unsigned char i2c_read(void)
{
unsigned char i;
lsb = SDA;
for (i = 0; i <= 7; i++) {
EP_DATA = EP_DATA << 1;
lsb = SDA;
SCL = 1;
SCL = 0;
}
if (EP_DATA == 13) {
SDA = 1;
SCL = 1;
SCL = 0;
SDA = 0;
i2c_stop();
return EP_DATA;
}
SDA = 0;
SCL = 1;
SCL = 0;
SDA = 1;
return EP_DATA;
}
I found the code on GitHub for I²C protocol for communication with DS1307 written by Embetronicx and I was unable to understand the use of the if condition if (EP_DATA == 13)
if (EP_DATA == 13) {
SDA = 1;
SCL = 1;
SCL = 0;
SDA = 0;
i2c_stop();
return EP_DATA
}
So, please can someone explain the use to that.
I don't think that this can be real working code.
This is not least because there are no delays between the clock edges. Unless the microcontroller executes these instructions at I2C compatible speeds (1940s hardware maybe?) then this will not work.
The particular if block you refer to can never be entered. Because ED_DATA is left shifted 8 times in the preceding loop, it can never equal 13.
If I was going to guess the meaning of 13 then I might suggest that it was meant to be the numerical value of '\r' a carriage return, delimiting the end of a line of ASCII text. Really though only the author of the code can tell you for certain why they wrote this.
Related
I have a PCA9535 GPIO Expander board connected through I²C to my Raspberry Pi. I am able to set the GPIO expander output pins (to high) using the i2cset command:
sudo i2cset 1 0x20 0x02 0xFF // 0x20 (gpio expander), and register 0x02
I came across a kernel driver for the PCA953X and loaded the kernel module gpio-pca953x.ko + modified the /boot/config.txt to include the dts overlay for the expander. When I run i2detect -y 1 (for i2c-1 bus), I can see "UU" at the 0x20 slave address, which corroborates that the driver is managing it.
If I wanted to do something comparable to what I did with the i2cset command programmatically, what API / interface should I use such that I make use of the PCA953X driver that's installed? As you can see from the code below, nothing there suggests that I would be utilizing PCA953X:
int file;
int adapter_nr = 1;
char filename[20];
snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
file = open(filename, O_RDWR);
if (file < 0) {
exit(1);
}
// Writes 0xFF to register 0x02
uint8_t cmd[2] = {0x02, 0XFF};
struct i2c_msg msg = {
.addr = 0x20,
.flags = 0,
.len = sizeof(cmd)/sizeof(uint8_t),
.buf = cmd
};
struct i2c_rdwr_ioctl_data tx = {
.msgs = &msg,
.nmsgs = 1
};
ioctl(file, I2C_RDWR, &tx);
Is there a programming route that involves utilizing the PCA953X driver? What does having that module actually do?
Thanks to #TomV for pointing it out in the comments. The PCA953X driver provides a new character device in the form of "gpiochipN". In my case, it was gpiochip2. I had not noticed this extra gpiochip in /dev as I was not looking for it. Because this is a character device, I was able to use ioctl to interact with the lines:
int fd, ret;
fd = open("/dev/gpiochip2", O_RDONLY);
if (fd < 0) {
printf("Unable to open: %s", strerror(errno));
return;
}
struct gpiohandle_request req;
req.lineoffsets[0] = 0;
req.lineoffsets[1] = 1;
req.lineoffsets[2] = 2;
req.lineoffsets[3] = 3;
req.lineoffsets[4] = 4;
req.lineoffsets[5] = 5;
req.lineoffsets[6] = 6;
req.lineoffsets[7] = 7;
req.lines = 8;
req.flags = GPIOHANDLE_REQUEST_OUTPUT;
ret = ioctl(fd, GPIO_GET_LINEHANDLE_IOCTL, &req);
if (-1 == ret) {
printf("Failed to get line handle:%s\n", strerror(ret));
close(fd);
return;
}
// Sets all 8 lines to high (equivalent to setting register 0x3 to 0b11111111 or 0xFF)
struct gpiohandle_data data;
data.values[0] = 1;
data.values[1] = 1;
data.values[2] = 1;
data.values[3] = 1;
data.values[4] = 1;
data.values[5] = 1;
data.values[6] = 1;
data.values[7] = 1;
ret = ioctl(req.fd, GPIOHANDLE_SET_LINE_VALUES_IOCTL, &data);
if (-1 == ret) {
printf("Failed to set line value\n");
}
else {
close(req.fd);
}
close(fd);
The above code interacts with the gpiochip2 character device, which is made possible by having the PCA953X installed. Without it, I would have to read and write to the registers through the i2c bus as was posted in my original question.
