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10.4. for loops

The loops we have written so far have a number of elements in common. All of them start by initializing a variable; they have a test, or condition, that depends on that variable; and inside the loop they do something to that variable, like increment it.

This type of loop is so common that there is an alternate loop statement, called for, that expresses it more concisely. The general syntax looks like this:

for (INITIALIZER; CONDITION; INCREMENTOR) {
  BODY
}

This statement is exactly equivalent to

INITIALIZER;
while (CONDITION) {
  BODY
  INCREMENTOR
}

except that it is more concise and, since it puts all the loop-related statements in one place, it is easier to read. For example:

int i;
for (i = 0; i < 4; i++) {
  cout << count[i] << endl;
}

is equivalent to

int i = 0;
while (i < 4) {
  cout << count[i] << endl;
  i++;
}

Run the active code below, which uses a for loop.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> count = {1,2,3,4}; int i; for (i = 0; i < 4; i++) { cout << count[i] << endl; } }

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Run the active code below, which uses a while loop.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> count = {1,2,3,4}; int i = 0; while (i < 4) { cout << count[i] << endl; i++; } }

Q-4: Where are the incrementors in for loops and while?

• in the BODIES of both loops
• Incorrect!
• in the BODY of a for loop, and in the statement of a while loop
• Incorrect!
• in the statement of a for loop, and in the BODY of a while loop
• Correct!
• in the statements of both loops
• Incorrect!

Construct the half_life() function that prints the first num half lives of the initial amount.

        void half_life(int initial_amount, int num) {
---
int half_life(int initial_amount, int num) {                         #paired
---
   int new_amount = initial_amount;
---
   for (int i = 0; i < num; i++) {
---
   for (int i = 0; i < num; i++) {                         #paired
---
      new_amount = new_amount / 2;
---
      new_amount / 2;                         #paired
---
      cout << new_amount << endl;
---
   return new_amount;                         #distractor
---
   }
}
        

Run the active code below, which uses a for loop with a negative change in the “INCREMENTOR”.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> count = {1,2,3,4}; int i; for (i = 3; i > -1; i--){ cout << count[i] << endl; } }

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