  • no ; straight after if, for or while
  • loops over arrays use i < size
  • every variable has a value before it is used
  • every case ends with break (unless you really want fall-through)
  • " %c" with a space when reading a character
  • strcmp for strings
  • zero compiler warnings with -Wall

Keep this list next to you for your next lab session — and enjoy spotting these bugs in your friends' code too.