I have made a custom board that features a STM32F469NI MCU coupled with AS4C4M32S SDRAM.
Following are the SDRAM timing and register set information:
hsdram1.Instance = FMC_SDRAM_DEVICE;
/* hsdram1.Init */
hsdram1.Init.SDBank = FMC_SDRAM_BANK1;
hsdram1.Init.ColumnBitsNumber = FMC_SDRAM_COLUMN_BITS_NUM_8;
hsdram1.Init.RowBitsNumber = FMC_SDRAM_ROW_BITS_NUM_12;
hsdram1.Init.MemoryDataWidth = FMC_SDRAM_MEM_BUS_WIDTH_32;
hsdram1.Init.InternalBankNumber = FMC_SDRAM_INTERN_BANKS_NUM_4;
hsdram1.Init.CASLatency = FMC_SDRAM_CAS_LATENCY_3;
hsdram1.Init.WriteProtection = FMC_SDRAM_WRITE_PROTECTION_DISABLE;
hsdram1.Init.SDClockPeriod = FMC_SDRAM_CLOCK_PERIOD_2;
hsdram1.Init.ReadBurst = FMC_SDRAM_RBURST_DISABLE;
hsdram1.Init.ReadPipeDelay = FMC_SDRAM_RPIPE_DELAY_0;
/* SdramTiming */
SdramTiming.LoadToActiveDelay = 2;
SdramTiming.ExitSelfRefreshDelay = 7;
SdramTiming.SelfRefreshTime = 4;
SdramTiming.RowCycleDelay = 6;
SdramTiming.WriteRecoveryTime = 2;
SdramTiming.RPDelay = 2;
SdramTiming.RCDDelay = 2;
if (HAL_SDRAM_Init(&hsdram1, &SdramTiming) != HAL_OK)
{
Error_Handler( );
}
/* USER CODE BEGIN FMC_Init 2 */
FMC_SDRAM_CommandTypeDef cmd = { 0 };
cmd.CommandMode = FMC_SDRAM_CMD_CLK_ENABLE;
cmd.CommandTarget = FMC_SDRAM_CMD_TARGET_BANK1;
cmd.AutoRefreshNumber = 1;
cmd.ModeRegisterDefinition = 0;
HAL_SDRAM_SendCommand(&hsdram1, &cmd, 0xFFFF);
HAL_Delay(100);
cmd.CommandMode = FMC_SDRAM_CMD_PALL;
cmd.CommandTarget = FMC_SDRAM_CMD_TARGET_BANK1;
cmd.AutoRefreshNumber = 1;
cmd.ModeRegisterDefinition = 0;
HAL_SDRAM_SendCommand(&hsdram1, &cmd, 0xFFFF);
cmd.CommandMode = FMC_SDRAM_CMD_AUTOREFRESH_MODE;
cmd.CommandTarget = FMC_SDRAM_CMD_TARGET_BANK1;
cmd.AutoRefreshNumber = 8;
cmd.ModeRegisterDefinition = 0;
HAL_SDRAM_SendCommand(&hsdram1, &cmd, 0xFFFF);
cmd.CommandMode = FMC_SDRAM_CMD_LOAD_MODE;
cmd.CommandTarget = FMC_SDRAM_CMD_TARGET_BANK1;
cmd.AutoRefreshNumber = 1;
cmd.ModeRegisterDefinition = (uint32_t) 0x230; //also tried 0x231
HAL_SDRAM_SendCommand(&hsdram1, &cmd, 0xFFFF);
//Setup SDRAM Refresh Timer Register with the delay between refresh cycles of the SDRAM, this is SDRAM specific.
HAL_SDRAM_ProgramRefreshRate(&hsdram1, 1404); //Tried 1386
This same code works when running on a STM32F469-I DISCOVERY Board but fails when It is run on my custom carrier board.
On my board, whenever I write to any SDRAM address, only the first byte is READ/WRITTEN to, for example when I write 0x00 to address 0xC0000000, the data at that address when monitored under debugging contains 0x48129100 i.e first 3 BYTES are random garbage and only the last byte is written correctly(0x00), and suppose If i write 0xAA to the same address the data there when monitored contains the value 0x481291AA.
This is true to every address that I read/write to/from the external SDRAM.
Only the last byte is written/read.
