STEP 4 Review: for/while, Arrays, and Strings

① Key-point cheat sheet

Review the essentials of STEP 4 in these tables .

🔁 Three parts of a for loop

for (int i = 0; i < 3; i++) {
    printf("%d ", i);
}
// Output: 0 1 2

Initialization / condition / update are separated by semicolons
A false condition ends the loop
Useful when the iteration count is known

🔄 while / do-while

// Check condition → execute 
while (cond) {
    // Zero or more iterations 
}

// Execute → check condition 
do {
    // At least one iteration 
} while (cond);

while: when the condition is false initially , the body never executes
do-while: at least one iteration executes (a trailing ; is required)
Useful when the number of iterations is unknown

🛑 break / continue

for (int i = 0; i < 5; i++) {
    if (i == 3) break;     // Exit the loop → 0 1 2 
    if (i == 2) continue;  // Skip this iteration only 
    printf("%d ", i);
}

break: the loop is exited completely
continue: skip the rest and move to the next iteration .

📦 Array basics

int a[5] = {10, 20, 30, 40, 50};
// Indices run from 0 to N-1 (0–4 here) 
printf("%d\n", a[0]); // 10
printf("%d\n", a[4]); // 50
// a[5] is out of bounds: undefined behavior

Indices run from 0 to N-1 (for N elements, the last is a[N-1])
Out-of-bounds access causes undefined behavior (possible crashes or meaningless values)
Works well with for: for(i=0; i<N; i++) ... a[i] ...

🔤 Strings (char arrays)

char s[] = "Hi";
// Memory: 'H' 'i' '\0' — terminator added automatically 

printf("%s\n", s);          // Hi
printf("%zu\n", strlen(s)); // 2 (excluding \0) 
printf("%zu\n", sizeof s); // 3 (array size including \0)

A C string is terminated by \0(a string literal includes it automatically)
strlen counts characters before \0; sizeof gives the array size in bytes
#include <string.h> declares strlen and related functions

② Six code-reading questions

Execute the code mentally before choosing an answer. Read the explanation even if you get it wrong.

Q1. A for loop's boundary (< versus <=)

for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}

What is the output?

1 2 3 4

1 2 3 4 5

0 1 2 3 4

2 3 4 5

Explanation: The condition i<=5 includes i=5. Starting at 1 gives five iterations: 1, 2, 3, 4, 5. With i<5 there would be four. Watch this off-by-one difference.

Q2. do-while executes at least once

int i = 10;
do {
    printf("%d ", i);
    i++;
} while (i < 3);

What is the output?

Nothing

10 11 12

Infinite loop

Explanation: do-while executes the body before checking the condition. It prints 10 and increments i to 11; 11<3 is false, so it stops. A while loop with the same initial condition would execute zero times.

Q3. Sum of array elements

int a[] = {2, 4, 6, 8};
int sum = 0;
for (int i = 0; i < 4; i++) sum += a[i];
printf("%d\n", sum);

What is the output?

Explanation: The running sum is 0 → 2 → 6 → 12 → 20. A standard array loop visits indices 0 through N-1.

Q4. Exiting with break

for (int i = 0; i < 5; i++) {
    if (i == 3) break;
    printf("%d ", i);
}

What is the output?

0 1 2 3 4

0 1 2

0 1 2 4

0 1 2 3

Explanation: At i=3, break exits the loop before printf runs. The output is 0 1 2. Replacing break with continue would skip only that iteration and print 0 1 2 4.

Q5. Accessing outside an array

int a[3] = {10, 20, 30};
for (int i = 0; i <= 3; i++) {
    printf("%d ", a[i]);
}

What happens in this code?

It reliably prints 10 20 30 without problems

Compilation error (bounds are checked)

It accesses a[3], causing undefined behavior (possible meaningless values or a crash)

a[3] is automatically treated as 0

Explanation: A three-element array has valid indices 0, 1, and 2. The condition i<=3 also accesses a[3], causing undefined behavior. C does not automatically check bounds; a crash, corrupted data, or apparently normal output are all possible. Use i < N.

Q6. String length

#include <stdio.h>
#include <string.h>

int main(void) {
    char s[10] = "Hello";
    printf("%zu\n", strlen(s));
    return 0;
}

What is the output?

10 (array capacity)

6 (including \0)

"Hello"

Explanation: strlen counts characters before the first null terminator. Hello contains five characters, so strlen(s) is 5. The terminator is excluded. In this example sizeof s is 10, the array capacity in bytes.

③ Common mistakes and tricky points

If an item feels familiar, return to its original lesson and review it.

1
<and <=caused an off-by-one iteration count
i<5 runs five times; i<=5 runs six. With an array, that can access outside its bounds.
2
Omitted an update, causing an infinite loop
while(i<5){...}inside i++ can leave the loop running forever if the update is omitted. Ensure something makes the condition false eventually.
3
Counted array indices from 1
C array indices start at 0 . For N elements, the valid indices are 0 〜 N-1.
4
Accessed outside array bounds
Compilation may succeed because C does not check bounds, but this causes undefined behavior . It may crash, corrupt another value, or appear to work.
5
Confused break and continue
break exits the loop; continue starts the next iteration. Confusing them changes the output.
6
Forgot the semicolon after do-while
do { ... } while (cond); needs its final semicolon, unlike an ordinary while loop.
7
Confused strlen and sizeof
strlen("Hi") is 2 (character count); sizeof "Hi" is 3 (bytes including \0). They serve different purposes.

① Key-point cheat sheet

🔁 Three parts of a for loop

🔄 while / do-while

🛑 break / continue

📦 Array basics

🔤 Strings (char arrays)

② Six code-reading questions

Q1. A for loop's boundary (< versus <=)

What is the output?

Q2. do-while executes at least once

What is the output?

Q3. Sum of array elements

What is the output?

Q4. Exiting with break

What is the output?

Q5. Accessing outside an array

What happens in this code?

Q6. String length

What is the output?

③ Common mistakes and tricky points

④ Lessons to revisit when you get stuck