The custom board schematic and PCB layout is identical to the STM32F469-I DISCO board.
Only difference is the RAM chip which instead of being MICRON one is Alliance Memory's AS4C4M32S .
What can be the issue of such a problem?
Reading SDRAM always uses at the full bit width, but writing requires to set the four signals BL0, BL1, BL2 and BL3 to say which bytes to write.
It sounds like only BL0 is currently working. Perhaps BL1-3 are not physically connected (hardware) or perhaps you just forgot to configure them (software).
(If it turns out to be a hardware problem then someone with the appropriate permissions will want to move this question and answer to the electronics stack exchange)
To be clear, I'm very new to STM32 and MBED programming, but is it possible to create valid VGA signal, using STM32 nucleo-F070RB board? I've picked up this standard, and my goal is to display "something" on screen. With this i mean I should have control which pixel i want to, well, turn on.
For demo, here is my (very crude) sketch:
#include <mbed.h>
DigitalOut led(LED1);
DigitalOut h_sync(PC_3);
DigitalOut v_sync(PC_2);
DigitalOut c_red(PC_0);
int main() {
h_sync = 1;
v_sync = 1;
int line_count = 0;
int color_red = 0;
while(1) {
wait_us(21);
h_sync = 0;
wait_us(3);
h_sync = 1;
wait_ns(2);
line_count++;
color_red++;
if (color_red == 16) { color_red = 0; c_red = !c_red; }
if (line_count == 601) v_sync = 0;
if (line_count == 605) v_sync = 1;
if (line_count == 628) { line_count = 0; c_red = 0; color_red = 0; }
}
}
I have connected V-Sync to V-Sync of my monitor, same with H-Sync and c_red (color red) through 560Ohm resistor to RED signal. And it (kind of) worked! It displayed red strips every 16 lines. Perfect, but I need to be able to control every pixel (if it's possible). I've seen some VGA libraries (maybe), but i really need to write it myself - something very crude. I just only want to have some sort of control over pixels, not the some super-super something (Just for me to learn :) ). And because I don't have much experience with STMs, after hours i was not able to "convince" my board to generate such "high-speed" signals, so, it is after all possible?
I was using MBED's Ticker function to generate the timing for each pixel, but it did not work - the fastest the Ticker went for me was something around few miliseconds, far too much. Can I use timer interrupts? Or something else?
I implemented a Galois Linear-Feedback Shift-Regiser in Verilog (and also in MATLAB, mainly to emulate the HDL design). It's been working great, and as of know I use MATLAB to calculate CRC-32 fields, and then include them in my HDL simulations to verify a data packet has arrived correctly (padding data with CRC-32), which produces good results.
The thing is I want to be able to calculate the CRC-32 I've implemented in software, because I'll be using a Raspberry Pi to input data through GPIO in my FPGA, and I haven't been able to do so. I've tried this online calculator, using the same parameters, but never get to yield the same result.
This is the MATLAB code I use to calculate my CRC-32:
N = 74*16;
data = [round(rand(1,N)) zeros(1,32)];
lfsr = ones(1,32);
next_lfsr = zeros(1,32);
for i = 1:length(data)
next_lfsr(1) = lfsr(2);
next_lfsr(2) = lfsr(3);
next_lfsr(3) = lfsr(4);
next_lfsr(4) = lfsr(5);
next_lfsr(5) = lfsr(6);
next_lfsr(6) = xor(lfsr(7),lfsr(1));
next_lfsr(7) = lfsr(8);
next_lfsr(8) = lfsr(9);
next_lfsr(9) = xor(lfsr(10),lfsr(1));
next_lfsr(10) = xor(lfsr(11),lfsr(1));
next_lfsr(11) = lfsr(12);
next_lfsr(12) = lfsr(13);
next_lfsr(13) = lfsr(14);
next_lfsr(14) = lfsr(15);
next_lfsr(15) = lfsr(16);
next_lfsr(16) = xor(lfsr(17), lfsr(1));
next_lfsr(17) = lfsr(18);
next_lfsr(18) = lfsr(19);
next_lfsr(19) = lfsr(20);
next_lfsr(20) = xor(lfsr(21),lfsr(1));
next_lfsr(21) = xor(lfsr(22),lfsr(1));
next_lfsr(22) = xor(lfsr(23),lfsr(1));
next_lfsr(23) = lfsr(24);
next_lfsr(24) = xor(lfsr(25), lfsr(1));
next_lfsr(25) = xor(lfsr(26), lfsr(1));
next_lfsr(26) = lfsr(27);
next_lfsr(27) = xor(lfsr(28), lfsr(1));
next_lfsr(28) = xor(lfsr(29), lfsr(1));
next_lfsr(29) = lfsr(30);
next_lfsr(30) = xor(lfsr(31), lfsr(1));
next_lfsr(31) = xor(lfsr(32), lfsr(1));
next_lfsr(32) = xor(data2(i), lfsr(1));
lfsr = next_lfsr;
end
crc32 = lfsr;
See I use a 32-zeroes padding to calculate the CRC-32 in the first place (whatever's left in the LFSR at the end is my CRC-32, and if I do the same replacing the zeroes with this CRC-32, my LFSR becomes empty at the end too, which means the verification passed).
The polynomial I'm using is the standard for CRC-32: 04C11DB7. See also that the order seems to be reversed, but that's just because it's mirrored to have the input in the MSB. The results of using this representation and a mirrored one are the same when the input is the same, only the result will be also mirrored.
Any ideas would be of great help.
Thanks in advance
Your CRC is not a CRC. The last 32 bits fed in don't actually participate in the calculation, other than being exclusive-or'ed into the result. That is, if you replace the last 32 bits of data with zeros, do your calculation, and then exclusive-or the last 32 bits of data with the resulting "crc32", then you will get the same result.
So you will never get it to match another CRC calculation, since it isn't a CRC.
This code in C replicates your function, where the data bits come from the series of n bytes at p, least significant bit first, and the result is a 32-bit value:
unsigned long notacrc(void const *p, unsigned n) {
unsigned char const *dat = p;
unsigned long reg = 0xffffffff;
while (n) {
for (unsigned k = 0; k < 8; k++)
reg = reg & 1 ? (reg >> 1) ^ 0xedb88320 : reg >> 1;
reg ^= (unsigned long)*dat++ << 24;
n--;
}
return reg;
}
You can immediately see that the last byte of data is simply exclusive-or'ed with the final register value. Less obvious is that the last four bytes are just exclusive-or'ed. This exactly equivalent version makes that evident:
unsigned long notacrc_xor(void const *p, unsigned n) {
unsigned char const *dat = p;
// initial register values
unsigned long const init[] = {
0xffffffff, 0x2dfd1072, 0xbe26ed00, 0x00be26ed, 0xdebb20e3};
unsigned xor = n > 3 ? 4 : n; // number of bytes merely xor'ed
unsigned long reg = init[xor];
while (n > xor) {
reg ^= *dat++;
for (unsigned k = 0; k < 8; k++)
reg = reg & 1 ? (reg >> 1) ^ 0xedb88320 : reg >> 1;
n--;
}
switch (n) {
case 4:
reg ^= *dat++;
case 3:
reg ^= (unsigned long)*dat++ << 8;
case 2:
reg ^= (unsigned long)*dat++ << 16;
case 1:
reg ^= (unsigned long)*dat++ << 24;
}
return reg;
}
There you can see that the last four bytes of the message, or all of the message if it is three or fewer bytes, is exclusive-or'ed with the final register value at the end.
An actual CRC must use all of the input data bits in determining when to exclusive-or the polynomial with the register. The inner part of that last function is what a CRC implementation looks like (though more efficient versions make use of pre-computed tables to process a byte or more at a time). Here is a function that computes an actual CRC:
unsigned long crc32_jam(void const *p, unsigned n) {
unsigned char const *dat = p;
unsigned long reg = 0xffffffff;
while (n) {
reg ^= *dat++;
for (unsigned k = 0; k < 8; k++)
reg = reg & 1 ? (reg >> 1) ^ 0xedb88320 : reg >> 1;
n--;
}
return reg;
}
That one is called crc32_jam because it implements a particular CRC called "JAMCRC". That CRC is the closest to what you attempted to implement.
If you want to use a real CRC, you will need to update your Verilog implementation.
Imagine a std:vector, say, with 100 things on it (0 to 99) currently. You are treating it as a loop. So the 105th item is index 4; forward 7 from index 98 is 5.
You want to delete N items after index position P.
So, delete 5 items after index 50; easy.
Or 5 items after index 99: as you delete 0 five times, or 4 through 0, noting that position at 99 will be erased from existence.
Worst, 5 items after index 97 - you have to deal with both modes of deletion.
What's the elegant and solid approach?
Here's a boring routine I wrote
-(void)knotRemovalHelper:(NSMutableArray*)original
after:(NSInteger)nn howManyToDelete:(NSInteger)desired
{
#define ORCO ((NSInteger)[original count])
static NSInteger kount, howManyUntilLoop, howManyExtraAferLoop;
if ( ... our array is NOT a loop ... )
// trivial, if messy...
{
for ( kount = 1; kount<=desired; ++kount )
{
if ( (nn+1) >= ORCO )
return;
[original removeObjectAtIndex:( nn+1 )];
}
return;
}
else // our array is a loop
// messy, confusing and inelegant. how to improve?
// here we go...
{
howManyUntilLoop = (ORCO-1) - nn;
if ( howManyUntilLoop > desired )
{
for ( kount = 1; kount<=desired; ++kount )
[original removeObjectAtIndex:( nn+1 )];
return;
}
howManyExtraAferLoop = desired - howManyUntilLoop;
for ( kount = 1; kount<=howManyUntilLoop; ++kount )
[original removeObjectAtIndex:( nn+1 )];
for ( kount = 1; kount<=howManyExtraAferLoop; ++kount )
[original removeObjectAtIndex:0];
return;
}
#undef ORCO
}
Update!
InVariant's second answer leads to the following excellent solution. "starting with" is much better than "starting after". So the routine now uses "start with". Invariant's second answer leads to this very simple solution...
N times do if P < currentsize remove P else remove 0
-(void)removeLoopilyFrom:(NSMutableArray*)ra
startingWithThisOne:(NSInteger)removeThisOneFirst
howManyToDelete:(NSInteger)countToDelete
{
// exception if removeThisOneFirst > ra highestIndex
// exception if countToDelete is > ra size
// so easy thanks to Invariant:
for ( do this countToDelete times )
{
if ( removeThisOneFirst < [ra count] )
[ra removeObjectAtIndex:removeThisOneFirst];
else
[ra removeObjectAtIndex:0];
}
}
Update!
Toolbox has pointed out the excellent idea of working to a new array - super KISS.
Here's an idea off the top of my head.
First, generate an array of integers representing the indices to remove. So "remove 5 from index 97" would generate [97,98,99,0,1]. This can be done with the application of a simple modulus operator.
Then, sort this array descending giving [99,98,97,1,0] and then remove the entries in that order.
Should work in all cases.
This solution seems to work, and it copies all remaining elements in the vector only once (to their final destination).
Assume kNumElements, kStartIndex, and kNumToRemove are defined as const size_t values.
vector<int> my_vec(kNumElements);
for (size_t i = 0; i < my_vec.size(); ++i) {
my_vec[i] = i;
}
for (size_t i = 0, cur = 0; i < my_vec.size(); ++i) {
// What is the "distance" from the current index to the start, taking
// into account the wrapping behavior?
size_t distance = (i + kNumElements - kStartIndex) % kNumElements;
// If it's not one of the ones to remove, then we keep it by copying it
// into its proper place.
if (distance >= kNumToRemove) {
my_vec[cur++] = my_vec[i];
}
}
my_vec.resize(kNumElements - kNumToRemove);
There's nothing wrong with two loop solutions as long as they're readable and don't do anything redundant. I don't know Objective-C syntax, but here's the pseudocode approach I'd take:
endIdx = after + howManyToDelete
if (Len <= after + howManyToDelete) //will have a second loop
firstloop = Len - after; //handle end in the first loop, beginning in second
else
firstpass = howManyToDelete; //the first loop will get them all
for (kount = 0; kount < firstpass; kount++)
remove after+1
for ( ; kount < howManyToDelete; kount++) //if firstpass < howManyToDelete, clean up leftovers
remove 0
This solution doesn't use mod, does the limit calculation outside the loop, and touches the relevant samples once each. The second for loop won't execute if all the samples were handled in the first loop.
The common way to do this in DSP is with a circular buffer. This is just a fixed length buffer with two associated counters:
//make sure BUFSIZE is a power of 2 for quick mod trick
#define BUFSIZE 1024
int CircBuf[BUFSIZE];
int InCtr, OutCtr;
void PutData(int *Buf, int count) {
int srcCtr;
int destCtr = InCtr & (BUFSIZE - 1); // if BUFSIZE is a power of 2, equivalent to and faster than destCtr = InCtr % BUFSIZE
for (srcCtr = 0; (srcCtr < count) && (destCtr < BUFSIZE); srcCtr++, destCtr++)
CircBuf[destCtr] = Buf[srcCtr];
for (destCtr = 0; srcCtr < count; srcCtr++, destCtr++)
CircBuf[destCtr] = Buf[srcCtr];
InCtr += count;
}
void GetData(int *Buf, int count) {
int srcCtr = OutCtr & (BUFSIZE - 1);
int destCtr = 0;
for (destCtr = 0; (srcCtr < BUFSIZE) && (destCtr < count); srcCtr++, destCtr++)
Buf[destCtr] = CircBuf[srcCtr];
for (srcCtr = 0; srcCtr < count; srcCtr++, destCtr++)
Buf[destCtr] = CircBuf[srcCtr];
OutCtr += count;
}
int BufferOverflow() {
return ((InCtr - OutCtr) > BUFSIZE);
}
This is pretty lightweight, but effective. And aside from the ctr = BigCtr & (SIZE-1) stuff, I'd argue it's highly readable. The only reason for the & trick is in old DSP environments, mod was an expensive operation so for something that ran often, like every time a buffer was ready for processing, you'd find ways to remove stuff like that. And if you were doing FFT's, your buffers were probably a power of 2 anyway.
These days, of course, you have 1 GHz processors and magically resizing arrays. You kids get off my lawn.
Another method:
N times do {remove entry at index P mod max(ArraySize, P)}
Example:
N=5, P=97, ArraySize=100
1: max(100, 97)=100 so remove at 97%100 = 97
2: max(99, 97)=99 so remove at 97%99 = 97 // array size is now 99
3: max(98, 97)=98 so remove at 97%98 = 97
4: max(97, 97)=97 so remove at 97%97 = 0
5: max(96, 97)=97 so remove at 97%97 = 0
I don't program iphone for know, so I image std::vector, it's quite easy, simple and elegant enough:
#include <iostream>
using std::cout;
#include <vector>
using std::vector;
#include <cassert> //no need for using, assert is macro
template<typename T>
void eraseCircularVector(vector<T> & vec, size_t position, size_t count)
{
assert(count <= vec.size());
if (count > 0)
{
position %= vec.size(); //normalize position
size_t positionEnd = (position + count) % vec.size();
if (positionEnd < position)
{
vec.erase(vec.begin() + position, vec.end());
vec.erase(vec.begin(), vec.begin() + positionEnd);
}
else
vec.erase(vec.begin() + position, vec.begin() + positionEnd);
}
}
int main()
{
vector<int> values;
for (int i = 0; i < 10; ++i)
values.push_back(i);
cout << "Values: ";
for (vector<int>::const_iterator cit = values.begin(); cit != values.end(); cit++)
cout << *cit << ' ';
cout << '\n';
eraseCircularVector(values, 5, 1); //remains 9: 0,1,2,3,4,6,7,8,9
eraseCircularVector(values, 16, 5); //remains 4: 3,4,6,7
cout << "Values: ";
for (vector<int>::const_iterator cit = values.begin(); cit != values.end(); cit++)
cout << *cit << ' ';
cout << '\n';
return 0;
}
However, you might consider:
creating new loop_vector class, if you use this kind of functionality enough
using list if you perform many deletions (or few deletions (not from end, that's simple pop_back) but large array)
If your container (NSMutableArray or whatever) is not list, but vector (i.e. resizable array), you most definitely don't want to delete items one by one, but whole range (e.g. std::vector's erase(begin, end)!
Edit: reacting to comment, to fully realize what must be done by vector, if you erase element other than the last one: it must copy all values after that element (e.g. 1000 items in array, you erase first, 999x copying (moving) of item, that is very costly).
Example:
#include <iostream>
#include <vector>
#include <ctime>
using namespace std;
int main()
{
clock_t start, end;
vector<int> vec;
const int items = 64 * 1024;
cout << "using " << items << " items in vector\n";
for (size_t i = 0; i < items; ++i) vec.push_back(i);
start = clock();
while (!vec.empty()) vec.erase(vec.begin());
end = clock();
cout << "Inefficient method took: "
<< (end - start) * 1.0 / CLOCKS_PER_SEC << " ms\n";
for (size_t i = 0; i < items; ++i) vec.push_back(i);
start = clock();
vec.erase(vec.begin(), vec.end());
end = clock();
cout << "Efficient method took: "
<< (end - start) * 1.0 / CLOCKS_PER_SEC << " ms\n";
return 0;
}
Produces output:
using 65536 items in vector
Inefficient method took: 1.705 ms
Efficient method took: 0 ms
Note it's very easy to get inefficient, look e.g. have at http://www.cplusplus.com/reference/stl/vector/erase/